v4.19.13 snapshot.
diff --git a/Documentation/spi/00-INDEX b/Documentation/spi/00-INDEX
new file mode 100644
index 0000000..8e4bb17
--- /dev/null
+++ b/Documentation/spi/00-INDEX
@@ -0,0 +1,16 @@
+00-INDEX
+ - this file.
+butterfly
+ - AVR Butterfly SPI driver overview and pin configuration.
+ep93xx_spi
+ - Basic EP93xx SPI driver configuration.
+pxa2xx
+ - PXA2xx SPI master controller build by spi_message fifo wq
+spidev
+ - Intro to the userspace API for spi devices
+spi-lm70llp
+ - Connecting an LM70-LLP sensor to the kernel via the SPI subsys.
+spi-sc18is602
+ - NXP SC18IS602/603 I2C-bus to SPI bridge
+spi-summary
+ - (Linux) SPI overview. If unsure about SPI or SPI in Linux, start here.
diff --git a/Documentation/spi/butterfly b/Documentation/spi/butterfly
new file mode 100644
index 0000000..9927af7
--- /dev/null
+++ b/Documentation/spi/butterfly
@@ -0,0 +1,68 @@
+spi_butterfly - parport-to-butterfly adapter driver
+===================================================
+
+This is a hardware and software project that includes building and using
+a parallel port adapter cable, together with an "AVR Butterfly" to run
+firmware for user interfacing and/or sensors. A Butterfly is a $US20
+battery powered card with an AVR microcontroller and lots of goodies:
+sensors, LCD, flash, toggle stick, and more. You can use AVR-GCC to
+develop firmware for this, and flash it using this adapter cable.
+
+You can make this adapter from an old printer cable and solder things
+directly to the Butterfly. Or (if you have the parts and skills) you
+can come up with something fancier, providing ciruit protection to the
+Butterfly and the printer port, or with a better power supply than two
+signal pins from the printer port. Or for that matter, you can use
+similar cables to talk to many AVR boards, even a breadboard.
+
+This is more powerful than "ISP programming" cables since it lets kernel
+SPI protocol drivers interact with the AVR, and could even let the AVR
+issue interrupts to them. Later, your protocol driver should work
+easily with a "real SPI controller", instead of this bitbanger.
+
+
+The first cable connections will hook Linux up to one SPI bus, with the
+AVR and a DataFlash chip; and to the AVR reset line. This is all you
+need to reflash the firmware, and the pins are the standard Atmel "ISP"
+connector pins (used also on non-Butterfly AVR boards). On the parport
+side this is like "sp12" programming cables.
+
+ Signal Butterfly Parport (DB-25)
+ ------ --------- ---------------
+ SCK = J403.PB1/SCK = pin 2/D0
+ RESET = J403.nRST = pin 3/D1
+ VCC = J403.VCC_EXT = pin 8/D6
+ MOSI = J403.PB2/MOSI = pin 9/D7
+ MISO = J403.PB3/MISO = pin 11/S7,nBUSY
+ GND = J403.GND = pin 23/GND
+
+Then to let Linux master that bus to talk to the DataFlash chip, you must
+(a) flash new firmware that disables SPI (set PRR.2, and disable pullups
+by clearing PORTB.[0-3]); (b) configure the mtd_dataflash driver; and
+(c) cable in the chipselect.
+
+ Signal Butterfly Parport (DB-25)
+ ------ --------- ---------------
+ VCC = J400.VCC_EXT = pin 7/D5
+ SELECT = J400.PB0/nSS = pin 17/C3,nSELECT
+ GND = J400.GND = pin 24/GND
+
+Or you could flash firmware making the AVR into an SPI slave (keeping the
+DataFlash in reset) and tweak the spi_butterfly driver to make it bind to
+the driver for your custom SPI-based protocol.
+
+The "USI" controller, using J405, can also be used for a second SPI bus.
+That would let you talk to the AVR using custom SPI-with-USI firmware,
+while letting either Linux or the AVR use the DataFlash. There are plenty
+of spare parport pins to wire this one up, such as:
+
+ Signal Butterfly Parport (DB-25)
+ ------ --------- ---------------
+ SCK = J403.PE4/USCK = pin 5/D3
+ MOSI = J403.PE5/DI = pin 6/D4
+ MISO = J403.PE6/DO = pin 12/S5,nPAPEROUT
+ GND = J403.GND = pin 22/GND
+
+ IRQ = J402.PF4 = pin 10/S6,ACK
+ GND = J402.GND(P2) = pin 25/GND
+
diff --git a/Documentation/spi/pxa2xx b/Documentation/spi/pxa2xx
new file mode 100644
index 0000000..13a0b7f
--- /dev/null
+++ b/Documentation/spi/pxa2xx
@@ -0,0 +1,235 @@
+PXA2xx SPI on SSP driver HOWTO
+===================================================
+This a mini howto on the pxa2xx_spi driver. The driver turns a PXA2xx
+synchronous serial port into a SPI master controller
+(see Documentation/spi/spi-summary). The driver has the following features
+
+- Support for any PXA2xx SSP
+- SSP PIO and SSP DMA data transfers.
