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+======================
+Writing an ALSA Driver
+======================
+
+:Author: Takashi Iwai <tiwai@suse.de>
+:Date: Oct 15, 2007
+:Edition: 0.3.7
+
+Preface
+=======
+
+This document describes how to write an `ALSA (Advanced Linux Sound
+Architecture) <http://www.alsa-project.org/>`__ driver. The document
+focuses mainly on PCI soundcards. In the case of other device types, the
+API might be different, too. However, at least the ALSA kernel API is
+consistent, and therefore it would be still a bit help for writing them.
+
+This document targets people who already have enough C language skills
+and have basic linux kernel programming knowledge. This document doesn't
+explain the general topic of linux kernel coding and doesn't cover
+low-level driver implementation details. It only describes the standard
+way to write a PCI sound driver on ALSA.
+
+If you are already familiar with the older ALSA ver.0.5.x API, you can
+check the drivers such as ``sound/pci/es1938.c`` or
+``sound/pci/maestro3.c`` which have also almost the same code-base in
+the ALSA 0.5.x tree, so you can compare the differences.
+
+This document is still a draft version. Any feedback and corrections,
+please!!
+
+File Tree Structure
+===================
+
+General
+-------
+
+The ALSA drivers are provided in two ways.
+
+One is the trees provided as a tarball or via cvs from the ALSA's ftp
+site, and another is the 2.6 (or later) Linux kernel tree. To
+synchronize both, the ALSA driver tree is split into two different
+trees: alsa-kernel and alsa-driver. The former contains purely the
+source code for the Linux 2.6 (or later) tree. This tree is designed
+only for compilation on 2.6 or later environment. The latter,
+alsa-driver, contains many subtle files for compiling ALSA drivers
+outside of the Linux kernel tree, wrapper functions for older 2.2 and
+2.4 kernels, to adapt the latest kernel API, and additional drivers
+which are still in development or in tests. The drivers in alsa-driver
+tree will be moved to alsa-kernel (and eventually to the 2.6 kernel
+tree) when they are finished and confirmed to work fine.
+
+The file tree structure of ALSA driver is depicted below. Both
+alsa-kernel and alsa-driver have almost the same file structure, except
+for “core” directory. It's named as “acore” in alsa-driver tree.
+
+::
+
+ sound
+ /core
+ /oss
+ /seq
+ /oss
+ /instr
+ /ioctl32
+ /include
+ /drivers
+ /mpu401
+ /opl3
+ /i2c
+ /l3
+ /synth
+ /emux
+ /pci
+ /(cards)
+ /isa
+ /(cards)
+ /arm
+ /ppc
+ /sparc
+ /usb
+ /pcmcia /(cards)
+ /oss
+
+
+core directory
+--------------
+
+This directory contains the middle layer which is the heart of ALSA
+drivers. In this directory, the native ALSA modules are stored. The
+sub-directories contain different modules and are dependent upon the
+kernel config.
+
+core/oss
+~~~~~~~~
+
+The codes for PCM and mixer OSS emulation modules are stored in this
+directory. The rawmidi OSS emulation is included in the ALSA rawmidi
+code since it's quite small. The sequencer code is stored in
+``core/seq/oss`` directory (see `below <#core-seq-oss>`__).
+
+core/ioctl32
+~~~~~~~~~~~~
+
+This directory contains the 32bit-ioctl wrappers for 64bit architectures
+such like x86-64, ppc64 and sparc64. For 32bit and alpha architectures,
+these are not compiled.
+
+core/seq
+~~~~~~~~
+
+This directory and its sub-directories are for the ALSA sequencer. This
+directory contains the sequencer core and primary sequencer modules such
+like snd-seq-midi, snd-seq-virmidi, etc. They are compiled only when
+``CONFIG_SND_SEQUENCER`` is set in the kernel config.
+
+core/seq/oss
+~~~~~~~~~~~~
+
+This contains the OSS sequencer emulation codes.
+
+core/seq/instr
+~~~~~~~~~~~~~~
+
+This directory contains the modules for the sequencer instrument layer.
+
+include directory
+-----------------
+
+This is the place for the public header files of ALSA drivers, which are
+to be exported to user-space, or included by several files at different
+directories. Basically, the private header files should not be placed in
+this directory, but you may still find files there, due to historical
+reasons :)
+
+drivers directory
+-----------------
+
+This directory contains code shared among different drivers on different
+architectures. They are hence supposed not to be architecture-specific.
+For example, the dummy pcm driver and the serial MIDI driver are found
+in this directory. In the sub-directories, there is code for components
+which are independent from bus and cpu architectures.
+
+drivers/mpu401
+~~~~~~~~~~~~~~
+
+The MPU401 and MPU401-UART modules are stored here.
+
+drivers/opl3 and opl4
+~~~~~~~~~~~~~~~~~~~~~
+
+The OPL3 and OPL4 FM-synth stuff is found here.
+
+i2c directory
+-------------
+
+This contains the ALSA i2c components.
+
+Although there is a standard i2c layer on Linux, ALSA has its own i2c
+code for some cards, because the soundcard needs only a simple operation
+and the standard i2c API is too complicated for such a purpose.
+
+i2c/l3
+~~~~~~
+
+This is a sub-directory for ARM L3 i2c.
+
+synth directory
+---------------
+
+This contains the synth middle-level modules.
+
+So far, there is only Emu8000/Emu10k1 synth driver under the
+``synth/emux`` sub-directory.
+
+pci directory
+-------------
+
+This directory and its sub-directories hold the top-level card modules
+for PCI soundcards and the code specific to the PCI BUS.
+
+The drivers compiled from a single file are stored directly in the pci
+directory, while the drivers with several source files are stored on
+their own sub-directory (e.g. emu10k1, ice1712).
+
+isa directory
+-------------
+
+This directory and its sub-directories hold the top-level card modules
+for ISA soundcards.
+
+arm, ppc, and sparc directories
+-------------------------------
+
+They are used for top-level card modules which are specific to one of
+these architectures.
+
+usb directory
+-------------
+
+This directory contains the USB-audio driver. In the latest version, the
+USB MIDI driver is integrated in the usb-audio driver.
+
+pcmcia directory
+----------------
+
+The PCMCIA, especially PCCard drivers will go here. CardBus drivers will
+be in the pci directory, because their API is identical to that of
+standard PCI cards.
+
+oss directory
+-------------
+
+The OSS/Lite source files are stored here in Linux 2.6 (or later) tree.
+In the ALSA driver tarball, this directory is empty, of course :)
+
+Basic Flow for PCI Drivers
+==========================
+
+Outline
+-------
+
+The minimum flow for PCI soundcards is as follows:
+
+- define the PCI ID table (see the section `PCI Entries`_).
+
+- create ``probe`` callback.
+
+- create ``remove`` callback.
+
+- create a :c:type:`struct pci_driver <pci_driver>` structure
+ containing the three pointers above.
+
+- create an ``init`` function just calling the
+ :c:func:`pci_register_driver()` to register the pci_driver
+ table defined above.
+
+- create an ``exit`` function to call the
+ :c:func:`pci_unregister_driver()` function.
+
+Full Code Example
+-----------------
+
+The code example is shown below. Some parts are kept unimplemented at
+this moment but will be filled in the next sections. The numbers in the
+comment lines of the :c:func:`snd_mychip_probe()` function refer
+to details explained in the following section.
+
+::
+
+ #include <linux/init.h>
+ #include <linux/pci.h>
+ #include <linux/slab.h>
+ #include <sound/core.h>
+ #include <sound/initval.h>
+
+ /* module parameters (see "Module Parameters") */
+ /* SNDRV_CARDS: maximum number of cards supported by this module */
+ static int index[SNDRV_CARDS] = SNDRV_DEFAULT_IDX;
+ static char *id[SNDRV_CARDS] = SNDRV_DEFAULT_STR;
+ static bool enable[SNDRV_CARDS] = SNDRV_DEFAULT_ENABLE_PNP;
+
+ /* definition of the chip-specific record */
+ struct mychip {
+ struct snd_card *card;
+ /* the rest of the implementation will be in section
+ * "PCI Resource Management"
+ */
+ };
+
+ /* chip-specific destructor
+ * (see "PCI Resource Management")
+ */
+ static int snd_mychip_free(struct mychip *chip)
+ {
+ .... /* will be implemented later... */
+ }
+
+ /* component-destructor
+ * (see "Management of Cards and Components")
+ */
+ static int snd_mychip_dev_free(struct snd_device *device)
+ {
+ return snd_mychip_free(device->device_data);
+ }
+
+ /* chip-specific constructor
+ * (see "Management of Cards and Components")
+ */
+ static int snd_mychip_create(struct snd_card *card,
+ struct pci_dev *pci,
+ struct mychip **rchip)
+ {
+ struct mychip *chip;
+ int err;
+ static struct snd_device_ops ops = {
+ .dev_free = snd_mychip_dev_free,
+ };
+
+ *rchip = NULL;
+
+ /* check PCI availability here
+ * (see "PCI Resource Management")
+ */
+ ....
+
+ /* allocate a chip-specific data with zero filled */
+ chip = kzalloc(sizeof(*chip), GFP_KERNEL);
+ if (chip == NULL)
+ return -ENOMEM;
+
+ chip->card = card;
+
+ /* rest of initialization here; will be implemented
+ * later, see "PCI Resource Management"
+ */
+ ....
+
+ err = snd_device_new(card, SNDRV_DEV_LOWLEVEL, chip, &ops);
+ if (err < 0) {
+ snd_mychip_free(chip);
+ return err;
+ }
+
+ *rchip = chip;
+ return 0;
+ }
+
+ /* constructor -- see "Driver Constructor" sub-section */
+ static int snd_mychip_probe(struct pci_dev *pci,
+ const struct pci_device_id *pci_id)
+ {
+ static int dev;
+ struct snd_card *card;
+ struct mychip *chip;
+ int err;
+
+ /* (1) */
+ if (dev >= SNDRV_CARDS)
+ return -ENODEV;
+ if (!enable[dev]) {
+ dev++;
+ return -ENOENT;
+ }
+
+ /* (2) */
+ err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
+ 0, &card);
+ if (err < 0)
+ return err;
+
+ /* (3) */
+ err = snd_mychip_create(card, pci, &chip);
+ if (err < 0) {
+ snd_card_free(card);
+ return err;
+ }
+
+ /* (4) */
+ strcpy(card->driver, "My Chip");
+ strcpy(card->shortname, "My Own Chip 123");
+ sprintf(card->longname, "%s at 0x%lx irq %i",
+ card->shortname, chip->ioport, chip->irq);
+
+ /* (5) */
+ .... /* implemented later */
+
+ /* (6) */
+ err = snd_card_register(card);
+ if (err < 0) {
+ snd_card_free(card);
+ return err;
+ }
+
+ /* (7) */
+ pci_set_drvdata(pci, card);
+ dev++;
+ return 0;
+ }
+
+ /* destructor -- see the "Destructor" sub-section */
+ static void snd_mychip_remove(struct pci_dev *pci)
+ {
+ snd_card_free(pci_get_drvdata(pci));
+ pci_set_drvdata(pci, NULL);
+ }
+
+
+
+Driver Constructor
+------------------
+
+The real constructor of PCI drivers is the ``probe`` callback. The
+``probe`` callback and other component-constructors which are called
+from the ``probe`` callback cannot be used with the ``__init`` prefix
+because any PCI device could be a hotplug device.
+
+In the ``probe`` callback, the following scheme is often used.
+
+1) Check and increment the device index.
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+ static int dev;
+ ....
+ if (dev >= SNDRV_CARDS)
+ return -ENODEV;
+ if (!enable[dev]) {
+ dev++;
+ return -ENOENT;
+ }
+
+
+where ``enable[dev]`` is the module option.
+
+Each time the ``probe`` callback is called, check the availability of
+the device. If not available, simply increment the device index and
+returns. dev will be incremented also later (`step 7
+<#set-the-pci-driver-data-and-return-zero>`__).
+
+2) Create a card instance
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+ struct snd_card *card;
+ int err;
+ ....
+ err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
+ 0, &card);
+
+
+The details will be explained in the section `Management of Cards and
+Components`_.
+
+3) Create a main component
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+In this part, the PCI resources are allocated.
+
+::
+
+ struct mychip *chip;
+ ....
+ err = snd_mychip_create(card, pci, &chip);
+ if (err < 0) {
+ snd_card_free(card);
+ return err;
+ }
+
+The details will be explained in the section `PCI Resource
+Management`_.
+
+4) Set the driver ID and name strings.
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+ strcpy(card->driver, "My Chip");
+ strcpy(card->shortname, "My Own Chip 123");
+ sprintf(card->longname, "%s at 0x%lx irq %i",
+ card->shortname, chip->ioport, chip->irq);
+
+The driver field holds the minimal ID string of the chip. This is used
+by alsa-lib's configurator, so keep it simple but unique. Even the
+same driver can have different driver IDs to distinguish the
+functionality of each chip type.
+
+The shortname field is a string shown as more verbose name. The longname
+field contains the information shown in ``/proc/asound/cards``.
+
+5) Create other components, such as mixer, MIDI, etc.
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Here you define the basic components such as `PCM <#PCM-Interface>`__,
+mixer (e.g. `AC97 <#API-for-AC97-Codec>`__), MIDI (e.g.
+`MPU-401 <#MIDI-MPU401-UART-Interface>`__), and other interfaces.
+Also, if you want a `proc file <#Proc-Interface>`__, define it here,
+too.
+
+6) Register the card instance.
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+ err = snd_card_register(card);
+ if (err < 0) {
+ snd_card_free(card);
+ return err;
+ }
+
+Will be explained in the section `Management of Cards and
+Components`_, too.
+
+7) Set the PCI driver data and return zero.
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+ pci_set_drvdata(pci, card);
+ dev++;
+ return 0;
+
+In the above, the card record is stored. This pointer is used in the
+remove callback and power-management callbacks, too.
+
+Destructor
+----------
+
+The destructor, remove callback, simply releases the card instance. Then
+the ALSA middle layer will release all the attached components
+automatically.
+
+It would be typically like the following:
+
+::
+
+ static void snd_mychip_remove(struct pci_dev *pci)
+ {
+ snd_card_free(pci_get_drvdata(pci));
+ pci_set_drvdata(pci, NULL);
+ }
+
+
+The above code assumes that the card pointer is set to the PCI driver
+data.
+
+Header Files
+------------
+
+For the above example, at least the following include files are
+necessary.
+
+::
+
+ #include <linux/init.h>
+ #include <linux/pci.h>
+ #include <linux/slab.h>
+ #include <sound/core.h>
+ #include <sound/initval.h>
+
+where the last one is necessary only when module options are defined
+in the source file. If the code is split into several files, the files
+without module options don't need them.
+
+In addition to these headers, you'll need ``<linux/interrupt.h>`` for
+interrupt handling, and ``<asm/io.h>`` for I/O access. If you use the
+:c:func:`mdelay()` or :c:func:`udelay()` functions, you'll need
+to include ``<linux/delay.h>`` too.
+
+The ALSA interfaces like the PCM and control APIs are defined in other
+``<sound/xxx.h>`` header files. They have to be included after
+``<sound/core.h>``.
+
+Management of Cards and Components
+==================================
+
+Card Instance
+-------------
+
+For each soundcard, a “card” record must be allocated.
+
+A card record is the headquarters of the soundcard. It manages the whole
+list of devices (components) on the soundcard, such as PCM, mixers,
+MIDI, synthesizer, and so on. Also, the card record holds the ID and the
+name strings of the card, manages the root of proc files, and controls
+the power-management states and hotplug disconnections. The component
+list on the card record is used to manage the correct release of
+resources at destruction.
+
+As mentioned above, to create a card instance, call
+:c:func:`snd_card_new()`.
+
+::
+
+ struct snd_card *card;
+ int err;
+ err = snd_card_new(&pci->dev, index, id, module, extra_size, &card);
+
+
+The function takes six arguments: the parent device pointer, the
+card-index number, the id string, the module pointer (usually
+``THIS_MODULE``), the size of extra-data space, and the pointer to
+return the card instance. The extra_size argument is used to allocate
+card->private_data for the chip-specific data. Note that these data are
+allocated by :c:func:`snd_card_new()`.
+
+The first argument, the pointer of struct :c:type:`struct device
+<device>`, specifies the parent device. For PCI devices, typically
+``&pci->`` is passed there.
+
+Components
+----------
+
+After the card is created, you can attach the components (devices) to
+the card instance. In an ALSA driver, a component is represented as a
+:c:type:`struct snd_device <snd_device>` object. A component
+can be a PCM instance, a control interface, a raw MIDI interface, etc.
+Each such instance has one component entry.
+
+A component can be created via :c:func:`snd_device_new()`
+function.
+
+::
+
+ snd_device_new(card, SNDRV_DEV_XXX, chip, &ops);
+
+This takes the card pointer, the device-level (``SNDRV_DEV_XXX``), the
+data pointer, and the callback pointers (``&ops``). The device-level
+defines the type of components and the order of registration and
+de-registration. For most components, the device-level is already
+defined. For a user-defined component, you can use
+``SNDRV_DEV_LOWLEVEL``.
+
+This function itself doesn't allocate the data space. The data must be
+allocated manually beforehand, and its pointer is passed as the
+argument. This pointer (``chip`` in the above example) is used as the
+identifier for the instance.
+
+Each pre-defined ALSA component such as ac97 and pcm calls
+:c:func:`snd_device_new()` inside its constructor. The destructor
+for each component is defined in the callback pointers. Hence, you don't
+need to take care of calling a destructor for such a component.
+
+If you wish to create your own component, you need to set the destructor
+function to the dev_free callback in the ``ops``, so that it can be
+released automatically via :c:func:`snd_card_free()`. The next
+example will show an implementation of chip-specific data.
+
+Chip-Specific Data
+------------------
+
+Chip-specific information, e.g. the I/O port address, its resource
+pointer, or the irq number, is stored in the chip-specific record.
+
+::
+
+ struct mychip {
+ ....
+ };
+
+
+In general, there are two ways of allocating the chip record.
+
+1. Allocating via :c:func:`snd_card_new()`.
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+As mentioned above, you can pass the extra-data-length to the 5th
+argument of :c:func:`snd_card_new()`, i.e.
+
+::
+
+ err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
+ sizeof(struct mychip), &card);
+
+:c:type:`struct mychip <mychip>` is the type of the chip record.
+
+In return, the allocated record can be accessed as
+
+::
+
+ struct mychip *chip = card->private_data;
+
+With this method, you don't have to allocate twice. The record is
+released together with the card instance.
+
+2. Allocating an extra device.
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+After allocating a card instance via :c:func:`snd_card_new()`
+(with ``0`` on the 4th arg), call :c:func:`kzalloc()`.
+
+::
+
+ struct snd_card *card;
+ struct mychip *chip;
+ err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
+ 0, &card);
+ .....
+ chip = kzalloc(sizeof(*chip), GFP_KERNEL);
+
+The chip record should have the field to hold the card pointer at least,
+
+::
+
+ struct mychip {
+ struct snd_card *card;
+ ....
+ };
+
+
+Then, set the card pointer in the returned chip instance.
+
+::
+
+ chip->card = card;
+
+Next, initialize the fields, and register this chip record as a
+low-level device with a specified ``ops``,
+
+::
+
+ static struct snd_device_ops ops = {
+ .dev_free = snd_mychip_dev_free,
+ };
+ ....
+ snd_device_new(card, SNDRV_DEV_LOWLEVEL, chip, &ops);
+
+:c:func:`snd_mychip_dev_free()` is the device-destructor
+function, which will call the real destructor.
+
+::
+
+ static int snd_mychip_dev_free(struct snd_device *device)
+ {
+ return snd_mychip_free(device->device_data);
+ }
+
+where :c:func:`snd_mychip_free()` is the real destructor.
+
+Registration and Release
+------------------------
+
+After all components are assigned, register the card instance by calling
+:c:func:`snd_card_register()`. Access to the device files is
+enabled at this point. That is, before
+:c:func:`snd_card_register()` is called, the components are safely
+inaccessible from external side. If this call fails, exit the probe
+function after releasing the card via :c:func:`snd_card_free()`.
+
+For releasing the card instance, you can call simply
+:c:func:`snd_card_free()`. As mentioned earlier, all components
+are released automatically by this call.
+
+For a device which allows hotplugging, you can use
+:c:func:`snd_card_free_when_closed()`. This one will postpone
+the destruction until all devices are closed.
+
+PCI Resource Management
+=======================
+
+Full Code Example
+-----------------
+
+In this section, we'll complete the chip-specific constructor,
+destructor and PCI entries. Example code is shown first, below.
+
+::
+
+ struct mychip {
+ struct snd_card *card;
+ struct pci_dev *pci;
+
+ unsigned long port;
+ int irq;
+ };
+
+ static int snd_mychip_free(struct mychip *chip)
+ {
+ /* disable hardware here if any */
+ .... /* (not implemented in this document) */
+
+ /* release the irq */
+ if (chip->irq >= 0)
+ free_irq(chip->irq, chip);
+ /* release the I/O ports & memory */
+ pci_release_regions(chip->pci);
+ /* disable the PCI entry */
+ pci_disable_device(chip->pci);
+ /* release the data */
+ kfree(chip);
+ return 0;
+ }
+
+ /* chip-specific constructor */
+ static int snd_mychip_create(struct snd_card *card,
+ struct pci_dev *pci,
+ struct mychip **rchip)
+ {
+ struct mychip *chip;
+ int err;
+ static struct snd_device_ops ops = {
+ .dev_free = snd_mychip_dev_free,
+ };
+
+ *rchip = NULL;
+
+ /* initialize the PCI entry */
+ err = pci_enable_device(pci);
+ if (err < 0)
+ return err;
+ /* check PCI availability (28bit DMA) */
+ if (pci_set_dma_mask(pci, DMA_BIT_MASK(28)) < 0 ||
+ pci_set_consistent_dma_mask(pci, DMA_BIT_MASK(28)) < 0) {
+ printk(KERN_ERR "error to set 28bit mask DMA\n");
+ pci_disable_device(pci);
+ return -ENXIO;
+ }
+
+ chip = kzalloc(sizeof(*chip), GFP_KERNEL);
+ if (chip == NULL) {
+ pci_disable_device(pci);
+ return -ENOMEM;
+ }
+
+ /* initialize the stuff */
+ chip->card = card;
+ chip->pci = pci;
+ chip->irq = -1;
+
+ /* (1) PCI resource allocation */
+ err = pci_request_regions(pci, "My Chip");
+ if (err < 0) {
+ kfree(chip);
+ pci_disable_device(pci);
+ return err;
+ }
+ chip->port = pci_resource_start(pci, 0);
+ if (request_irq(pci->irq, snd_mychip_interrupt,
+ IRQF_SHARED, KBUILD_MODNAME, chip)) {
+ printk(KERN_ERR "cannot grab irq %d\n", pci->irq);
+ snd_mychip_free(chip);
+ return -EBUSY;
+ }
+ chip->irq = pci->irq;
+
+ /* (2) initialization of the chip hardware */
+ .... /* (not implemented in this document) */
+
+ err = snd_device_new(card, SNDRV_DEV_LOWLEVEL, chip, &ops);
+ if (err < 0) {
+ snd_mychip_free(chip);
+ return err;
+ }
+
+ *rchip = chip;
+ return 0;
+ }
+
+ /* PCI IDs */
+ static struct pci_device_id snd_mychip_ids[] = {
+ { PCI_VENDOR_ID_FOO, PCI_DEVICE_ID_BAR,
+ PCI_ANY_ID, PCI_ANY_ID, 0, 0, 0, },
+ ....
+ { 0, }
+ };
+ MODULE_DEVICE_TABLE(pci, snd_mychip_ids);
+
+ /* pci_driver definition */
+ static struct pci_driver driver = {
+ .name = KBUILD_MODNAME,
+ .id_table = snd_mychip_ids,
+ .probe = snd_mychip_probe,
+ .remove = snd_mychip_remove,
+ };
+
+ /* module initialization */
+ static int __init alsa_card_mychip_init(void)
+ {
+ return pci_register_driver(&driver);
+ }
+
+ /* module clean up */
+ static void __exit alsa_card_mychip_exit(void)
+ {
+ pci_unregister_driver(&driver);
+ }
+
+ module_init(alsa_card_mychip_init)
+ module_exit(alsa_card_mychip_exit)
+
+ EXPORT_NO_SYMBOLS; /* for old kernels only */
+
+Some Hafta's
+------------
+
+The allocation of PCI resources is done in the ``probe`` function, and
+usually an extra :c:func:`xxx_create()` function is written for this
+purpose.
+
+In the case of PCI devices, you first have to call the
+:c:func:`pci_enable_device()` function before allocating
+resources. Also, you need to set the proper PCI DMA mask to limit the
+accessed I/O range. In some cases, you might need to call
+:c:func:`pci_set_master()` function, too.
+
+Suppose the 28bit mask, and the code to be added would be like:
+
+::
+
+ err = pci_enable_device(pci);
+ if (err < 0)
+ return err;
+ if (pci_set_dma_mask(pci, DMA_BIT_MASK(28)) < 0 ||
+ pci_set_consistent_dma_mask(pci, DMA_BIT_MASK(28)) < 0) {
+ printk(KERN_ERR "error to set 28bit mask DMA\n");
+ pci_disable_device(pci);
+ return -ENXIO;
+ }
+
+
+Resource Allocation
+-------------------
+
+The allocation of I/O ports and irqs is done via standard kernel
+functions. Unlike ALSA ver.0.5.x., there are no helpers for that. And
+these resources must be released in the destructor function (see below).
+Also, on ALSA 0.9.x, you don't need to allocate (pseudo-)DMA for PCI
+like in ALSA 0.5.x.
+
+Now assume that the PCI device has an I/O port with 8 bytes and an
+interrupt. Then :c:type:`struct mychip <mychip>` will have the
+following fields:
+
+::
+
+ struct mychip {
+ struct snd_card *card;
+
+ unsigned long port;
+ int irq;
+ };
+
+
+For an I/O port (and also a memory region), you need to have the
+resource pointer for the standard resource management. For an irq, you
+have to keep only the irq number (integer). But you need to initialize
+this number as -1 before actual allocation, since irq 0 is valid. The
+port address and its resource pointer can be initialized as null by
+:c:func:`kzalloc()` automatically, so you don't have to take care of
+resetting them.
+
+The allocation of an I/O port is done like this:
+
+::
+
+ err = pci_request_regions(pci, "My Chip");
+ if (err < 0) {
+ kfree(chip);
+ pci_disable_device(pci);
+ return err;
+ }
+ chip->port = pci_resource_start(pci, 0);
+
+It will reserve the I/O port region of 8 bytes of the given PCI device.
+The returned value, ``chip->res_port``, is allocated via
+:c:func:`kmalloc()` by :c:func:`request_region()`. The pointer
+must be released via :c:func:`kfree()`, but there is a problem with
+this. This issue will be explained later.
+
+The allocation of an interrupt source is done like this:
+
+::
+
+ if (request_irq(pci->irq, snd_mychip_interrupt,
+ IRQF_SHARED, KBUILD_MODNAME, chip)) {
+ printk(KERN_ERR "cannot grab irq %d\n", pci->irq);
+ snd_mychip_free(chip);
+ return -EBUSY;
+ }
+ chip->irq = pci->irq;
+
+where :c:func:`snd_mychip_interrupt()` is the interrupt handler
+defined `later <#pcm-interface-interrupt-handler>`__. Note that
+``chip->irq`` should be defined only when :c:func:`request_irq()`
+succeeded.
+
+On the PCI bus, interrupts can be shared. Thus, ``IRQF_SHARED`` is used
+as the interrupt flag of :c:func:`request_irq()`.
+
+The last argument of :c:func:`request_irq()` is the data pointer
+passed to the interrupt handler. Usually, the chip-specific record is
+used for that, but you can use what you like, too.
+
+I won't give details about the interrupt handler at this point, but at
+least its appearance can be explained now. The interrupt handler looks
+usually like the following:
+
+::
+
+ static irqreturn_t snd_mychip_interrupt(int irq, void *dev_id)
+ {
+ struct mychip *chip = dev_id;
+ ....
+ return IRQ_HANDLED;
+ }
+
+
+Now let's write the corresponding destructor for the resources above.
+The role of destructor is simple: disable the hardware (if already
+activated) and release the resources. So far, we have no hardware part,
+so the disabling code is not written here.
+
+To release the resources, the “check-and-release” method is a safer way.
+For the interrupt, do like this:
+
+::
+
+ if (chip->irq >= 0)
+ free_irq(chip->irq, chip);
+
+Since the irq number can start from 0, you should initialize
+``chip->irq`` with a negative value (e.g. -1), so that you can check
+the validity of the irq number as above.
+
+When you requested I/O ports or memory regions via
+:c:func:`pci_request_region()` or
+:c:func:`pci_request_regions()` like in this example, release the
+resource(s) using the corresponding function,
+:c:func:`pci_release_region()` or
+:c:func:`pci_release_regions()`.
+
+::
+
+ pci_release_regions(chip->pci);
+
+When you requested manually via :c:func:`request_region()` or
+:c:func:`request_mem_region()`, you can release it via
+:c:func:`release_resource()`. Suppose that you keep the resource
+pointer returned from :c:func:`request_region()` in
+chip->res_port, the release procedure looks like:
+
+::
+
+ release_and_free_resource(chip->res_port);
+
+Don't forget to call :c:func:`pci_disable_device()` before the
+end.
+
+And finally, release the chip-specific record.
+
+::
+
+ kfree(chip);
+
+We didn't implement the hardware disabling part in the above. If you
+need to do this, please note that the destructor may be called even
+before the initialization of the chip is completed. It would be better
+to have a flag to skip hardware disabling if the hardware was not
+initialized yet.
+
+When the chip-data is assigned to the card using
+:c:func:`snd_device_new()` with ``SNDRV_DEV_LOWLELVEL`` , its
+destructor is called at the last. That is, it is assured that all other
+components like PCMs and controls have already been released. You don't
+have to stop PCMs, etc. explicitly, but just call low-level hardware
+stopping.
+
+The management of a memory-mapped region is almost as same as the
+management of an I/O port. You'll need three fields like the
+following:
+
+::
+
+ struct mychip {
+ ....
+ unsigned long iobase_phys;
+ void __iomem *iobase_virt;
+ };
+
+and the allocation would be like below:
+
+::
+
+ if ((err = pci_request_regions(pci, "My Chip")) < 0) {
+ kfree(chip);
+ return err;
+ }
+ chip->iobase_phys = pci_resource_start(pci, 0);
+ chip->iobase_virt = ioremap_nocache(chip->iobase_phys,
+ pci_resource_len(pci, 0));
+
+and the corresponding destructor would be:
+
+::
+
+ static int snd_mychip_free(struct mychip *chip)
+ {
+ ....
+ if (chip->iobase_virt)
+ iounmap(chip->iobase_virt);
+ ....
+ pci_release_regions(chip->pci);
+ ....
+ }
+
+PCI Entries
+-----------
+
+So far, so good. Let's finish the missing PCI stuff. At first, we need a
+:c:type:`struct pci_device_id <pci_device_id>` table for
+this chipset. It's a table of PCI vendor/device ID number, and some
+masks.
+
+For example,
+
+::
+
+ static struct pci_device_id snd_mychip_ids[] = {
+ { PCI_VENDOR_ID_FOO, PCI_DEVICE_ID_BAR,
+ PCI_ANY_ID, PCI_ANY_ID, 0, 0, 0, },
+ ....
+ { 0, }
+ };
+ MODULE_DEVICE_TABLE(pci, snd_mychip_ids);
+
+The first and second fields of the :c:type:`struct pci_device_id
+<pci_device_id>` structure are the vendor and device IDs. If you
+have no reason to filter the matching devices, you can leave the
+remaining fields as above. The last field of the :c:type:`struct
+pci_device_id <pci_device_id>` struct contains private data
+for this entry. You can specify any value here, for example, to define
+specific operations for supported device IDs. Such an example is found
+in the intel8x0 driver.
+
+The last entry of this list is the terminator. You must specify this
+all-zero entry.
+
+Then, prepare the :c:type:`struct pci_driver <pci_driver>`
+record:
+
+::
+
+ static struct pci_driver driver = {
+ .name = KBUILD_MODNAME,
+ .id_table = snd_mychip_ids,
+ .probe = snd_mychip_probe,
+ .remove = snd_mychip_remove,
+ };
+
+The ``probe`` and ``remove`` functions have already been defined in
+the previous sections. The ``name`` field is the name string of this
+device. Note that you must not use a slash “/” in this string.
+
+And at last, the module entries:
+
+::
+
+ static int __init alsa_card_mychip_init(void)
+ {
+ return pci_register_driver(&driver);
+ }
+
+ static void __exit alsa_card_mychip_exit(void)
+ {
+ pci_unregister_driver(&driver);
+ }
+
+ module_init(alsa_card_mychip_init)
+ module_exit(alsa_card_mychip_exit)
+
+Note that these module entries are tagged with ``__init`` and ``__exit``
+prefixes.
+
+Oh, one thing was forgotten. If you have no exported symbols, you need
+to declare it in 2.2 or 2.4 kernels (it's not necessary in 2.6 kernels).
+
+::
+
+ EXPORT_NO_SYMBOLS;
+
+That's all!
+
+PCM Interface
+=============
+
+General
+-------
+
+The PCM middle layer of ALSA is quite powerful and it is only necessary
+for each driver to implement the low-level functions to access its
+hardware.
+
+For accessing to the PCM layer, you need to include ``<sound/pcm.h>``
+first. In addition, ``<sound/pcm_params.h>`` might be needed if you
+access to some functions related with hw_param.
+
+Each card device can have up to four pcm instances. A pcm instance
+corresponds to a pcm device file. The limitation of number of instances
+comes only from the available bit size of the Linux's device numbers.
+Once when 64bit device number is used, we'll have more pcm instances
+available.
+
+A pcm instance consists of pcm playback and capture streams, and each
+pcm stream consists of one or more pcm substreams. Some soundcards
+support multiple playback functions. For example, emu10k1 has a PCM
+playback of 32 stereo substreams. In this case, at each open, a free
+substream is (usually) automatically chosen and opened. Meanwhile, when
+only one substream exists and it was already opened, the successful open
+will either block or error with ``EAGAIN`` according to the file open
+mode. But you don't have to care about such details in your driver. The
+PCM middle layer will take care of such work.
+
+Full Code Example
+-----------------
+
+The example code below does not include any hardware access routines but
+shows only the skeleton, how to build up the PCM interfaces.
+
+::
+
+ #include <sound/pcm.h>
+ ....
+
+ /* hardware definition */
+ static struct snd_pcm_hardware snd_mychip_playback_hw = {
+ .info = (SNDRV_PCM_INFO_MMAP |
+ SNDRV_PCM_INFO_INTERLEAVED |
+ SNDRV_PCM_INFO_BLOCK_TRANSFER |
+ SNDRV_PCM_INFO_MMAP_VALID),
+ .formats = SNDRV_PCM_FMTBIT_S16_LE,
+ .rates = SNDRV_PCM_RATE_8000_48000,
+ .rate_min = 8000,
+ .rate_max = 48000,
+ .channels_min = 2,
+ .channels_max = 2,
+ .buffer_bytes_max = 32768,
+ .period_bytes_min = 4096,
+ .period_bytes_max = 32768,
+ .periods_min = 1,
+ .periods_max = 1024,
+ };
+
+ /* hardware definition */
+ static struct snd_pcm_hardware snd_mychip_capture_hw = {
+ .info = (SNDRV_PCM_INFO_MMAP |
+ SNDRV_PCM_INFO_INTERLEAVED |
+ SNDRV_PCM_INFO_BLOCK_TRANSFER |
+ SNDRV_PCM_INFO_MMAP_VALID),
+ .formats = SNDRV_PCM_FMTBIT_S16_LE,
+ .rates = SNDRV_PCM_RATE_8000_48000,
+ .rate_min = 8000,
+ .rate_max = 48000,
+ .channels_min = 2,
+ .channels_max = 2,
+ .buffer_bytes_max = 32768,
+ .period_bytes_min = 4096,
+ .period_bytes_max = 32768,
+ .periods_min = 1,
+ .periods_max = 1024,
+ };
+
+ /* open callback */
+ static int snd_mychip_playback_open(struct snd_pcm_substream *substream)
+ {
+ struct mychip *chip = snd_pcm_substream_chip(substream);
+ struct snd_pcm_runtime *runtime = substream->runtime;
+
+ runtime->hw = snd_mychip_playback_hw;
+ /* more hardware-initialization will be done here */
+ ....