+- External and Internal (SSPFRM) chip selects.
+- Per slave device (chip) configuration.
+- Full suspend, freeze, resume support.
+
+The driver is built around a "spi_message" fifo serviced by workqueue and a
+tasklet. The workqueue, "pump_messages", drives message fifo and the tasklet
+(pump_transfer) is responsible for queuing SPI transactions and setting up and
+launching the dma/interrupt driven transfers.
+
+Declaring PXA2xx Master Controllers
+-----------------------------------
+Typically a SPI master is defined in the arch/.../mach-*/board-*.c as a
+"platform device". The master configuration is passed to the driver via a table
+found in include/linux/spi/pxa2xx_spi.h:
+
+struct pxa2xx_spi_master {
+ u16 num_chipselect;
+ u8 enable_dma;
+};
+
+The "pxa2xx_spi_master.num_chipselect" field is used to determine the number of
+slave device (chips) attached to this SPI master.
+
+The "pxa2xx_spi_master.enable_dma" field informs the driver that SSP DMA should
+be used. This caused the driver to acquire two DMA channels: rx_channel and
+tx_channel. The rx_channel has a higher DMA service priority the tx_channel.
+See the "PXA2xx Developer Manual" section "DMA Controller".
+
+NSSP MASTER SAMPLE
+------------------
+Below is a sample configuration using the PXA255 NSSP.
+
+static struct resource pxa_spi_nssp_resources[] = {
+ [0] = {
+ .start = __PREG(SSCR0_P(2)), /* Start address of NSSP */
+ .end = __PREG(SSCR0_P(2)) + 0x2c, /* Range of registers */
+ .flags = IORESOURCE_MEM,
+ },
+ [1] = {
+ .start = IRQ_NSSP, /* NSSP IRQ */
+ .end = IRQ_NSSP,
+ .flags = IORESOURCE_IRQ,
+ },
+};
+
+static struct pxa2xx_spi_master pxa_nssp_master_info = {
+ .num_chipselect = 1, /* Matches the number of chips attached to NSSP */
+ .enable_dma = 1, /* Enables NSSP DMA */
+};
+
+static struct platform_device pxa_spi_nssp = {
+ .name = "pxa2xx-spi", /* MUST BE THIS VALUE, so device match driver */
+ .id = 2, /* Bus number, MUST MATCH SSP number 1..n */
+ .resource = pxa_spi_nssp_resources,
+ .num_resources = ARRAY_SIZE(pxa_spi_nssp_resources),
+ .dev = {
+ .platform_data = &pxa_nssp_master_info, /* Passed to driver */
+ },
+};
+
+static struct platform_device *devices[] __initdata = {
+ &pxa_spi_nssp,
+};
+
+static void __init board_init(void)
+{
+ (void)platform_add_device(devices, ARRAY_SIZE(devices));
+}
+
+Declaring Slave Devices
+-----------------------
+Typically each SPI slave (chip) is defined in the arch/.../mach-*/board-*.c
+using the "spi_board_info" structure found in "linux/spi/spi.h". See
+"Documentation/spi/spi-summary" for additional information.
+
+Each slave device attached to the PXA must provide slave specific configuration
+information via the structure "pxa2xx_spi_chip" found in
+"include/linux/spi/pxa2xx_spi.h". The pxa2xx_spi master controller driver
+will uses the configuration whenever the driver communicates with the slave
+device. All fields are optional.
+
+struct pxa2xx_spi_chip {
+ u8 tx_threshold;
+ u8 rx_threshold;
+ u8 dma_burst_size;
+ u32 timeout;
+ u8 enable_loopback;
+ void (*cs_control)(u32 command);
+};
+
+The "pxa2xx_spi_chip.tx_threshold" and "pxa2xx_spi_chip.rx_threshold" fields are
+used to configure the SSP hardware fifo. These fields are critical to the
+performance of pxa2xx_spi driver and misconfiguration will result in rx
+fifo overruns (especially in PIO mode transfers). Good default values are
+
+ .tx_threshold = 8,
+ .rx_threshold = 8,
+
+The range is 1 to 16 where zero indicates "use default".