+ return 0;
+ }
+
+ /* close callback */
+ static int snd_mychip_playback_close(struct snd_pcm_substream *substream)
+ {
+ struct mychip *chip = snd_pcm_substream_chip(substream);
+ /* the hardware-specific codes will be here */
+ ....
+ return 0;
+
+ }
+
+ /* open callback */
+ static int snd_mychip_capture_open(struct snd_pcm_substream *substream)
+ {
+ struct mychip *chip = snd_pcm_substream_chip(substream);
+ struct snd_pcm_runtime *runtime = substream->runtime;
+
+ runtime->hw = snd_mychip_capture_hw;
+ /* more hardware-initialization will be done here */
+ ....
+ return 0;
+ }
+
+ /* close callback */
+ static int snd_mychip_capture_close(struct snd_pcm_substream *substream)
+ {
+ struct mychip *chip = snd_pcm_substream_chip(substream);
+ /* the hardware-specific codes will be here */
+ ....
+ return 0;
+
+ }
+
+ /* hw_params callback */
+ static int snd_mychip_pcm_hw_params(struct snd_pcm_substream *substream,
+ struct snd_pcm_hw_params *hw_params)
+ {
+ return snd_pcm_lib_malloc_pages(substream,
+ params_buffer_bytes(hw_params));
+ }
+
+ /* hw_free callback */
+ static int snd_mychip_pcm_hw_free(struct snd_pcm_substream *substream)
+ {
+ return snd_pcm_lib_free_pages(substream);
+ }
+
+ /* prepare callback */
+ static int snd_mychip_pcm_prepare(struct snd_pcm_substream *substream)
+ {
+ struct mychip *chip = snd_pcm_substream_chip(substream);
+ struct snd_pcm_runtime *runtime = substream->runtime;
+
+ /* set up the hardware with the current configuration
+ * for example...
+ */
+ mychip_set_sample_format(chip, runtime->format);
+ mychip_set_sample_rate(chip, runtime->rate);
+ mychip_set_channels(chip, runtime->channels);
+ mychip_set_dma_setup(chip, runtime->dma_addr,
+ chip->buffer_size,
+ chip->period_size);
+ return 0;
+ }
+
+ /* trigger callback */
+ static int snd_mychip_pcm_trigger(struct snd_pcm_substream *substream,
+ int cmd)
+ {
+ switch (cmd) {
+ case SNDRV_PCM_TRIGGER_START:
+ /* do something to start the PCM engine */
+ ....
+ break;
+ case SNDRV_PCM_TRIGGER_STOP:
+ /* do something to stop the PCM engine */
+ ....
+ break;
+ default:
+ return -EINVAL;
+ }
+ }
+
+ /* pointer callback */
+ static snd_pcm_uframes_t
+ snd_mychip_pcm_pointer(struct snd_pcm_substream *substream)
+ {
+ struct mychip *chip = snd_pcm_substream_chip(substream);
+ unsigned int current_ptr;
+
+ /* get the current hardware pointer */
+ current_ptr = mychip_get_hw_pointer(chip);
+ return current_ptr;
+ }
+
+ /* operators */
+ static struct snd_pcm_ops snd_mychip_playback_ops = {
+ .open = snd_mychip_playback_open,
+ .close = snd_mychip_playback_close,
+ .ioctl = snd_pcm_lib_ioctl,
+ .hw_params = snd_mychip_pcm_hw_params,
+ .hw_free = snd_mychip_pcm_hw_free,
+ .prepare = snd_mychip_pcm_prepare,
+ .trigger = snd_mychip_pcm_trigger,
+ .pointer = snd_mychip_pcm_pointer,
+ };
+
+ /* operators */
+ static struct snd_pcm_ops snd_mychip_capture_ops = {
+ .open = snd_mychip_capture_open,
+ .close = snd_mychip_capture_close,
+ .ioctl = snd_pcm_lib_ioctl,
+ .hw_params = snd_mychip_pcm_hw_params,
+ .hw_free = snd_mychip_pcm_hw_free,
+ .prepare = snd_mychip_pcm_prepare,
+ .trigger = snd_mychip_pcm_trigger,
+ .pointer = snd_mychip_pcm_pointer,
+ };
+
+ /*
+ * definitions of capture are omitted here...
+ */
+
+ /* create a pcm device */
+ static int snd_mychip_new_pcm(struct mychip *chip)
+ {
+ struct snd_pcm *pcm;
+ int err;
+
+ err = snd_pcm_new(chip->card, "My Chip", 0, 1, 1, &pcm);
+ if (err < 0)
+ return err;
+ pcm->private_data = chip;
+ strcpy(pcm->name, "My Chip");
+ chip->pcm = pcm;
+ /* set operators */
+ snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_PLAYBACK,
+ &snd_mychip_playback_ops);
+ snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_CAPTURE,
+ &snd_mychip_capture_ops);
+ /* pre-allocation of buffers */
+ /* NOTE: this may fail */
+ snd_pcm_lib_preallocate_pages_for_all(pcm, SNDRV_DMA_TYPE_DEV,
+ snd_dma_pci_data(chip->pci),
+ 64*1024, 64*1024);
+ return 0;
+ }
+
+
+PCM Constructor
+---------------
+
+A pcm instance is allocated by the :c:func:`snd_pcm_new()`
+function. It would be better to create a constructor for pcm, namely,
+
+::
+
+ static int snd_mychip_new_pcm(struct mychip *chip)
+ {
+ struct snd_pcm *pcm;
+ int err;
+
+ err = snd_pcm_new(chip->card, "My Chip", 0, 1, 1, &pcm);
+ if (err < 0)
+ return err;
+ pcm->private_data = chip;
+ strcpy(pcm->name, "My Chip");
+ chip->pcm = pcm;
+ ....
+ return 0;
+ }
+
+The :c:func:`snd_pcm_new()` function takes four arguments. The
+first argument is the card pointer to which this pcm is assigned, and
+the second is the ID string.
+
+The third argument (``index``, 0 in the above) is the index of this new
+pcm. It begins from zero. If you create more than one pcm instances,
+specify the different numbers in this argument. For example, ``index =
+1`` for the second PCM device.
+
+The fourth and fifth arguments are the number of substreams for playback
+and capture, respectively. Here 1 is used for both arguments. When no
+playback or capture substreams are available, pass 0 to the
+corresponding argument.
+
+If a chip supports multiple playbacks or captures, you can specify more
+numbers, but they must be handled properly in open/close, etc.
+callbacks. When you need to know which substream you are referring to,
+then it can be obtained from :c:type:`struct snd_pcm_substream
+<snd_pcm_substream>` data passed to each callback as follows:
+
+::
+
+ struct snd_pcm_substream *substream;
+ int index = substream->number;
+
+
+After the pcm is created, you need to set operators for each pcm stream.
+
+::
+
+ snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_PLAYBACK,
+ &snd_mychip_playback_ops);
+ snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_CAPTURE,
+ &snd_mychip_capture_ops);
+
+The operators are defined typically like this:
+
+::
+
+ static struct snd_pcm_ops snd_mychip_playback_ops = {
+ .open = snd_mychip_pcm_open,
+ .close = snd_mychip_pcm_close,
+ .ioctl = snd_pcm_lib_ioctl,
+ .hw_params = snd_mychip_pcm_hw_params,
+ .hw_free = snd_mychip_pcm_hw_free,
+ .prepare = snd_mychip_pcm_prepare,
+ .trigger = snd_mychip_pcm_trigger,
+ .pointer = snd_mychip_pcm_pointer,
+ };
+
+All the callbacks are described in the Operators_ subsection.
+
+After setting the operators, you probably will want to pre-allocate the
+buffer. For the pre-allocation, simply call the following:
+
+::
+
+ snd_pcm_lib_preallocate_pages_for_all(pcm, SNDRV_DMA_TYPE_DEV,
+ snd_dma_pci_data(chip->pci),
+ 64*1024, 64*1024);
+
+It will allocate a buffer up to 64kB as default. Buffer management
+details will be described in the later section `Buffer and Memory
+Management`_.
+
+Additionally, you can set some extra information for this pcm in
+``pcm->info_flags``. The available values are defined as
+``SNDRV_PCM_INFO_XXX`` in ``<sound/asound.h>``, which is used for the
+hardware definition (described later). When your soundchip supports only
+half-duplex, specify like this:
+
+::
+
+ pcm->info_flags = SNDRV_PCM_INFO_HALF_DUPLEX;
+
+
+... And the Destructor?
+-----------------------
+
+The destructor for a pcm instance is not always necessary. Since the pcm
+device will be released by the middle layer code automatically, you
+don't have to call the destructor explicitly.
+
+The destructor would be necessary if you created special records
+internally and needed to release them. In such a case, set the
+destructor function to ``pcm->private_free``:
+
+::
+
+ static void mychip_pcm_free(struct snd_pcm *pcm)
+ {
+ struct mychip *chip = snd_pcm_chip(pcm);
+ /* free your own data */
+ kfree(chip->my_private_pcm_data);
+ /* do what you like else */
+ ....
+ }
+
+ static int snd_mychip_new_pcm(struct mychip *chip)
+ {
+ struct snd_pcm *pcm;
+ ....
+ /* allocate your own data */
+ chip->my_private_pcm_data = kmalloc(...);
+ /* set the destructor */
+ pcm->private_data = chip;
+ pcm->private_free = mychip_pcm_free;
+ ....
+ }
+
+
+
+Runtime Pointer - The Chest of PCM Information
+----------------------------------------------
+
+When the PCM substream is opened, a PCM runtime instance is allocated
+and assigned to the substream. This pointer is accessible via
+``substream->runtime``. This runtime pointer holds most information you
+need to control the PCM: the copy of hw_params and sw_params
+configurations, the buffer pointers, mmap records, spinlocks, etc.
+
+The definition of runtime instance is found in ``<sound/pcm.h>``. Here
+are the contents of this file:
+
+::
+
+ struct _snd_pcm_runtime {
+ /* -- Status -- */
+ struct snd_pcm_substream *trigger_master;
+ snd_timestamp_t trigger_tstamp; /* trigger timestamp */
+ int overrange;
+ snd_pcm_uframes_t avail_max;
+ snd_pcm_uframes_t hw_ptr_base; /* Position at buffer restart */
+ snd_pcm_uframes_t hw_ptr_interrupt; /* Position at interrupt time*/
+
+ /* -- HW params -- */
+ snd_pcm_access_t access; /* access mode */
+ snd_pcm_format_t format; /* SNDRV_PCM_FORMAT_* */
+ snd_pcm_subformat_t subformat; /* subformat */
+ unsigned int rate; /* rate in Hz */
+ unsigned int channels; /* channels */
+ snd_pcm_uframes_t period_size; /* period size */
+ unsigned int periods; /* periods */
+ snd_pcm_uframes_t buffer_size; /* buffer size */
+ unsigned int tick_time; /* tick time */
+ snd_pcm_uframes_t min_align; /* Min alignment for the format */
+ size_t byte_align;
+ unsigned int frame_bits;
+ unsigned int sample_bits;
+ unsigned int info;
+ unsigned int rate_num;
+ unsigned int rate_den;
+
+ /* -- SW params -- */
+ struct timespec tstamp_mode; /* mmap timestamp is updated */
+ unsigned int period_step;
+ unsigned int sleep_min; /* min ticks to sleep */
+ snd_pcm_uframes_t start_threshold;
+ snd_pcm_uframes_t stop_threshold;
+ snd_pcm_uframes_t silence_threshold; /* Silence filling happens when
+ noise is nearest than this */
+ snd_pcm_uframes_t silence_size; /* Silence filling size */
+ snd_pcm_uframes_t boundary; /* pointers wrap point */
+
+ snd_pcm_uframes_t silenced_start;
+ snd_pcm_uframes_t silenced_size;
+
+ snd_pcm_sync_id_t sync; /* hardware synchronization ID */
+
+ /* -- mmap -- */
+ volatile struct snd_pcm_mmap_status *status;
+ volatile struct snd_pcm_mmap_control *control;
+ atomic_t mmap_count;
+
+ /* -- locking / scheduling -- */
+ spinlock_t lock;
+ wait_queue_head_t sleep;
+ struct timer_list tick_timer;
+ struct fasync_struct *fasync;
+
+ /* -- private section -- */
+ void *private_data;
+ void (*private_free)(struct snd_pcm_runtime *runtime);
+
+ /* -- hardware description -- */
+ struct snd_pcm_hardware hw;
+ struct snd_pcm_hw_constraints hw_constraints;
+
+ /* -- timer -- */
+ unsigned int timer_resolution; /* timer resolution */
+
+ /* -- DMA -- */
+ unsigned char *dma_area; /* DMA area */
+ dma_addr_t dma_addr; /* physical bus address (not accessible from main CPU) */
+ size_t dma_bytes; /* size of DMA area */
+
+ struct snd_dma_buffer *dma_buffer_p; /* allocated buffer */
+
+ #if defined(CONFIG_SND_PCM_OSS) || defined(CONFIG_SND_PCM_OSS_MODULE)
+ /* -- OSS things -- */
+ struct snd_pcm_oss_runtime oss;
+ #endif
+ };
+
+
+For the operators (callbacks) of each sound driver, most of these
+records are supposed to be read-only. Only the PCM middle-layer changes
+/ updates them. The exceptions are the hardware description (hw) DMA
+buffer information and the private data. Besides, if you use the
+standard buffer allocation method via
+:c:func:`snd_pcm_lib_malloc_pages()`, you don't need to set the
+DMA buffer information by yourself.
+
+In the sections below, important records are explained.
+
+Hardware Description
+~~~~~~~~~~~~~~~~~~~~
+
+The hardware descriptor (:c:type:`struct snd_pcm_hardware
+<snd_pcm_hardware>`) contains the definitions of the fundamental
+hardware configuration. Above all, you'll need to define this in the
+`PCM open callback`_. Note that the runtime instance holds the copy of
+the descriptor, not the pointer to the existing descriptor. That is,
+in the open callback, you can modify the copied descriptor
+(``runtime->hw``) as you need. For example, if the maximum number of
+channels is 1 only on some chip models, you can still use the same
+hardware descriptor and change the channels_max later:
+
+::
+
+ struct snd_pcm_runtime *runtime = substream->runtime;
+ ...
+ runtime->hw = snd_mychip_playback_hw; /* common definition */
+ if (chip->model == VERY_OLD_ONE)
+ runtime->hw.channels_max = 1;
+
+Typically, you'll have a hardware descriptor as below:
+
+::
+
+ static struct snd_pcm_hardware snd_mychip_playback_hw = {
+ .info = (SNDRV_PCM_INFO_MMAP |
+ SNDRV_PCM_INFO_INTERLEAVED |
+ SNDRV_PCM_INFO_BLOCK_TRANSFER |
+ SNDRV_PCM_INFO_MMAP_VALID),
+ .formats = SNDRV_PCM_FMTBIT_S16_LE,
+ .rates = SNDRV_PCM_RATE_8000_48000,
+ .rate_min = 8000,
+ .rate_max = 48000,
+ .channels_min = 2,
+ .channels_max = 2,
+ .buffer_bytes_max = 32768,
+ .period_bytes_min = 4096,
+ .period_bytes_max = 32768,
+ .periods_min = 1,
+ .periods_max = 1024,
+ };
+
+- The ``info`` field contains the type and capabilities of this
+ pcm. The bit flags are defined in ``<sound/asound.h>`` as
+ ``SNDRV_PCM_INFO_XXX``. Here, at least, you have to specify whether
+ the mmap is supported and which interleaved format is
+ supported. When the hardware supports mmap, add the
+ ``SNDRV_PCM_INFO_MMAP`` flag here. When the hardware supports the
+ interleaved or the non-interleaved formats,
+ ``SNDRV_PCM_INFO_INTERLEAVED`` or ``SNDRV_PCM_INFO_NONINTERLEAVED``
+ flag must be set, respectively. If both are supported, you can set
+ both, too.
+
+ In the above example, ``MMAP_VALID`` and ``BLOCK_TRANSFER`` are
+ specified for the OSS mmap mode. Usually both are set. Of course,
+ ``MMAP_VALID`` is set only if the mmap is really supported.
+
+ The other possible flags are ``SNDRV_PCM_INFO_PAUSE`` and
+ ``SNDRV_PCM_INFO_RESUME``. The ``PAUSE`` bit means that the pcm
+ supports the “pause” operation, while the ``RESUME`` bit means that
+ the pcm supports the full “suspend/resume” operation. If the
+ ``PAUSE`` flag is set, the ``trigger`` callback below must handle
+ the corresponding (pause push/release) commands. The suspend/resume
+ trigger commands can be defined even without the ``RESUME``
+ flag. See `Power Management`_ section for details.
+
+ When the PCM substreams can be synchronized (typically,
+ synchronized start/stop of a playback and a capture streams), you
+ can give ``SNDRV_PCM_INFO_SYNC_START``, too. In this case, you'll
+ need to check the linked-list of PCM substreams in the trigger
+ callback. This will be described in the later section.
+
+- ``formats`` field contains the bit-flags of supported formats
+ (``SNDRV_PCM_FMTBIT_XXX``). If the hardware supports more than one
+ format, give all or'ed bits. In the example above, the signed 16bit
+ little-endian format is specified.
+
+- ``rates`` field contains the bit-flags of supported rates
+ (``SNDRV_PCM_RATE_XXX``). When the chip supports continuous rates,
+ pass ``CONTINUOUS`` bit additionally. The pre-defined rate bits are
+ provided only for typical rates. If your chip supports
+ unconventional rates, you need to add the ``KNOT`` bit and set up
+ the hardware constraint manually (explained later).
+
+- ``rate_min`` and ``rate_max`` define the minimum and maximum sample
+ rate. This should correspond somehow to ``rates`` bits.
+
+- ``channel_min`` and ``channel_max`` define, as you might already
+ expected, the minimum and maximum number of channels.
+
+- ``buffer_bytes_max`` defines the maximum buffer size in
+ bytes. There is no ``buffer_bytes_min`` field, since it can be
+ calculated from the minimum period size and the minimum number of
+ periods. Meanwhile, ``period_bytes_min`` and define the minimum and
+ maximum size of the period in bytes. ``periods_max`` and
+ ``periods_min`` define the maximum and minimum number of periods in
+ the buffer.
+
+ The “period” is a term that corresponds to a fragment in the OSS
+ world. The period defines the size at which a PCM interrupt is
+ generated. This size strongly depends on the hardware. Generally,
+ the smaller period size will give you more interrupts, that is,
+ more controls. In the case of capture, this size defines the input
+ latency. On the other hand, the whole buffer size defines the
+ output latency for the playback direction.
+
+- There is also a field ``fifo_size``. This specifies the size of the
+ hardware FIFO, but currently it is neither used in the driver nor
+ in the alsa-lib. So, you can ignore this field.
+
+PCM Configurations
+~~~~~~~~~~~~~~~~~~
+
+Ok, let's go back again to the PCM runtime records. The most
+frequently referred records in the runtime instance are the PCM
+configurations. The PCM configurations are stored in the runtime
+instance after the application sends ``hw_params`` data via
+alsa-lib. There are many fields copied from hw_params and sw_params
+structs. For example, ``format`` holds the format type chosen by the
+application. This field contains the enum value
+``SNDRV_PCM_FORMAT_XXX``.
+
+One thing to be noted is that the configured buffer and period sizes
+are stored in “frames” in the runtime. In the ALSA world, ``1 frame =
+channels \* samples-size``. For conversion between frames and bytes,
+you can use the :c:func:`frames_to_bytes()` and
+:c:func:`bytes_to_frames()` helper functions.
+
+::
+
+ period_bytes = frames_to_bytes(runtime, runtime->period_size);
+
+Also, many software parameters (sw_params) are stored in frames, too.
+Please check the type of the field. ``snd_pcm_uframes_t`` is for the
+frames as unsigned integer while ``snd_pcm_sframes_t`` is for the
+frames as signed integer.
+
+DMA Buffer Information
+~~~~~~~~~~~~~~~~~~~~~~
+
+The DMA buffer is defined by the following four fields, ``dma_area``,
+``dma_addr``, ``dma_bytes`` and ``dma_private``. The ``dma_area``
+holds the buffer pointer (the logical address). You can call
+:c:func:`memcpy()` from/to this pointer. Meanwhile, ``dma_addr`` holds
+the physical address of the buffer. This field is specified only when
+the buffer is a linear buffer. ``dma_bytes`` holds the size of buffer
+in bytes. ``dma_private`` is used for the ALSA DMA allocator.
+
+If you use a standard ALSA function,
+:c:func:`snd_pcm_lib_malloc_pages()`, for allocating the buffer,
+these fields are set by the ALSA middle layer, and you should *not*
+change them by yourself. You can read them but not write them. On the
+other hand, if you want to allocate the buffer by yourself, you'll
+need to manage it in hw_params callback. At least, ``dma_bytes`` is
+mandatory. ``dma_area`` is necessary when the buffer is mmapped. If
+your driver doesn't support mmap, this field is not
+necessary. ``dma_addr`` is also optional. You can use dma_private as
+you like, too.
+
+Running Status
+~~~~~~~~~~~~~~
+
+The running status can be referred via ``runtime->status``. This is
+the pointer to the :c:type:`struct snd_pcm_mmap_status
+<snd_pcm_mmap_status>` record. For example, you can get the current
+DMA hardware pointer via ``runtime->status->hw_ptr``.
+
+The DMA application pointer can be referred via ``runtime->control``,
+which points to the :c:type:`struct snd_pcm_mmap_control
+<snd_pcm_mmap_control>` record. However, accessing directly to
+this value is not recommended.
+
+Private Data
+~~~~~~~~~~~~
+
+You can allocate a record for the substream and store it in
+``runtime->private_data``. Usually, this is done in the `PCM open
+callback`_. Don't mix this with ``pcm->private_data``. The
+``pcm->private_data`` usually points to the chip instance assigned
+statically at the creation of PCM, while the ``runtime->private_data``
+points to a dynamic data structure created at the PCM open
+callback.
+
+::
+
+ static int snd_xxx_open(struct snd_pcm_substream *substream)
+ {
+ struct my_pcm_data *data;
+ ....
+ data = kmalloc(sizeof(*data), GFP_KERNEL);
+ substream->runtime->private_data = data;
+ ....
+ }
+
+
+The allocated object must be released in the `close callback`_.
+
+Operators
+---------
+
+OK, now let me give details about each pcm callback (``ops``). In
+general, every callback must return 0 if successful, or a negative
+error number such as ``-EINVAL``. To choose an appropriate error
+number, it is advised to check what value other parts of the kernel
+return when the same kind of request fails.
+
+The callback function takes at least the argument with :c:type:`struct
+snd_pcm_substream <snd_pcm_substream>` pointer. To retrieve the chip
+record from the given substream instance, you can use the following
+macro.
+
+::
+
+ int xxx() {
+ struct mychip *chip = snd_pcm_substream_chip(substream);
+ ....
+ }
+
+The macro reads ``substream->private_data``, which is a copy of
+``pcm->private_data``. You can override the former if you need to
+assign different data records per PCM substream. For example, the
+cmi8330 driver assigns different ``private_data`` for playback and
+capture directions, because it uses two different codecs (SB- and
+AD-compatible) for different directions.
+
+PCM open callback
+~~~~~~~~~~~~~~~~~
+
+::
+
+ static int snd_xxx_open(struct snd_pcm_substream *substream);
+
+This is called when a pcm substream is opened.
+
+At least, here you have to initialize the ``runtime->hw``
+record. Typically, this is done by like this:
+
+::
+
+ static int snd_xxx_open(struct snd_pcm_substream *substream)
+ {
+ struct mychip *chip = snd_pcm_substream_chip(substream);
+ struct snd_pcm_runtime *runtime = substream->runtime;
+
+ runtime->hw = snd_mychip_playback_hw;
+ return 0;
+ }
+
+where ``snd_mychip_playback_hw`` is the pre-defined hardware
+description.
+
+You can allocate a private data in this callback, as described in
+`Private Data`_ section.
+
+If the hardware configuration needs more constraints, set the hardware
+constraints here, too. See Constraints_ for more details.
+
+close callback
+~~~~~~~~~~~~~~
+
+::
+
+ static int snd_xxx_close(struct snd_pcm_substream *substream);
+
+
+Obviously, this is called when a pcm substream is closed.
+
+Any private instance for a pcm substream allocated in the ``open``
+callback will be released here.
+
+::
+
+ static int snd_xxx_close(struct snd_pcm_substream *substream)
+ {
+ ....
+ kfree(substream->runtime->private_data);
+ ....
+ }
+
+ioctl callback
+~~~~~~~~~~~~~~
+
+This is used for any special call to pcm ioctls. But usually you can
+pass a generic ioctl callback, :c:func:`snd_pcm_lib_ioctl()`.
+
+hw_params callback
+~~~~~~~~~~~~~~~~~~~
+
+::
+
+ static int snd_xxx_hw_params(struct snd_pcm_substream *substream,
+ struct snd_pcm_hw_params *hw_params);
+
+This is called when the hardware parameter (``hw_params``) is set up
+by the application, that is, once when the buffer size, the period
+size, the format, etc. are defined for the pcm substream.
+
+Many hardware setups should be done in this callback, including the
+allocation of buffers.
+
+Parameters to be initialized are retrieved by
+:c:func:`params_xxx()` macros. To allocate buffer, you can call a
+helper function,
+
+::
+
+ snd_pcm_lib_malloc_pages(substream, params_buffer_bytes(hw_params));
+
+:c:func:`snd_pcm_lib_malloc_pages()` is available only when the
+DMA buffers have been pre-allocated. See the section `Buffer Types`_
+for more details.
+
+Note that this and ``prepare`` callbacks may be called multiple times
+per initialization. For example, the OSS emulation may call these
+callbacks at each change via its ioctl.
+
+Thus, you need to be careful not to allocate the same buffers many
+times, which will lead to memory leaks! Calling the helper function
+above many times is OK. It will release the previous buffer
+automatically when it was already allocated.
+
+Another note is that this callback is non-atomic (schedulable) as
+default, i.e. when no ``nonatomic`` flag set. This is important,
+because the ``trigger`` callback is atomic (non-schedulable). That is,
+mutexes or any schedule-related functions are not available in
+``trigger`` callback. Please see the subsection Atomicity_ for
+details.
+
+hw_free callback
+~~~~~~~~~~~~~~~~~
+
+::
+
+ static int snd_xxx_hw_free(struct snd_pcm_substream *substream);
+
+This is called to release the resources allocated via
+``hw_params``. For example, releasing the buffer via
+:c:func:`snd_pcm_lib_malloc_pages()` is done by calling the
+following:
+
+::
+
+ snd_pcm_lib_free_pages(substream);
+
+This function is always called before the close callback is called.
+Also, the callback may be called multiple times, too. Keep track
+whether the resource was already released.
+
+prepare callback
+~~~~~~~~~~~~~~~~
+
+::
+
+ static int snd_xxx_prepare(struct snd_pcm_substream *substream);
+
+This callback is called when the pcm is “prepared”. You can set the
+format type, sample rate, etc. here. The difference from ``hw_params``
+is that the ``prepare`` callback will be called each time
+:c:func:`snd_pcm_prepare()` is called, i.e. when recovering after
+underruns, etc.
+
+Note that this callback is now non-atomic. You can use
+schedule-related functions safely in this callback.
+
+In this and the following callbacks, you can refer to the values via
+the runtime record, ``substream->runtime``. For example, to get the
+current rate, format or channels, access to ``runtime->rate``,
+``runtime->format`` or ``runtime->channels``, respectively. The
+physical address of the allocated buffer is set to
+``runtime->dma_area``. The buffer and period sizes are in
+``runtime->buffer_size`` and ``runtime->period_size``, respectively.
+
+Be careful that this callback will be called many times at each setup,
+too.
+
+trigger callback
+~~~~~~~~~~~~~~~~
+
+::
+
+ static int snd_xxx_trigger(struct snd_pcm_substream *substream, int cmd);
+
+This is called when the pcm is started, stopped or paused.
+
+Which action is specified in the second argument,
+``SNDRV_PCM_TRIGGER_XXX`` in ``<sound/pcm.h>``. At least, the ``START``
+and ``STOP`` commands must be defined in this callback.
+
+::
+
+ switch (cmd) {
+ case SNDRV_PCM_TRIGGER_START:
+ /* do something to start the PCM engine */
+ break;
+ case SNDRV_PCM_TRIGGER_STOP:
+ /* do something to stop the PCM engine */
+ break;
+ default:
+ return -EINVAL;
+ }
+
+When the pcm supports the pause operation (given in the info field of
+the hardware table), the ``PAUSE_PUSH`` and ``PAUSE_RELEASE`` commands
+must be handled here, too. The former is the command to pause the pcm,
+and the latter to restart the pcm again.
+
+When the pcm supports the suspend/resume operation, regardless of full
+or partial suspend/resume support, the ``SUSPEND`` and ``RESUME``
+commands must be handled, too. These commands are issued when the
+power-management status is changed. Obviously, the ``SUSPEND`` and
+``RESUME`` commands suspend and resume the pcm substream, and usually,
+they are identical to the ``STOP`` and ``START`` commands, respectively.
+See the `Power Management`_ section for details.
+
+As mentioned, this callback is atomic as default unless ``nonatomic``
+flag set, and you cannot call functions which may sleep. The
+``trigger`` callback should be as minimal as possible, just really
+triggering the DMA. The other stuff should be initialized
+``hw_params`` and ``prepare`` callbacks properly beforehand.
+
+pointer callback
+~~~~~~~~~~~~~~~~
+
+::
+
+ static snd_pcm_uframes_t snd_xxx_pointer(struct snd_pcm_substream *substream)
+
+This callback is called when the PCM middle layer inquires the current
+hardware position on the buffer. The position must be returned in
+frames, ranging from 0 to ``buffer_size - 1``.
+
+This is called usually from the buffer-update routine in the pcm
+middle layer, which is invoked when :c:func:`snd_pcm_period_elapsed()`
+is called in the interrupt routine. Then the pcm middle layer updates
+the position and calculates the available space, and wakes up the
+sleeping poll threads, etc.
+
+This callback is also atomic as default.
+
+copy_user, copy_kernel and fill_silence ops
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+These callbacks are not mandatory, and can be omitted in most cases.
+These callbacks are used when the hardware buffer cannot be in the
+normal memory space. Some chips have their own buffer on the hardware
+which is not mappable. In such a case, you have to transfer the data
+manually from the memory buffer to the hardware buffer. Or, if the
+buffer is non-contiguous on both physical and virtual memory spaces,
+these callbacks must be defined, too.
+
+If these two callbacks are defined, copy and set-silence operations
+are done by them. The detailed will be described in the later section
+`Buffer and Memory Management`_.
+
+ack callback
+~~~~~~~~~~~~
+
+This callback is also not mandatory. This callback is called when the
+``appl_ptr`` is updated in read or write operations. Some drivers like
+emu10k1-fx and cs46xx need to track the current ``appl_ptr`` for the
+internal buffer, and this callback is useful only for such a purpose.
+
+This callback is atomic as default.
+
+page callback
+~~~~~~~~~~~~~
+
+This callback is optional too. This callback is used mainly for
+non-contiguous buffers. The mmap calls this callback to get the page
+address. Some examples will be explained in the later section `Buffer
+and Memory Management`_, too.
+
+PCM Interrupt Handler
+---------------------
+
+The rest of pcm stuff is the PCM interrupt handler. The role of PCM
+interrupt handler in the sound driver is to update the buffer position
+and to tell the PCM middle layer when the buffer position goes across
+the prescribed period size. To inform this, call the
+:c:func:`snd_pcm_period_elapsed()` function.
+
+There are several types of sound chips to generate the interrupts.
+
+Interrupts at the period (fragment) boundary
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+This is the most frequently found type: the hardware generates an
+interrupt at each period boundary. In this case, you can call
+:c:func:`snd_pcm_period_elapsed()` at each interrupt.
+
+:c:func:`snd_pcm_period_elapsed()` takes the substream pointer as
+its argument. Thus, you need to keep the substream pointer accessible
+from the chip instance. For example, define ``substream`` field in the
+chip record to hold the current running substream pointer, and set the
+pointer value at ``open`` callback (and reset at ``close`` callback).
+
+If you acquire a spinlock in the interrupt handler, and the lock is used
+in other pcm callbacks, too, then you have to release the lock before
+calling :c:func:`snd_pcm_period_elapsed()`, because
+:c:func:`snd_pcm_period_elapsed()` calls other pcm callbacks
+inside.
+
+Typical code would be like:
+
+::
+
+
+ static irqreturn_t snd_mychip_interrupt(int irq, void *dev_id)
+ {
+ struct mychip *chip = dev_id;
+ spin_lock(&chip->lock);
+ ....
+ if (pcm_irq_invoked(chip)) {
+ /* call updater, unlock before it */
+ spin_unlock(&chip->lock);
+ snd_pcm_period_elapsed(chip->substream);
+ spin_lock(&chip->lock);
+ /* acknowledge the interrupt if necessary */
+ }
+ ....
+ spin_unlock(&chip->lock);
+ return IRQ_HANDLED;
+ }
+
+
+
+High frequency timer interrupts
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+This happens when the hardware doesn't generate interrupts at the period
+boundary but issues timer interrupts at a fixed timer rate (e.g. es1968
+or ymfpci drivers). In this case, you need to check the current hardware
+position and accumulate the processed sample length at each interrupt.
+When the accumulated size exceeds the period size, call
+:c:func:`snd_pcm_period_elapsed()` and reset the accumulator.
+
+Typical code would be like the following.
+
+::
+
+
+ static irqreturn_t snd_mychip_interrupt(int irq, void *dev_id)
+ {
+ struct mychip *chip = dev_id;
+ spin_lock(&chip->lock);
+ ....
+ if (pcm_irq_invoked(chip)) {
+ unsigned int last_ptr, size;
+ /* get the current hardware pointer (in frames) */
+ last_ptr = get_hw_ptr(chip);
+ /* calculate the processed frames since the
+ * last update
+ */
+ if (last_ptr < chip->last_ptr)
+ size = runtime->buffer_size + last_ptr
+ - chip->last_ptr;
+ else
+ size = last_ptr - chip->last_ptr;
+ /* remember the last updated point */
+ chip->last_ptr = last_ptr;
+ /* accumulate the size */
+ chip->size += size;
+ /* over the period boundary? */
+ if (chip->size >= runtime->period_size) {
+ /* reset the accumulator */
+ chip->size %= runtime->period_size;
+ /* call updater */
+ spin_unlock(&chip->lock);
+ snd_pcm_period_elapsed(substream);
+ spin_lock(&chip->lock);
+ }
+ /* acknowledge the interrupt if necessary */
+ }
+ ....
+ spin_unlock(&chip->lock);
+ return IRQ_HANDLED;
+ }
+
+
+
+On calling :c:func:`snd_pcm_period_elapsed()`
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+In both cases, even if more than one period are elapsed, you don't have
+to call :c:func:`snd_pcm_period_elapsed()` many times. Call only
+once. And the pcm layer will check the current hardware pointer and
+update to the latest status.
+
+Atomicity
+---------
+
+One of the most important (and thus difficult to debug) problems in
+kernel programming are race conditions. In the Linux kernel, they are
+usually avoided via spin-locks, mutexes or semaphores. In general, if a
+race condition can happen in an interrupt handler, it has to be managed
+atomically, and you have to use a spinlock to protect the critical
+session. If the critical section is not in interrupt handler code and if
+taking a relatively long time to execute is acceptable, you should use
+mutexes or semaphores instead.
+
+As already seen, some pcm callbacks are atomic and some are not. For
+example, the ``hw_params`` callback is non-atomic, while ``trigger``
+callback is atomic. This means, the latter is called already in a
+spinlock held by the PCM middle layer. Please take this atomicity into
+account when you choose a locking scheme in the callbacks.
+
+In the atomic callbacks, you cannot use functions which may call
+:c:func:`schedule()` or go to :c:func:`sleep()`. Semaphores and
+mutexes can sleep, and hence they cannot be used inside the atomic
+callbacks (e.g. ``trigger`` callback). To implement some delay in such a
+callback, please use :c:func:`udelay()` or :c:func:`mdelay()`.
+
+All three atomic callbacks (trigger, pointer, and ack) are called with
+local interrupts disabled.
+
+The recent changes in PCM core code, however, allow all PCM operations
+to be non-atomic. This assumes that the all caller sides are in
+non-atomic contexts. For example, the function
+:c:func:`snd_pcm_period_elapsed()` is called typically from the
+interrupt handler. But, if you set up the driver to use a threaded
+interrupt handler, this call can be in non-atomic context, too. In such
+a case, you can set ``nonatomic`` filed of :c:type:`struct snd_pcm
+<snd_pcm>` object after creating it. When this flag is set, mutex
+and rwsem are used internally in the PCM core instead of spin and
+rwlocks, so that you can call all PCM functions safely in a non-atomic
+context.
+
+Constraints
+-----------
+
+If your chip supports unconventional sample rates, or only the limited
+samples, you need to set a constraint for the condition.
+
+For example, in order to restrict the sample rates in the some supported
+values, use :c:func:`snd_pcm_hw_constraint_list()`. You need to
+call this function in the open callback.
+
+::
+
+ static unsigned int rates[] =
+ {4000, 10000, 22050, 44100};
+ static struct snd_pcm_hw_constraint_list constraints_rates = {
+ .count = ARRAY_SIZE(rates),
+ .list = rates,
+ .mask = 0,
+ };
+
+ static int snd_mychip_pcm_open(struct snd_pcm_substream *substream)
+ {
+ int err;
+ ....
+ err = snd_pcm_hw_constraint_list(substream->runtime, 0,
+ SNDRV_PCM_HW_PARAM_RATE,
+ &constraints_rates);
+ if (err < 0)
+ return err;
+ ....
+ }
+
+
+
+There are many different constraints. Look at ``sound/pcm.h`` for a
+complete list. You can even define your own constraint rules. For
+example, let's suppose my_chip can manage a substream of 1 channel if
+and only if the format is ``S16_LE``, otherwise it supports any format
+specified in the :c:type:`struct snd_pcm_hardware
+<snd_pcm_hardware>` structure (or in any other
+constraint_list). You can build a rule like this:
+
+::
+
+ static int hw_rule_channels_by_format(struct snd_pcm_hw_params *params,
+ struct snd_pcm_hw_rule *rule)
+ {
+ struct snd_interval *c = hw_param_interval(params,
+ SNDRV_PCM_HW_PARAM_CHANNELS);
+ struct snd_mask *f = hw_param_mask(params, SNDRV_PCM_HW_PARAM_FORMAT);
+ struct snd_interval ch;
+
+ snd_interval_any(&ch);
+ if (f->bits[0] == SNDRV_PCM_FMTBIT_S16_LE) {
+ ch.min = ch.max = 1;
+ ch.integer = 1;
+ return snd_interval_refine(c, &ch);
+ }
+ return 0;
+ }
+
+
+Then you need to call this function to add your rule:
+
+::
+
+ snd_pcm_hw_rule_add(substream->runtime, 0, SNDRV_PCM_HW_PARAM_CHANNELS,
+ hw_rule_channels_by_format, NULL,
+ SNDRV_PCM_HW_PARAM_FORMAT, -1);
+
+The rule function is called when an application sets the PCM format, and
+it refines the number of channels accordingly. But an application may
+set the number of channels before setting the format. Thus you also need
+to define the inverse rule:
+
+::
+
+ static int hw_rule_format_by_channels(struct snd_pcm_hw_params *params,
+ struct snd_pcm_hw_rule *rule)
+ {
+ struct snd_interval *c = hw_param_interval(params,
+ SNDRV_PCM_HW_PARAM_CHANNELS);
+ struct snd_mask *f = hw_param_mask(params, SNDRV_PCM_HW_PARAM_FORMAT);
+ struct snd_mask fmt;
+
+ snd_mask_any(&fmt); /* Init the struct */
+ if (c->min < 2) {
+ fmt.bits[0] &= SNDRV_PCM_FMTBIT_S16_LE;
+ return snd_mask_refine(f, &fmt);
+ }
+ return 0;
+ }
+
+
+... and in the open callback:
+
+::
+
+ snd_pcm_hw_rule_add(substream->runtime, 0, SNDRV_PCM_HW_PARAM_FORMAT,
+ hw_rule_format_by_channels, NULL,
+ SNDRV_PCM_HW_PARAM_CHANNELS, -1);
+
+I won't give more details here, rather I would like to say, “Luke, use
+the source.”
+
+Control Interface
+=================
+
+General
+-------
+
+The control interface is used widely for many switches, sliders, etc.
+which are accessed from user-space. Its most important use is the mixer
+interface. In other words, since ALSA 0.9.x, all the mixer stuff is
+implemented on the control kernel API.
+
+ALSA has a well-defined AC97 control module. If your chip supports only
+the AC97 and nothing else, you can skip this section.
+
+The control API is defined in ``<sound/control.h>``. Include this file
+if you want to add your own controls.
+
+Definition of Controls
+----------------------
+
+To create a new control, you need to define the following three
+callbacks: ``info``, ``get`` and ``put``. Then, define a
+:c:type:`struct snd_kcontrol_new <snd_kcontrol_new>` record, such as:
+
+::
+
+
+ static struct snd_kcontrol_new my_control = {
+ .iface = SNDRV_CTL_ELEM_IFACE_MIXER,
+ .name = "PCM Playback Switch",
+ .index = 0,
+ .access = SNDRV_CTL_ELEM_ACCESS_READWRITE,
+ .private_value = 0xffff,
+ .info = my_control_info,
+ .get = my_control_get,
+ .put = my_control_put
+ };
+
+
+The ``iface`` field specifies the control type,
+``SNDRV_CTL_ELEM_IFACE_XXX``, which is usually ``MIXER``. Use ``CARD``
+for global controls that are not logically part of the mixer. If the
+control is closely associated with some specific device on the sound
+card, use ``HWDEP``, ``PCM``, ``RAWMIDI``, ``TIMER``, or ``SEQUENCER``,
+and specify the device number with the ``device`` and ``subdevice``
+fields.
+
+The ``name`` is the name identifier string. Since ALSA 0.9.x, the
+control name is very important, because its role is classified from
+its name. There are pre-defined standard control names. The details
+are described in the `Control Names`_ subsection.
+
+The ``index`` field holds the index number of this control. If there
+are several different controls with the same name, they can be
+distinguished by the index number. This is the case when several
+codecs exist on the card. If the index is zero, you can omit the
+definition above.
+
+The ``access`` field contains the access type of this control. Give
+the combination of bit masks, ``SNDRV_CTL_ELEM_ACCESS_XXX``,
+there. The details will be explained in the `Access Flags`_
+subsection.
+
+The ``private_value`` field contains an arbitrary long integer value
+for this record. When using the generic ``info``, ``get`` and ``put``
+callbacks, you can pass a value through this field. If several small
+numbers are necessary, you can combine them in bitwise. Or, it's
+possible to give a pointer (casted to unsigned long) of some record to
+this field, too.
+
+The ``tlv`` field can be used to provide metadata about the control;
+see the `Metadata`_ subsection.
+
+The other three are `Control Callbacks`_.
+
+Control Names
+-------------
+
+There are some standards to define the control names. A control is
+usually defined from the three parts as “SOURCE DIRECTION FUNCTION”.
+
+The first, ``SOURCE``, specifies the source of the control, and is a
+string such as “Master”, “PCM”, “CD” and “Line”. There are many
+pre-defined sources.
+
+The second, ``DIRECTION``, is one of the following strings according to
+the direction of the control: “Playback”, “Capture”, “Bypass Playback”
+and “Bypass Capture”. Or, it can be omitted, meaning both playback and
+capture directions.
+
+The third, ``FUNCTION``, is one of the following strings according to
+the function of the control: “Switch”, “Volume” and “Route”.
+
+The example of control names are, thus, “Master Capture Switch” or “PCM
+Playback Volume”.
+
+There are some exceptions:
+
+Global capture and playback
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+“Capture Source”, “Capture Switch” and “Capture Volume” are used for the
+global capture (input) source, switch and volume. Similarly, “Playback
+Switch” and “Playback Volume” are used for the global output gain switch
+and volume.
+
+Tone-controls
+~~~~~~~~~~~~~
+
+tone-control switch and volumes are specified like “Tone Control - XXX”,
+e.g. “Tone Control - Switch”, “Tone Control - Bass”, “Tone Control -
+Center”.
+
+3D controls
+~~~~~~~~~~~
+
+3D-control switches and volumes are specified like “3D Control - XXX”,
+e.g. “3D Control - Switch”, “3D Control - Center”, “3D Control - Space”.
+
+Mic boost
+~~~~~~~~~
+
+Mic-boost switch is set as “Mic Boost” or “Mic Boost (6dB)”.
+
+More precise information can be found in
+``Documentation/sound/designs/control-names.rst``.
+
+Access Flags
+------------
+
+The access flag is the bitmask which specifies the access type of the
+given control. The default access type is
+``SNDRV_CTL_ELEM_ACCESS_READWRITE``, which means both read and write are
+allowed to this control. When the access flag is omitted (i.e. = 0), it
+is considered as ``READWRITE`` access as default.
+
+When the control is read-only, pass ``SNDRV_CTL_ELEM_ACCESS_READ``
+instead. In this case, you don't have to define the ``put`` callback.
+Similarly, when the control is write-only (although it's a rare case),
+you can use the ``WRITE`` flag instead, and you don't need the ``get``
+callback.
+
+If the control value changes frequently (e.g. the VU meter),
+``VOLATILE`` flag should be given. This means that the control may be
+changed without `Change notification`_. Applications should poll such
+a control constantly.
+
+When the control is inactive, set the ``INACTIVE`` flag, too. There are
+``LOCK`` and ``OWNER`` flags to change the write permissions.
+
+Control Callbacks
+-----------------
+
+info callback
+~~~~~~~~~~~~~
+
+The ``info`` callback is used to get detailed information on this
+control. This must store the values of the given :c:type:`struct
+snd_ctl_elem_info <snd_ctl_elem_info>` object. For example,
+for a boolean control with a single element:
+
+::
+
+
+ static int snd_myctl_mono_info(struct snd_kcontrol *kcontrol,
+ struct snd_ctl_elem_info *uinfo)
+ {
+ uinfo->type = SNDRV_CTL_ELEM_TYPE_BOOLEAN;
+ uinfo->count = 1;
+ uinfo->value.integer.min = 0;
+ uinfo->value.integer.max = 1;
+ return 0;
+ }
+
+
+
+The ``type`` field specifies the type of the control. There are
+``BOOLEAN``, ``INTEGER``, ``ENUMERATED``, ``BYTES``, ``IEC958`` and
+``INTEGER64``. The ``count`` field specifies the number of elements in
+this control. For example, a stereo volume would have count = 2. The
+``value`` field is a union, and the values stored are depending on the
+type. The boolean and integer types are identical.
+
+The enumerated type is a bit different from others. You'll need to set
+the string for the currently given item index.
+
+::
+
+ static int snd_myctl_enum_info(struct snd_kcontrol *kcontrol,
+ struct snd_ctl_elem_info *uinfo)
+ {
+ static char *texts[4] = {
+ "First", "Second", "Third", "Fourth"
+ };
+ uinfo->type = SNDRV_CTL_ELEM_TYPE_ENUMERATED;
+ uinfo->count = 1;
+ uinfo->value.enumerated.items = 4;
+ if (uinfo->value.enumerated.item > 3)
+ uinfo->value.enumerated.item = 3;
+ strcpy(uinfo->value.enumerated.name,
+ texts[uinfo->value.enumerated.item]);
+ return 0;
+ }
+
+The above callback can be simplified with a helper function,
+:c:func:`snd_ctl_enum_info()`. The final code looks like below.
+(You can pass ``ARRAY_SIZE(texts)`` instead of 4 in the third argument;
+it's a matter of taste.)
+
+::
+
+ static int snd_myctl_enum_info(struct snd_kcontrol *kcontrol,
+ struct snd_ctl_elem_info *uinfo)
+ {
+ static char *texts[4] = {
+ "First", "Second", "Third", "Fourth"
+ };
+ return snd_ctl_enum_info(uinfo, 1, 4, texts);
+ }
+
+
+Some common info callbacks are available for your convenience:
+:c:func:`snd_ctl_boolean_mono_info()` and
+:c:func:`snd_ctl_boolean_stereo_info()`. Obviously, the former
+is an info callback for a mono channel boolean item, just like
+:c:func:`snd_myctl_mono_info()` above, and the latter is for a
+stereo channel boolean item.
+
+get callback
+~~~~~~~~~~~~
+
+This callback is used to read the current value of the control and to
+return to user-space.
+
+For example,
+
+::
+
+
+ static int snd_myctl_get(struct snd_kcontrol *kcontrol,
+ struct snd_ctl_elem_value *ucontrol)
+ {
+ struct mychip *chip = snd_kcontrol_chip(kcontrol);
+ ucontrol->value.integer.value[0] = get_some_value(chip);
+ return 0;
+ }
+
+
+
+The ``value`` field depends on the type of control as well as on the
+info callback. For example, the sb driver uses this field to store the
+register offset, the bit-shift and the bit-mask. The ``private_value``
+field is set as follows:
+
+::
+
+ .private_value = reg | (shift << 16) | (mask << 24)
+
+and is retrieved in callbacks like
+
+::
+
+ static int snd_sbmixer_get_single(struct snd_kcontrol *kcontrol,
+ struct snd_ctl_elem_value *ucontrol)
+ {
+ int reg = kcontrol->private_value & 0xff;
+ int shift = (kcontrol->private_value >> 16) & 0xff;
+ int mask = (kcontrol->private_value >> 24) & 0xff;
+ ....
+ }
+
+In the ``get`` callback, you have to fill all the elements if the
+control has more than one elements, i.e. ``count > 1``. In the example
+above, we filled only one element (``value.integer.value[0]``) since
+it's assumed as ``count = 1``.
+
+put callback
+~~~~~~~~~~~~
+
+This callback is used to write a value from user-space.
+
+For example,
+
+::
+
+
+ static int snd_myctl_put(struct snd_kcontrol *kcontrol,
+ struct snd_ctl_elem_value *ucontrol)
+ {
+ struct mychip *chip = snd_kcontrol_chip(kcontrol);
+ int changed = 0;
+ if (chip->current_value !=
+ ucontrol->value.integer.value[0]) {
+ change_current_value(chip,
+ ucontrol->value.integer.value[0]);
+ changed = 1;
+ }
+ return changed;
+ }
+
+
+
+As seen above, you have to return 1 if the value is changed. If the
+value is not changed, return 0 instead. If any fatal error happens,
+return a negative error code as usual.
+
+As in the ``get`` callback, when the control has more than one
+elements, all elements must be evaluated in this callback, too.
+
+Callbacks are not atomic
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+All these three callbacks are basically not atomic.
+
+Control Constructor
+-------------------
+
+When everything is ready, finally we can create a new control. To create
+a control, there are two functions to be called,
+:c:func:`snd_ctl_new1()` and :c:func:`snd_ctl_add()`.
+
+In the simplest way, you can do like this:
+
+::
+
+ err = snd_ctl_add(card, snd_ctl_new1(&my_control, chip));
+ if (err < 0)
+ return err;
+
+where ``my_control`` is the :c:type:`struct snd_kcontrol_new
+<snd_kcontrol_new>` object defined above, and chip is the object
+pointer to be passed to kcontrol->private_data which can be referred
+to in callbacks.
+
+:c:func:`snd_ctl_new1()` allocates a new :c:type:`struct
+snd_kcontrol <snd_kcontrol>` instance, and
+:c:func:`snd_ctl_add()` assigns the given control component to the
+card.
+
+Change Notification
+-------------------
+
+If you need to change and update a control in the interrupt routine, you
+can call :c:func:`snd_ctl_notify()`. For example,
+
+::
+
+ snd_ctl_notify(card, SNDRV_CTL_EVENT_MASK_VALUE, id_pointer);
+
+This function takes the card pointer, the event-mask, and the control id
+pointer for the notification. The event-mask specifies the types of
+notification, for example, in the above example, the change of control
+values is notified. The id pointer is the pointer of :c:type:`struct
+snd_ctl_elem_id <snd_ctl_elem_id>` to be notified. You can
+find some examples in ``es1938.c`` or ``es1968.c`` for hardware volume
+interrupts.
+
+Metadata
+--------
+
+To provide information about the dB values of a mixer control, use on of
+the ``DECLARE_TLV_xxx`` macros from ``<sound/tlv.h>`` to define a
+variable containing this information, set the ``tlv.p`` field to point to
+this variable, and include the ``SNDRV_CTL_ELEM_ACCESS_TLV_READ`` flag
+in the ``access`` field; like this:
+
+::
+
+ static DECLARE_TLV_DB_SCALE(db_scale_my_control, -4050, 150, 0);
+
+ static struct snd_kcontrol_new my_control = {
+ ...
+ .access = SNDRV_CTL_ELEM_ACCESS_READWRITE |
+ SNDRV_CTL_ELEM_ACCESS_TLV_READ,
+ ...
+ .tlv.p = db_scale_my_control,
+ };
+
+
+The :c:func:`DECLARE_TLV_DB_SCALE()` macro defines information
+about a mixer control where each step in the control's value changes the
+dB value by a constant dB amount. The first parameter is the name of the
+variable to be defined. The second parameter is the minimum value, in
+units of 0.01 dB. The third parameter is the step size, in units of 0.01
+dB. Set the fourth parameter to 1 if the minimum value actually mutes
+the control.
+
+The :c:func:`DECLARE_TLV_DB_LINEAR()` macro defines information
+about a mixer control where the control's value affects the output
+linearly. The first parameter is the name of the variable to be defined.
+The second parameter is the minimum value, in units of 0.01 dB. The
+third parameter is the maximum value, in units of 0.01 dB. If the
+minimum value mutes the control, set the second parameter to
+``TLV_DB_GAIN_MUTE``.
+
+API for AC97 Codec
+==================
+
+General
+-------
+
+The ALSA AC97 codec layer is a well-defined one, and you don't have to
+write much code to control it. Only low-level control routines are
+necessary. The AC97 codec API is defined in ``<sound/ac97_codec.h>``.
+
+Full Code Example
+-----------------
+
+::
+
+ struct mychip {
+ ....
+ struct snd_ac97 *ac97;
+ ....
+ };
+
+ static unsigned short snd_mychip_ac97_read(struct snd_ac97 *ac97,
+ unsigned short reg)
+ {
+ struct mychip *chip = ac97->private_data;
+ ....
+ /* read a register value here from the codec */
+ return the_register_value;
+ }
+
+ static void snd_mychip_ac97_write(struct snd_ac97 *ac97,
+ unsigned short reg, unsigned short val)
+ {
+ struct mychip *chip = ac97->private_data;
+ ....
+ /* write the given register value to the codec */
+ }
+
+ static int snd_mychip_ac97(struct mychip *chip)
+ {
+ struct snd_ac97_bus *bus;
+ struct snd_ac97_template ac97;
+ int err;
+ static struct snd_ac97_bus_ops ops = {
+ .write = snd_mychip_ac97_write,
+ .read = snd_mychip_ac97_read,
+ };
+
+ err = snd_ac97_bus(chip->card, 0, &ops, NULL, &bus);
+ if (err < 0)
+ return err;
+ memset(&ac97, 0, sizeof(ac97));
+ ac97.private_data = chip;
+ return snd_ac97_mixer(bus, &ac97, &chip->ac97);
+ }
+
+
+AC97 Constructor
+----------------
+
+To create an ac97 instance, first call :c:func:`snd_ac97_bus()`
+with an ``ac97_bus_ops_t`` record with callback functions.
+
+::
+
+ struct snd_ac97_bus *bus;
+ static struct snd_ac97_bus_ops ops = {
+ .write = snd_mychip_ac97_write,
+ .read = snd_mychip_ac97_read,
+ };
+
+ snd_ac97_bus(card, 0, &ops, NULL, &pbus);
+
+The bus record is shared among all belonging ac97 instances.
+
+And then call :c:func:`snd_ac97_mixer()` with an :c:type:`struct
+snd_ac97_template <snd_ac97_template>` record together with
+the bus pointer created above.
+
+::
+
+ struct snd_ac97_template ac97;
+ int err;
+
+ memset(&ac97, 0, sizeof(ac97));
+ ac97.private_data = chip;
+ snd_ac97_mixer(bus, &ac97, &chip->ac97);
+
+where chip->ac97 is a pointer to a newly created ``ac97_t``
+instance. In this case, the chip pointer is set as the private data,
+so that the read/write callback functions can refer to this chip
+instance. This instance is not necessarily stored in the chip
+record. If you need to change the register values from the driver, or
+need the suspend/resume of ac97 codecs, keep this pointer to pass to
+the corresponding functions.
+
+AC97 Callbacks
+--------------
+
+The standard callbacks are ``read`` and ``write``. Obviously they
+correspond to the functions for read and write accesses to the
+hardware low-level codes.
+
+The ``read`` callback returns the register value specified in the
+argument.
+
+::
+
+ static unsigned short snd_mychip_ac97_read(struct snd_ac97 *ac97,
+ unsigned short reg)
+ {
+ struct mychip *chip = ac97->private_data;
+ ....
+ return the_register_value;
+ }
+
+Here, the chip can be cast from ``ac97->private_data``.
+
+Meanwhile, the ``write`` callback is used to set the register
+value
+
+::
+
+ static void snd_mychip_ac97_write(struct snd_ac97 *ac97,
+ unsigned short reg, unsigned short val)
+
+
+These callbacks are non-atomic like the control API callbacks.
+
+There are also other callbacks: ``reset``, ``wait`` and ``init``.
+
+The ``reset`` callback is used to reset the codec. If the chip
+requires a special kind of reset, you can define this callback.
+
+The ``wait`` callback is used to add some waiting time in the standard
+initialization of the codec. If the chip requires the extra waiting
+time, define this callback.
+
+The ``init`` callback is used for additional initialization of the
+codec.
+
+Updating Registers in The Driver
+--------------------------------
+
+If you need to access to the codec from the driver, you can call the
+following functions: :c:func:`snd_ac97_write()`,
+:c:func:`snd_ac97_read()`, :c:func:`snd_ac97_update()` and
+:c:func:`snd_ac97_update_bits()`.
+
+Both :c:func:`snd_ac97_write()` and
+:c:func:`snd_ac97_update()` functions are used to set a value to
+the given register (``AC97_XXX``). The difference between them is that
+:c:func:`snd_ac97_update()` doesn't write a value if the given
+value has been already set, while :c:func:`snd_ac97_write()`
+always rewrites the value.
+
+::
+
+ snd_ac97_write(ac97, AC97_MASTER, 0x8080);
+ snd_ac97_update(ac97, AC97_MASTER, 0x8080);
+
+:c:func:`snd_ac97_read()` is used to read the value of the given
+register. For example,
+
+::
+
+ value = snd_ac97_read(ac97, AC97_MASTER);
+
+:c:func:`snd_ac97_update_bits()` is used to update some bits in
+the given register.
+
+::
+
+ snd_ac97_update_bits(ac97, reg, mask, value);
+
+Also, there is a function to change the sample rate (of a given register
+such as ``AC97_PCM_FRONT_DAC_RATE``) when VRA or DRA is supported by the
+codec: :c:func:`snd_ac97_set_rate()`.
+
+::
+
+ snd_ac97_set_rate(ac97, AC97_PCM_FRONT_DAC_RATE, 44100);
+
+
+The following registers are available to set the rate:
+``AC97_PCM_MIC_ADC_RATE``, ``AC97_PCM_FRONT_DAC_RATE``,
+``AC97_PCM_LR_ADC_RATE``, ``AC97_SPDIF``. When ``AC97_SPDIF`` is
+specified, the register is not really changed but the corresponding
+IEC958 status bits will be updated.
+
+Clock Adjustment
+----------------
+
+In some chips, the clock of the codec isn't 48000 but using a PCI clock
+(to save a quartz!). In this case, change the field ``bus->clock`` to
+the corresponding value. For example, intel8x0 and es1968 drivers have
+their own function to read from the clock.
+
+Proc Files
+----------
+
+The ALSA AC97 interface will create a proc file such as
+``/proc/asound/card0/codec97#0/ac97#0-0`` and ``ac97#0-0+regs``. You
+can refer to these files to see the current status and registers of
+the codec.
+
+Multiple Codecs
+---------------
+
+When there are several codecs on the same card, you need to call
+:c:func:`snd_ac97_mixer()` multiple times with ``ac97.num=1`` or
+greater. The ``num`` field specifies the codec number.
+
+If you set up multiple codecs, you either need to write different
+callbacks for each codec or check ``ac97->num`` in the callback
+routines.
+
+MIDI (MPU401-UART) Interface
+============================
+
+General
+-------
+
+Many soundcards have built-in MIDI (MPU401-UART) interfaces. When the
+soundcard supports the standard MPU401-UART interface, most likely you
+can use the ALSA MPU401-UART API. The MPU401-UART API is defined in
+``<sound/mpu401.h>``.
+
+Some soundchips have a similar but slightly different implementation of
+mpu401 stuff. For example, emu10k1 has its own mpu401 routines.
+
+MIDI Constructor
+----------------
+
+To create a rawmidi object, call :c:func:`snd_mpu401_uart_new()`.
+
+::
+
+ struct snd_rawmidi *rmidi;
+ snd_mpu401_uart_new(card, 0, MPU401_HW_MPU401, port, info_flags,
+ irq, &rmidi);
+
+
+The first argument is the card pointer, and the second is the index of
+this component. You can create up to 8 rawmidi devices.
+
+The third argument is the type of the hardware, ``MPU401_HW_XXX``. If
+it's not a special one, you can use ``MPU401_HW_MPU401``.
+
+The 4th argument is the I/O port address. Many backward-compatible
+MPU401 have an I/O port such as 0x330. Or, it might be a part of its own
+PCI I/O region. It depends on the chip design.
+
+The 5th argument is a bitflag for additional information. When the I/O
+port address above is part of the PCI I/O region, the MPU401 I/O port
+might have been already allocated (reserved) by the driver itself. In
+such a case, pass a bit flag ``MPU401_INFO_INTEGRATED``, and the
+mpu401-uart layer will allocate the I/O ports by itself.
+
+When the controller supports only the input or output MIDI stream, pass
+the ``MPU401_INFO_INPUT`` or ``MPU401_INFO_OUTPUT`` bitflag,
+respectively. Then the rawmidi instance is created as a single stream.
+
+``MPU401_INFO_MMIO`` bitflag is used to change the access method to MMIO
+(via readb and writeb) instead of iob and outb. In this case, you have
+to pass the iomapped address to :c:func:`snd_mpu401_uart_new()`.
+
+When ``MPU401_INFO_TX_IRQ`` is set, the output stream isn't checked in
+the default interrupt handler. The driver needs to call
+:c:func:`snd_mpu401_uart_interrupt_tx()` by itself to start
+processing the output stream in the irq handler.
+
+If the MPU-401 interface shares its interrupt with the other logical
+devices on the card, set ``MPU401_INFO_IRQ_HOOK`` (see
+`below <#MIDI-Interrupt-Handler>`__).
+
+Usually, the port address corresponds to the command port and port + 1
+corresponds to the data port. If not, you may change the ``cport``
+field of :c:type:`struct snd_mpu401 <snd_mpu401>` manually afterward.
+However, :c:type:`struct snd_mpu401 <snd_mpu401>` pointer is
+not returned explicitly by :c:func:`snd_mpu401_uart_new()`. You
+need to cast ``rmidi->private_data`` to :c:type:`struct snd_mpu401
+<snd_mpu401>` explicitly,
+
+::
+
+ struct snd_mpu401 *mpu;
+ mpu = rmidi->private_data;
+
+and reset the ``cport`` as you like:
+
+::
+
+ mpu->cport = my_own_control_port;
+
+The 6th argument specifies the ISA irq number that will be allocated. If
+no interrupt is to be allocated (because your code is already allocating
+a shared interrupt, or because the device does not use interrupts), pass
+-1 instead. For a MPU-401 device without an interrupt, a polling timer
+will be used instead.
+
+MIDI Interrupt Handler
+----------------------
+
+When the interrupt is allocated in
+:c:func:`snd_mpu401_uart_new()`, an exclusive ISA interrupt
+handler is automatically used, hence you don't have anything else to do
+than creating the mpu401 stuff. Otherwise, you have to set
+``MPU401_INFO_IRQ_HOOK``, and call
+:c:func:`snd_mpu401_uart_interrupt()` explicitly from your own
+interrupt handler when it has determined that a UART interrupt has
+occurred.
+
+In this case, you need to pass the private_data of the returned rawmidi
+object from :c:func:`snd_mpu401_uart_new()` as the second
+argument of :c:func:`snd_mpu401_uart_interrupt()`.
+
+::
+
+ snd_mpu401_uart_interrupt(irq, rmidi->private_data, regs);
+
+
+RawMIDI Interface
+=================
+
+Overview
+--------
+
+The raw MIDI interface is used for hardware MIDI ports that can be
+accessed as a byte stream. It is not used for synthesizer chips that do
+not directly understand MIDI.
+
+ALSA handles file and buffer management. All you have to do is to write
+some code to move data between the buffer and the hardware.
+
+The rawmidi API is defined in ``<sound/rawmidi.h>``.
+
+RawMIDI Constructor
+-------------------
+
+To create a rawmidi device, call the :c:func:`snd_rawmidi_new()`
+function:
+
+::
+
+ struct snd_rawmidi *rmidi;
+ err = snd_rawmidi_new(chip->card, "MyMIDI", 0, outs, ins, &rmidi);
+ if (err < 0)
+ return err;
+ rmidi->private_data = chip;
+ strcpy(rmidi->name, "My MIDI");
+ rmidi->info_flags = SNDRV_RAWMIDI_INFO_OUTPUT |
+ SNDRV_RAWMIDI_INFO_INPUT |
+ SNDRV_RAWMIDI_INFO_DUPLEX;
+
+The first argument is the card pointer, the second argument is the ID
+string.
+
+The third argument is the index of this component. You can create up to
+8 rawmidi devices.
+
+The fourth and fifth arguments are the number of output and input
+substreams, respectively, of this device (a substream is the equivalent
+of a MIDI port).
+
+Set the ``info_flags`` field to specify the capabilities of the
+device. Set ``SNDRV_RAWMIDI_INFO_OUTPUT`` if there is at least one
+output port, ``SNDRV_RAWMIDI_INFO_INPUT`` if there is at least one
+input port, and ``SNDRV_RAWMIDI_INFO_DUPLEX`` if the device can handle
+output and input at the same time.
+
+After the rawmidi device is created, you need to set the operators
+(callbacks) for each substream. There are helper functions to set the
+operators for all the substreams of a device:
+
+::
+
+ snd_rawmidi_set_ops(rmidi, SNDRV_RAWMIDI_STREAM_OUTPUT, &snd_mymidi_output_ops);
+ snd_rawmidi_set_ops(rmidi, SNDRV_RAWMIDI_STREAM_INPUT, &snd_mymidi_input_ops);
+
+The operators are usually defined like this:
+
+::
+
+ static struct snd_rawmidi_ops snd_mymidi_output_ops = {
+ .open = snd_mymidi_output_open,
+ .close = snd_mymidi_output_close,
+ .trigger = snd_mymidi_output_trigger,
+ };
+
+These callbacks are explained in the `RawMIDI Callbacks`_ section.
+
+If there are more than one substream, you should give a unique name to
+each of them:
+
+::
+
+ struct snd_rawmidi_substream *substream;
+ list_for_each_entry(substream,
+ &rmidi->streams[SNDRV_RAWMIDI_STREAM_OUTPUT].substreams,
+ list {
+ sprintf(substream->name, "My MIDI Port %d", substream->number + 1);
+ }
+ /* same for SNDRV_RAWMIDI_STREAM_INPUT */
+
+RawMIDI Callbacks
+-----------------
+
+In all the callbacks, the private data that you've set for the rawmidi
+device can be accessed as ``substream->rmidi->private_data``.
+
+If there is more than one port, your callbacks can determine the port
+index from the struct snd_rawmidi_substream data passed to each
+callback:
+
+::
+
+ struct snd_rawmidi_substream *substream;
+ int index = substream->number;
+
+RawMIDI open callback
+~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+ static int snd_xxx_open(struct snd_rawmidi_substream *substream);
+
+
+This is called when a substream is opened. You can initialize the
+hardware here, but you shouldn't start transmitting/receiving data yet.
+
+RawMIDI close callback
+~~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+ static int snd_xxx_close(struct snd_rawmidi_substream *substream);
+
+Guess what.
+
+The ``open`` and ``close`` callbacks of a rawmidi device are
+serialized with a mutex, and can sleep.
+
+Rawmidi trigger callback for output substreams
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+ static void snd_xxx_output_trigger(struct snd_rawmidi_substream *substream, int up);
+
+
+This is called with a nonzero ``up`` parameter when there is some data
+in the substream buffer that must be transmitted.
+
+To read data from the buffer, call
+:c:func:`snd_rawmidi_transmit_peek()`. It will return the number
+of bytes that have been read; this will be less than the number of bytes
+requested when there are no more data in the buffer. After the data have
+been transmitted successfully, call
+:c:func:`snd_rawmidi_transmit_ack()` to remove the data from the
+substream buffer:
+
+::
+
+ unsigned char data;
+ while (snd_rawmidi_transmit_peek(substream, &data, 1) == 1) {
+ if (snd_mychip_try_to_transmit(data))
+ snd_rawmidi_transmit_ack(substream, 1);
+ else
+ break; /* hardware FIFO full */
+ }
+
+If you know beforehand that the hardware will accept data, you can use
+the :c:func:`snd_rawmidi_transmit()` function which reads some
+data and removes them from the buffer at once:
+
+::
+
+ while (snd_mychip_transmit_possible()) {
+ unsigned char data;
+ if (snd_rawmidi_transmit(substream, &data, 1) != 1)
+ break; /* no more data */
+ snd_mychip_transmit(data);
+ }
+
+If you know beforehand how many bytes you can accept, you can use a
+buffer size greater than one with the
+:c:func:`snd_rawmidi_transmit\*()` functions.
+
+The ``trigger`` callback must not sleep. If the hardware FIFO is full
+before the substream buffer has been emptied, you have to continue
+transmitting data later, either in an interrupt handler, or with a
+timer if the hardware doesn't have a MIDI transmit interrupt.
+
+The ``trigger`` callback is called with a zero ``up`` parameter when
+the transmission of data should be aborted.
+
+RawMIDI trigger callback for input substreams
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+ static void snd_xxx_input_trigger(struct snd_rawmidi_substream *substream, int up);
+
+
+This is called with a nonzero ``up`` parameter to enable receiving data,
+or with a zero ``up`` parameter do disable receiving data.
+
+The ``trigger`` callback must not sleep; the actual reading of data
+from the device is usually done in an interrupt handler.
+
+When data reception is enabled, your interrupt handler should call
+:c:func:`snd_rawmidi_receive()` for all received data:
+
+::
+
+ void snd_mychip_midi_interrupt(...)
+ {
+ while (mychip_midi_available()) {
+ unsigned char data;
+ data = mychip_midi_read();
+ snd_rawmidi_receive(substream, &data, 1);
+ }
+ }
+
+
+drain callback
+~~~~~~~~~~~~~~
+
+::
+
+ static void snd_xxx_drain(struct snd_rawmidi_substream *substream);
+
+
+This is only used with output substreams. This function should wait
+until all data read from the substream buffer have been transmitted.
+This ensures that the device can be closed and the driver unloaded
+without losing data.
+
+This callback is optional. If you do not set ``drain`` in the struct
+snd_rawmidi_ops structure, ALSA will simply wait for 50 milliseconds
+instead.
+
+Miscellaneous Devices
+=====================
+
+FM OPL3
+-------
+
+The FM OPL3 is still used in many chips (mainly for backward
+compatibility). ALSA has a nice OPL3 FM control layer, too. The OPL3 API
+is defined in ``<sound/opl3.h>``.
+
+FM registers can be directly accessed through the direct-FM API, defined
+in ``<sound/asound_fm.h>``. In ALSA native mode, FM registers are
+accessed through the Hardware-Dependent Device direct-FM extension API,
+whereas in OSS compatible mode, FM registers can be accessed with the
+OSS direct-FM compatible API in ``/dev/dmfmX`` device.
+
+To create the OPL3 component, you have two functions to call. The first
+one is a constructor for the ``opl3_t`` instance.
+
+::
+
+ struct snd_opl3 *opl3;
+ snd_opl3_create(card, lport, rport, OPL3_HW_OPL3_XXX,
+ integrated, &opl3);
+
+The first argument is the card pointer, the second one is the left port
+address, and the third is the right port address. In most cases, the
+right port is placed at the left port + 2.
+
+The fourth argument is the hardware type.
+
+When the left and right ports have been already allocated by the card
+driver, pass non-zero to the fifth argument (``integrated``). Otherwise,
+the opl3 module will allocate the specified ports by itself.
+
+When the accessing the hardware requires special method instead of the
+standard I/O access, you can create opl3 instance separately with
+:c:func:`snd_opl3_new()`.
+
+::
+
+ struct snd_opl3 *opl3;
+ snd_opl3_new(card, OPL3_HW_OPL3_XXX, &opl3);
+
+Then set ``command``, ``private_data`` and ``private_free`` for the
+private access function, the private data and the destructor. The
+``l_port`` and ``r_port`` are not necessarily set. Only the command
+must be set properly. You can retrieve the data from the
+``opl3->private_data`` field.
+
+After creating the opl3 instance via :c:func:`snd_opl3_new()`,
+call :c:func:`snd_opl3_init()` to initialize the chip to the
+proper state. Note that :c:func:`snd_opl3_create()` always calls
+it internally.
+
+If the opl3 instance is created successfully, then create a hwdep device
+for this opl3.
+
+::
+
+ struct snd_hwdep *opl3hwdep;
+ snd_opl3_hwdep_new(opl3, 0, 1, &opl3hwdep);
+
+The first argument is the ``opl3_t`` instance you created, and the
+second is the index number, usually 0.
+
+The third argument is the index-offset for the sequencer client assigned
+to the OPL3 port. When there is an MPU401-UART, give 1 for here (UART
+always takes 0).
+
+Hardware-Dependent Devices
+--------------------------
+
+Some chips need user-space access for special controls or for loading
+the micro code. In such a case, you can create a hwdep
+(hardware-dependent) device. The hwdep API is defined in
+``<sound/hwdep.h>``. You can find examples in opl3 driver or
+``isa/sb/sb16_csp.c``.
+
+The creation of the ``hwdep`` instance is done via
+:c:func:`snd_hwdep_new()`.
+
+::
+
+ struct snd_hwdep *hw;
+ snd_hwdep_new(card, "My HWDEP", 0, &hw);
+
+where the third argument is the index number.
+
+You can then pass any pointer value to the ``private_data``. If you
+assign a private data, you should define the destructor, too. The
+destructor function is set in the ``private_free`` field.
+
+::
+
+ struct mydata *p = kmalloc(sizeof(*p), GFP_KERNEL);
+ hw->private_data = p;
+ hw->private_free = mydata_free;
+
+and the implementation of the destructor would be:
+
+::
+
+ static void mydata_free(struct snd_hwdep *hw)
+ {
+ struct mydata *p = hw->private_data;
+ kfree(p);
+ }
+
+The arbitrary file operations can be defined for this instance. The file
+operators are defined in the ``ops`` table. For example, assume that
+this chip needs an ioctl.
+
+::
+
+ hw->ops.open = mydata_open;
+ hw->ops.ioctl = mydata_ioctl;
+ hw->ops.release = mydata_release;
+
+And implement the callback functions as you like.
+
+IEC958 (S/PDIF)
+---------------
+
+Usually the controls for IEC958 devices are implemented via the control
+interface. There is a macro to compose a name string for IEC958
+controls, :c:func:`SNDRV_CTL_NAME_IEC958()` defined in
+``<include/asound.h>``.
+
+There are some standard controls for IEC958 status bits. These controls
+use the type ``SNDRV_CTL_ELEM_TYPE_IEC958``, and the size of element is
+fixed as 4 bytes array (value.iec958.status[x]). For the ``info``
+callback, you don't specify the value field for this type (the count
+field must be set, though).
+
+“IEC958 Playback Con Mask” is used to return the bit-mask for the IEC958
+status bits of consumer mode. Similarly, “IEC958 Playback Pro Mask”
+returns the bitmask for professional mode. They are read-only controls,
+and are defined as MIXER controls (iface =
+``SNDRV_CTL_ELEM_IFACE_MIXER``).
+
+Meanwhile, “IEC958 Playback Default” control is defined for getting and
+setting the current default IEC958 bits. Note that this one is usually
+defined as a PCM control (iface = ``SNDRV_CTL_ELEM_IFACE_PCM``),
+although in some places it's defined as a MIXER control.
+
+In addition, you can define the control switches to enable/disable or to
+set the raw bit mode. The implementation will depend on the chip, but
+the control should be named as “IEC958 xxx”, preferably using the
+:c:func:`SNDRV_CTL_NAME_IEC958()` macro.
+
+You can find several cases, for example, ``pci/emu10k1``,
+``pci/ice1712``, or ``pci/cmipci.c``.
+
+Buffer and Memory Management
+============================
+
+Buffer Types
+------------
+
+ALSA provides several different buffer allocation functions depending on
+the bus and the architecture. All these have a consistent API. The
+allocation of physically-contiguous pages is done via
+:c:func:`snd_malloc_xxx_pages()` function, where xxx is the bus
+type.
+
+The allocation of pages with fallback is
+:c:func:`snd_malloc_xxx_pages_fallback()`. This function tries
+to allocate the specified pages but if the pages are not available, it
+tries to reduce the page sizes until enough space is found.
+
+The release the pages, call :c:func:`snd_free_xxx_pages()`
+function.
+
+Usually, ALSA drivers try to allocate and reserve a large contiguous
+physical space at the time the module is loaded for the later use. This
+is called “pre-allocation”. As already written, you can call the
+following function at pcm instance construction time (in the case of PCI
+bus).
+
+::
+
+ snd_pcm_lib_preallocate_pages_for_all(pcm, SNDRV_DMA_TYPE_DEV,
+ snd_dma_pci_data(pci), size, max);
+
+where ``size`` is the byte size to be pre-allocated and the ``max`` is
+the maximum size to be changed via the ``prealloc`` proc file. The
+allocator will try to get an area as large as possible within the
+given size.
+
+The second argument (type) and the third argument (device pointer) are
+dependent on the bus. In the case of the ISA bus, pass
+:c:func:`snd_dma_isa_data()` as the third argument with
+``SNDRV_DMA_TYPE_DEV`` type. For the continuous buffer unrelated to the
+bus can be pre-allocated with ``SNDRV_DMA_TYPE_CONTINUOUS`` type and the
+``snd_dma_continuous_data(GFP_KERNEL)`` device pointer, where
+``GFP_KERNEL`` is the kernel allocation flag to use. For the PCI
+scatter-gather buffers, use ``SNDRV_DMA_TYPE_DEV_SG`` with
+``snd_dma_pci_data(pci)`` (see the `Non-Contiguous Buffers`_
+section).
+
+Once the buffer is pre-allocated, you can use the allocator in the
+``hw_params`` callback:
+
+::
+
+ snd_pcm_lib_malloc_pages(substream, size);
+
+Note that you have to pre-allocate to use this function.
+
+External Hardware Buffers
+-------------------------
+
+Some chips have their own hardware buffers and the DMA transfer from the
+host memory is not available. In such a case, you need to either 1)
+copy/set the audio data directly to the external hardware buffer, or 2)
+make an intermediate buffer and copy/set the data from it to the
+external hardware buffer in interrupts (or in tasklets, preferably).
+
+The first case works fine if the external hardware buffer is large
+enough. This method doesn't need any extra buffers and thus is more
+effective. You need to define the ``copy_user`` and ``copy_kernel``
+callbacks for the data transfer, in addition to ``fill_silence``
+callback for playback. However, there is a drawback: it cannot be
+mmapped. The examples are GUS's GF1 PCM or emu8000's wavetable PCM.
+
+The second case allows for mmap on the buffer, although you have to
+handle an interrupt or a tasklet to transfer the data from the
+intermediate buffer to the hardware buffer. You can find an example in
+the vxpocket driver.
+
+Another case is when the chip uses a PCI memory-map region for the
+buffer instead of the host memory. In this case, mmap is available only
+on certain architectures like the Intel one. In non-mmap mode, the data
+cannot be transferred as in the normal way. Thus you need to define the
+``copy_user``, ``copy_kernel`` and ``fill_silence`` callbacks as well,
+as in the cases above. The examples are found in ``rme32.c`` and
+``rme96.c``.
+
+The implementation of the ``copy_user``, ``copy_kernel`` and
+``silence`` callbacks depends upon whether the hardware supports
+interleaved or non-interleaved samples. The ``copy_user`` callback is
+defined like below, a bit differently depending whether the direction
+is playback or capture:
+
+::
+
+ static int playback_copy_user(struct snd_pcm_substream *substream,
+ int channel, unsigned long pos,
+ void __user *src, unsigned long count);
+ static int capture_copy_user(struct snd_pcm_substream *substream,
+ int channel, unsigned long pos,
+ void __user *dst, unsigned long count);
+
+In the case of interleaved samples, the second argument (``channel``) is
+not used. The third argument (``pos``) points the current position
+offset in bytes.
+
+The meaning of the fourth argument is different between playback and
+capture. For playback, it holds the source data pointer, and for
+capture, it's the destination data pointer.
+
+The last argument is the number of bytes to be copied.
+
+What you have to do in this callback is again different between playback
+and capture directions. In the playback case, you copy the given amount
+of data (``count``) at the specified pointer (``src``) to the specified
+offset (``pos``) on the hardware buffer. When coded like memcpy-like
+way, the copy would be like:
+
+::
+
+ my_memcpy_from_user(my_buffer + pos, src, count);
+
+For the capture direction, you copy the given amount of data (``count``)
+at the specified offset (``pos``) on the hardware buffer to the
+specified pointer (``dst``).
+
+::
+
+ my_memcpy_to_user(dst, my_buffer + pos, count);
+
+Here the functions are named as ``from_user`` and ``to_user`` because
+it's the user-space buffer that is passed to these callbacks. That
+is, the callback is supposed to copy from/to the user-space data
+directly to/from the hardware buffer.
+
+Careful readers might notice that these callbacks receive the
+arguments in bytes, not in frames like other callbacks. It's because
+it would make coding easier like the examples above, and also it makes
+easier to unify both the interleaved and non-interleaved cases, as
+explained in the following.
+
+In the case of non-interleaved samples, the implementation will be a bit
+more complicated. The callback is called for each channel, passed by
+the second argument, so totally it's called for N-channels times per
+transfer.
+
+The meaning of other arguments are almost same as the interleaved
+case. The callback is supposed to copy the data from/to the given
+user-space buffer, but only for the given channel. For the detailed
+implementations, please check ``isa/gus/gus_pcm.c`` or
+"pci/rme9652/rme9652.c" as examples.
+
+The above callbacks are the copy from/to the user-space buffer. There
+are some cases where we want copy from/to the kernel-space buffer
+instead. In such a case, ``copy_kernel`` callback is called. It'd
+look like:
+
+::
+
+ static int playback_copy_kernel(struct snd_pcm_substream *substream,
+ int channel, unsigned long pos,
+ void *src, unsigned long count);
+ static int capture_copy_kernel(struct snd_pcm_substream *substream,
+ int channel, unsigned long pos,
+ void *dst, unsigned long count);
+
+As found easily, the only difference is that the buffer pointer is
+without ``__user`` prefix; that is, a kernel-buffer pointer is passed
+in the fourth argument. Correspondingly, the implementation would be
+a version without the user-copy, such as:
+
+::
+
+ my_memcpy(my_buffer + pos, src, count);
+
+Usually for the playback, another callback ``fill_silence`` is
+defined. It's implemented in a similar way as the copy callbacks
+above:
+
+::
+
+ static int silence(struct snd_pcm_substream *substream, int channel,
+ unsigned long pos, unsigned long count);
+
+The meanings of arguments are the same as in the ``copy_user`` and
+``copy_kernel`` callbacks, although there is no buffer pointer
+argument. In the case of interleaved samples, the channel argument has
+no meaning, as well as on ``copy_*`` callbacks.
+
+The role of ``fill_silence`` callback is to set the given amount
+(``count``) of silence data at the specified offset (``pos``) on the
+hardware buffer. Suppose that the data format is signed (that is, the
+silent-data is 0), and the implementation using a memset-like function
+would be like:
+
+::
+
+ my_memset(my_buffer + pos, 0, count);
+
+In the case of non-interleaved samples, again, the implementation
+becomes a bit more complicated, as it's called N-times per transfer
+for each channel. See, for example, ``isa/gus/gus_pcm.c``.
+
+Non-Contiguous Buffers
+----------------------
+
+If your hardware supports the page table as in emu10k1 or the buffer
+descriptors as in via82xx, you can use the scatter-gather (SG) DMA. ALSA
+provides an interface for handling SG-buffers. The API is provided in
+``<sound/pcm.h>``.
+
+For creating the SG-buffer handler, call
+:c:func:`snd_pcm_lib_preallocate_pages()` or
+:c:func:`snd_pcm_lib_preallocate_pages_for_all()` with
+``SNDRV_DMA_TYPE_DEV_SG`` in the PCM constructor like other PCI
+pre-allocator. You need to pass ``snd_dma_pci_data(pci)``, where pci is
+the :c:type:`struct pci_dev <pci_dev>` pointer of the chip as
+well. The ``struct snd_sg_buf`` instance is created as
+``substream->dma_private``. You can cast the pointer like:
+
+::
+
+ struct snd_sg_buf *sgbuf = (struct snd_sg_buf *)substream->dma_private;
+
+Then call :c:func:`snd_pcm_lib_malloc_pages()` in the ``hw_params``
+callback as well as in the case of normal PCI buffer. The SG-buffer
+handler will allocate the non-contiguous kernel pages of the given size
+and map them onto the virtually contiguous memory. The virtual pointer
+is addressed in runtime->dma_area. The physical address
+(``runtime->dma_addr``) is set to zero, because the buffer is
+physically non-contiguous. The physical address table is set up in
+``sgbuf->table``. You can get the physical address at a certain offset
+via :c:func:`snd_pcm_sgbuf_get_addr()`.
+
+When a SG-handler is used, you need to set
+:c:func:`snd_pcm_sgbuf_ops_page()` as the ``page`` callback. (See
+`page callback`_ section.)
+
+To release the data, call :c:func:`snd_pcm_lib_free_pages()` in
+the ``hw_free`` callback as usual.
+
+Vmalloc'ed Buffers
+------------------
+
+It's possible to use a buffer allocated via :c:func:`vmalloc()`, for
+example, for an intermediate buffer. Since the allocated pages are not
+contiguous, you need to set the ``page`` callback to obtain the physical
+address at every offset.
+
+The implementation of ``page`` callback would be like this:
+
+::
+
+ #include <linux/vmalloc.h>
+
+ /* get the physical page pointer on the given offset */
+ static struct page *mychip_page(struct snd_pcm_substream *substream,
+ unsigned long offset)
+ {
+ void *pageptr = substream->runtime->dma_area + offset;
+ return vmalloc_to_page(pageptr);
+ }
+
+Proc Interface
+==============
+
+ALSA provides an easy interface for procfs. The proc files are very
+useful for debugging. I recommend you set up proc files if you write a
+driver and want to get a running status or register dumps. The API is
+found in ``<sound/info.h>``.
+
+To create a proc file, call :c:func:`snd_card_proc_new()`.
+
+::
+
+ struct snd_info_entry *entry;
+ int err = snd_card_proc_new(card, "my-file", &entry);
+
+where the second argument specifies the name of the proc file to be
+created. The above example will create a file ``my-file`` under the
+card directory, e.g. ``/proc/asound/card0/my-file``.
+
+Like other components, the proc entry created via
+:c:func:`snd_card_proc_new()` will be registered and released
+automatically in the card registration and release functions.
+
+When the creation is successful, the function stores a new instance in
+the pointer given in the third argument. It is initialized as a text
+proc file for read only. To use this proc file as a read-only text file
+as it is, set the read callback with a private data via
+:c:func:`snd_info_set_text_ops()`.
+
+::
+
+ snd_info_set_text_ops(entry, chip, my_proc_read);
+
+where the second argument (``chip``) is the private data to be used in
+the callbacks. The third parameter specifies the read buffer size and
+the fourth (``my_proc_read``) is the callback function, which is
+defined like
+
+::
+
+ static void my_proc_read(struct snd_info_entry *entry,
+ struct snd_info_buffer *buffer);
+
+In the read callback, use :c:func:`snd_iprintf()` for output
+strings, which works just like normal :c:func:`printf()`. For
+example,
+
+::
+
+ static void my_proc_read(struct snd_info_entry *entry,
+ struct snd_info_buffer *buffer)
+ {
+ struct my_chip *chip = entry->private_data;
+
+ snd_iprintf(buffer, "This is my chip!\n");
+ snd_iprintf(buffer, "Port = %ld\n", chip->port);
+ }
+
+The file permissions can be changed afterwards. As default, it's set as
+read only for all users. If you want to add write permission for the
+user (root as default), do as follows:
+
+::
+
+ entry->mode = S_IFREG | S_IRUGO | S_IWUSR;
+
+and set the write buffer size and the callback
+
+::
+
+ entry->c.text.write = my_proc_write;
+
+For the write callback, you can use :c:func:`snd_info_get_line()`
+to get a text line, and :c:func:`snd_info_get_str()` to retrieve
+a string from the line. Some examples are found in
+``core/oss/mixer_oss.c``, core/oss/and ``pcm_oss.c``.
+
+For a raw-data proc-file, set the attributes as follows:
+
+::
+
+ static struct snd_info_entry_ops my_file_io_ops = {
+ .read = my_file_io_read,
+ };
+
+ entry->content = SNDRV_INFO_CONTENT_DATA;
+ entry->private_data = chip;
+ entry->c.ops = &my_file_io_ops;
+ entry->size = 4096;
+ entry->mode = S_IFREG | S_IRUGO;
+
+For the raw data, ``size`` field must be set properly. This specifies
+the maximum size of the proc file access.
+
+The read/write callbacks of raw mode are more direct than the text mode.
+You need to use a low-level I/O functions such as
+:c:func:`copy_from/to_user()` to transfer the data.
+
+::
+
+ static ssize_t my_file_io_read(struct snd_info_entry *entry,
+ void *file_private_data,
+ struct file *file,
+ char *buf,
+ size_t count,
+ loff_t pos)
+ {
+ if (copy_to_user(buf, local_data + pos, count))
+ return -EFAULT;
+ return count;
+ }
+
+If the size of the info entry has been set up properly, ``count`` and
+``pos`` are guaranteed to fit within 0 and the given size. You don't
+have to check the range in the callbacks unless any other condition is
+required.
+
+Power Management
+================
+
+If the chip is supposed to work with suspend/resume functions, you need
+to add power-management code to the driver. The additional code for
+power-management should be ifdef-ed with ``CONFIG_PM``.
+
+If the driver *fully* supports suspend/resume that is, the device can be
+properly resumed to its state when suspend was called, you can set the
+``SNDRV_PCM_INFO_RESUME`` flag in the pcm info field. Usually, this is
+possible when the registers of the chip can be safely saved and restored
+to RAM. If this is set, the trigger callback is called with
+``SNDRV_PCM_TRIGGER_RESUME`` after the resume callback completes.
+
+Even if the driver doesn't support PM fully but partial suspend/resume
+is still possible, it's still worthy to implement suspend/resume
+callbacks. In such a case, applications would reset the status by
+calling :c:func:`snd_pcm_prepare()` and restart the stream
+appropriately. Hence, you can define suspend/resume callbacks below but
+don't set ``SNDRV_PCM_INFO_RESUME`` info flag to the PCM.
+
+Note that the trigger with SUSPEND can always be called when
+:c:func:`snd_pcm_suspend_all()` is called, regardless of the
+``SNDRV_PCM_INFO_RESUME`` flag. The ``RESUME`` flag affects only the
+behavior of :c:func:`snd_pcm_resume()`. (Thus, in theory,
+``SNDRV_PCM_TRIGGER_RESUME`` isn't needed to be handled in the trigger
+callback when no ``SNDRV_PCM_INFO_RESUME`` flag is set. But, it's better
+to keep it for compatibility reasons.)
+
+In the earlier version of ALSA drivers, a common power-management layer
+was provided, but it has been removed. The driver needs to define the
+suspend/resume hooks according to the bus the device is connected to. In
+the case of PCI drivers, the callbacks look like below:
+
+::
+
+ #ifdef CONFIG_PM
+ static int snd_my_suspend(struct pci_dev *pci, pm_message_t state)
+ {
+ .... /* do things for suspend */
+ return 0;
+ }
+ static int snd_my_resume(struct pci_dev *pci)
+ {
+ .... /* do things for suspend */
+ return 0;
+ }
+ #endif
+
+The scheme of the real suspend job is as follows.
+
+1. Retrieve the card and the chip data.
+
+2. Call :c:func:`snd_power_change_state()` with
+ ``SNDRV_CTL_POWER_D3hot`` to change the power status.
+
+3. Call :c:func:`snd_pcm_suspend_all()` to suspend the running
+ PCM streams.
+
+4. If AC97 codecs are used, call :c:func:`snd_ac97_suspend()` for
+ each codec.
+
+5. Save the register values if necessary.
+
+6. Stop the hardware if necessary.
+
+7. Disable the PCI device by calling
+ :c:func:`pci_disable_device()`. Then, call
+ :c:func:`pci_save_state()` at last.
+
+A typical code would be like:
+
+::
+
+ static int mychip_suspend(struct pci_dev *pci, pm_message_t state)
+ {
+ /* (1) */
+ struct snd_card *card = pci_get_drvdata(pci);
+ struct mychip *chip = card->private_data;
+ /* (2) */
+ snd_power_change_state(card, SNDRV_CTL_POWER_D3hot);
+ /* (3) */
+ snd_pcm_suspend_all(chip->pcm);
+ /* (4) */
+ snd_ac97_suspend(chip->ac97);
+ /* (5) */
+ snd_mychip_save_registers(chip);
+ /* (6) */
+ snd_mychip_stop_hardware(chip);
+ /* (7) */
+ pci_disable_device(pci);
+ pci_save_state(pci);
+ return 0;
+ }
+
+
+The scheme of the real resume job is as follows.
+
+1. Retrieve the card and the chip data.
+
+2. Set up PCI. First, call :c:func:`pci_restore_state()`. Then
+ enable the pci device again by calling
+ :c:func:`pci_enable_device()`. Call
+ :c:func:`pci_set_master()` if necessary, too.
+
+3. Re-initialize the chip.
+
+4. Restore the saved registers if necessary.
+
+5. Resume the mixer, e.g. calling :c:func:`snd_ac97_resume()`.
+
+6. Restart the hardware (if any).
+
+7. Call :c:func:`snd_power_change_state()` with
+ ``SNDRV_CTL_POWER_D0`` to notify the processes.
+
+A typical code would be like:
+
+::
+
+ static int mychip_resume(struct pci_dev *pci)
+ {
+ /* (1) */
+ struct snd_card *card = pci_get_drvdata(pci);
+ struct mychip *chip = card->private_data;
+ /* (2) */
+ pci_restore_state(pci);
+ pci_enable_device(pci);
+ pci_set_master(pci);
+ /* (3) */
+ snd_mychip_reinit_chip(chip);
+ /* (4) */
+ snd_mychip_restore_registers(chip);
+ /* (5) */
+ snd_ac97_resume(chip->ac97);
+ /* (6) */
+ snd_mychip_restart_chip(chip);
+ /* (7) */
+ snd_power_change_state(card, SNDRV_CTL_POWER_D0);
+ return 0;
+ }
+
+As shown in the above, it's better to save registers after suspending
+the PCM operations via :c:func:`snd_pcm_suspend_all()` or
+:c:func:`snd_pcm_suspend()`. It means that the PCM streams are
+already stopped when the register snapshot is taken. But, remember that
+you don't have to restart the PCM stream in the resume callback. It'll
+be restarted via trigger call with ``SNDRV_PCM_TRIGGER_RESUME`` when
+necessary.
+
+OK, we have all callbacks now. Let's set them up. In the initialization
+of the card, make sure that you can get the chip data from the card
+instance, typically via ``private_data`` field, in case you created the
+chip data individually.
+
+::
+
+ static int snd_mychip_probe(struct pci_dev *pci,
+ const struct pci_device_id *pci_id)
+ {
+ ....
+ struct snd_card *card;
+ struct mychip *chip;
+ int err;
+ ....
+ err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
+ 0, &card);
+ ....
+ chip = kzalloc(sizeof(*chip), GFP_KERNEL);
+ ....
+ card->private_data = chip;
+ ....
+ }
+
+When you created the chip data with :c:func:`snd_card_new()`, it's
+anyway accessible via ``private_data`` field.
+
+::
+
+ static int snd_mychip_probe(struct pci_dev *pci,
+ const struct pci_device_id *pci_id)
+ {
+ ....
+ struct snd_card *card;
+ struct mychip *chip;
+ int err;
+ ....
+ err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
+ sizeof(struct mychip), &card);
+ ....
+ chip = card->private_data;
+ ....
+ }
+
+If you need a space to save the registers, allocate the buffer for it
+here, too, since it would be fatal if you cannot allocate a memory in
+the suspend phase. The allocated buffer should be released in the
+corresponding destructor.
+
+And next, set suspend/resume callbacks to the pci_driver.
+
+::
+
+ static struct pci_driver driver = {
+ .name = KBUILD_MODNAME,
+ .id_table = snd_my_ids,
+ .probe = snd_my_probe,
+ .remove = snd_my_remove,
+ #ifdef CONFIG_PM
+ .suspend = snd_my_suspend,
+ .resume = snd_my_resume,
+ #endif
+ };
+
+Module Parameters
+=================
+
+There are standard module options for ALSA. At least, each module should
+have the ``index``, ``id`` and ``enable`` options.
+
+If the module supports multiple cards (usually up to 8 = ``SNDRV_CARDS``
+cards), they should be arrays. The default initial values are defined
+already as constants for easier programming:
+
+::
+
+ static int index[SNDRV_CARDS] = SNDRV_DEFAULT_IDX;
+ static char *id[SNDRV_CARDS] = SNDRV_DEFAULT_STR;
+ static int enable[SNDRV_CARDS] = SNDRV_DEFAULT_ENABLE_PNP;
+
+If the module supports only a single card, they could be single
+variables, instead. ``enable`` option is not always necessary in this
+case, but it would be better to have a dummy option for compatibility.
+
+The module parameters must be declared with the standard
+``module_param()()``, ``module_param_array()()`` and
+:c:func:`MODULE_PARM_DESC()` macros.
+
+The typical coding would be like below:
+
+::
+
+ #define CARD_NAME "My Chip"
+
+ module_param_array(index, int, NULL, 0444);
+ MODULE_PARM_DESC(index, "Index value for " CARD_NAME " soundcard.");
+ module_param_array(id, charp, NULL, 0444);
+ MODULE_PARM_DESC(id, "ID string for " CARD_NAME " soundcard.");
+ module_param_array(enable, bool, NULL, 0444);
+ MODULE_PARM_DESC(enable, "Enable " CARD_NAME " soundcard.");
+
+Also, don't forget to define the module description, classes, license
+and devices. Especially, the recent modprobe requires to define the
+module license as GPL, etc., otherwise the system is shown as “tainted”.
+
+::
+
+ MODULE_DESCRIPTION("My Chip");
+ MODULE_LICENSE("GPL");
+ MODULE_SUPPORTED_DEVICE("{{Vendor,My Chip Name}}");
+
+
+How To Put Your Driver Into ALSA Tree
+=====================================
+
+General
+-------
+
+So far, you've learned how to write the driver codes. And you might have
+a question now: how to put my own driver into the ALSA driver tree? Here
+(finally :) the standard procedure is described briefly.
+
+Suppose that you create a new PCI driver for the card “xyz”. The card
+module name would be snd-xyz. The new driver is usually put into the
+alsa-driver tree, ``alsa-driver/pci`` directory in the case of PCI
+cards. Then the driver is evaluated, audited and tested by developers
+and users. After a certain time, the driver will go to the alsa-kernel
+tree (to the corresponding directory, such as ``alsa-kernel/pci``) and
+eventually will be integrated into the Linux 2.6 tree (the directory
+would be ``linux/sound/pci``).
+
+In the following sections, the driver code is supposed to be put into
+alsa-driver tree. The two cases are covered: a driver consisting of a
+single source file and one consisting of several source files.
+
+Driver with A Single Source File
+--------------------------------
+
+1. Modify alsa-driver/pci/Makefile
+
+ Suppose you have a file xyz.c. Add the following two lines
+
+::
+
+ snd-xyz-objs := xyz.o
+ obj-$(CONFIG_SND_XYZ) += snd-xyz.o
+
+2. Create the Kconfig entry
+
+ Add the new entry of Kconfig for your xyz driver. config SND_XYZ
+ tristate "Foobar XYZ" depends on SND select SND_PCM help Say Y here
+ to include support for Foobar XYZ soundcard. To compile this driver
+ as a module, choose M here: the module will be called snd-xyz. the
+ line, select SND_PCM, specifies that the driver xyz supports PCM. In
+ addition to SND_PCM, the following components are supported for
+ select command: SND_RAWMIDI, SND_TIMER, SND_HWDEP,
+ SND_MPU401_UART, SND_OPL3_LIB, SND_OPL4_LIB, SND_VX_LIB,
+ SND_AC97_CODEC. Add the select command for each supported
+ component.
+
+ Note that some selections imply the lowlevel selections. For example,
+ PCM includes TIMER, MPU401_UART includes RAWMIDI, AC97_CODEC
+ includes PCM, and OPL3_LIB includes HWDEP. You don't need to give
+ the lowlevel selections again.
+
+ For the details of Kconfig script, refer to the kbuild documentation.
+
+3. Run cvscompile script to re-generate the configure script and build
+ the whole stuff again.
+
+Drivers with Several Source Files
+---------------------------------
+
+Suppose that the driver snd-xyz have several source files. They are
+located in the new subdirectory, pci/xyz.
+
+1. Add a new directory (``xyz``) in ``alsa-driver/pci/Makefile`` as
+ below
+
+::
+
+ obj-$(CONFIG_SND) += xyz/
+
+
+2. Under the directory ``xyz``, create a Makefile
+
+::
+
+ ifndef SND_TOPDIR
+ SND_TOPDIR=../..
+ endif
+
+ include $(SND_TOPDIR)/toplevel.config
+ include $(SND_TOPDIR)/Makefile.conf
+
+ snd-xyz-objs := xyz.o abc.o def.o
+
+ obj-$(CONFIG_SND_XYZ) += snd-xyz.o
+
+ include $(SND_TOPDIR)/Rules.make
+
+3. Create the Kconfig entry
+
+ This procedure is as same as in the last section.
+
+4. Run cvscompile script to re-generate the configure script and build
+ the whole stuff again.
+
+Useful Functions
+================
+
+:c:func:`snd_printk()` and friends
+---------------------------------------
+
+ALSA provides a verbose version of the :c:func:`printk()` function.
+If a kernel config ``CONFIG_SND_VERBOSE_PRINTK`` is set, this function
+prints the given message together with the file name and the line of the
+caller. The ``KERN_XXX`` prefix is processed as well as the original
+:c:func:`printk()` does, so it's recommended to add this prefix,
+e.g. snd_printk(KERN_ERR "Oh my, sorry, it's extremely bad!\\n");
+
+There are also :c:func:`printk()`'s for debugging.
+:c:func:`snd_printd()` can be used for general debugging purposes.
+If ``CONFIG_SND_DEBUG`` is set, this function is compiled, and works
+just like :c:func:`snd_printk()`. If the ALSA is compiled without
+the debugging flag, it's ignored.
+
+:c:func:`snd_printdd()` is compiled in only when
+``CONFIG_SND_DEBUG_VERBOSE`` is set. Please note that
+``CONFIG_SND_DEBUG_VERBOSE`` is not set as default even if you configure
+the alsa-driver with ``--with-debug=full`` option. You need to give
+explicitly ``--with-debug=detect`` option instead.
+
+:c:func:`snd_BUG()`
+------------------------
+
+It shows the ``BUG?`` message and stack trace as well as
+:c:func:`snd_BUG_ON()` at the point. It's useful to show that a
+fatal error happens there.
+
+When no debug flag is set, this macro is ignored.
+
+:c:func:`snd_BUG_ON()`
+----------------------------
+
+:c:func:`snd_BUG_ON()` macro is similar with
+:c:func:`WARN_ON()` macro. For example, snd_BUG_ON(!pointer); or
+it can be used as the condition, if (snd_BUG_ON(non_zero_is_bug))
+return -EINVAL;
+
+The macro takes an conditional expression to evaluate. When
+``CONFIG_SND_DEBUG``, is set, if the expression is non-zero, it shows
+the warning message such as ``BUG? (xxx)`` normally followed by stack
+trace. In both cases it returns the evaluated value.
+
+Acknowledgments
+===============
+
+I would like to thank Phil Kerr for his help for improvement and
+corrections of this document.
+
+Kevin Conder reformatted the original plain-text to the DocBook format.
+
+Giuliano Pochini corrected typos and contributed the example codes in
+the hardware constraints section.