+
+The "pxa2xx_spi_chip.dma_burst_size" field is used to configure PXA2xx DMA
+engine and is related the "spi_device.bits_per_word" field. Read and understand
+the PXA2xx "Developer Manual" sections on the DMA controller and SSP Controllers
+to determine the correct value. An SSP configured for byte-wide transfers would
+use a value of 8. The driver will determine a reasonable default if
+dma_burst_size == 0.
+
+The "pxa2xx_spi_chip.timeout" fields is used to efficiently handle
+trailing bytes in the SSP receiver fifo. The correct value for this field is
+dependent on the SPI bus speed ("spi_board_info.max_speed_hz") and the specific
+slave device. Please note that the PXA2xx SSP 1 does not support trailing byte
+timeouts and must busy-wait any trailing bytes.
+
+The "pxa2xx_spi_chip.enable_loopback" field is used to place the SSP porting
+into internal loopback mode. In this mode the SSP controller internally
+connects the SSPTX pin to the SSPRX pin. This is useful for initial setup
+testing.
+
+The "pxa2xx_spi_chip.cs_control" field is used to point to a board specific
+function for asserting/deasserting a slave device chip select. If the field is
+NULL, the pxa2xx_spi master controller driver assumes that the SSP port is
+configured to use SSPFRM instead.
+
+NOTE: the SPI driver cannot control the chip select if SSPFRM is used, so the
+chipselect is dropped after each spi_transfer. Most devices need chip select
+asserted around the complete message. Use SSPFRM as a GPIO (through cs_control)
+to accommodate these chips.
+
+
+NSSP SLAVE SAMPLE
+-----------------
+The pxa2xx_spi_chip structure is passed to the pxa2xx_spi driver in the
+"spi_board_info.controller_data" field. Below is a sample configuration using
+the PXA255 NSSP.
+
+/* Chip Select control for the CS8415A SPI slave device */
+static void cs8415a_cs_control(u32 command)
+{
+ if (command & PXA2XX_CS_ASSERT)
+ GPCR(2) = GPIO_bit(2);
+ else
+ GPSR(2) = GPIO_bit(2);
+}
+
+/* Chip Select control for the CS8405A SPI slave device */
+static void cs8405a_cs_control(u32 command)
+{
+ if (command & PXA2XX_CS_ASSERT)
+ GPCR(3) = GPIO_bit(3);
+ else
+ GPSR(3) = GPIO_bit(3);
+}
+
+static struct pxa2xx_spi_chip cs8415a_chip_info = {
+ .tx_threshold = 8, /* SSP hardward FIFO threshold */
+ .rx_threshold = 8, /* SSP hardward FIFO threshold */
+ .dma_burst_size = 8, /* Byte wide transfers used so 8 byte bursts */
+ .timeout = 235, /* See Intel documentation */
+ .cs_control = cs8415a_cs_control, /* Use external chip select */
+};
+
+static struct pxa2xx_spi_chip cs8405a_chip_info = {
+ .tx_threshold = 8, /* SSP hardward FIFO threshold */
+ .rx_threshold = 8, /* SSP hardward FIFO threshold */
+ .dma_burst_size = 8, /* Byte wide transfers used so 8 byte bursts */
+ .timeout = 235, /* See Intel documentation */
+ .cs_control = cs8405a_cs_control, /* Use external chip select */
+};
+
+static struct spi_board_info streetracer_spi_board_info[] __initdata = {
+ {
+ .modalias = "cs8415a", /* Name of spi_driver for this device */
+ .max_speed_hz = 3686400, /* Run SSP as fast a possbile */
+ .bus_num = 2, /* Framework bus number */
+ .chip_select = 0, /* Framework chip select */
+ .platform_data = NULL; /* No spi_driver specific config */
+ .controller_data = &cs8415a_chip_info, /* Master chip config */
+ .irq = STREETRACER_APCI_IRQ, /* Slave device interrupt */
+ },
+ {
+ .modalias = "cs8405a", /* Name of spi_driver for this device */
+ .max_speed_hz = 3686400, /* Run SSP as fast a possbile */
+ .bus_num = 2, /* Framework bus number */
+ .chip_select = 1, /* Framework chip select */
+ .controller_data = &cs8405a_chip_info, /* Master chip config */
+ .irq = STREETRACER_APCI_IRQ, /* Slave device interrupt */
+ },
+};
+
+static void __init streetracer_init(void)
+{
+ spi_register_board_info(streetracer_spi_board_info,
+ ARRAY_SIZE(streetracer_spi_board_info));
+}
+
+
+DMA and PIO I/O Support
+-----------------------
+The pxa2xx_spi driver supports both DMA and interrupt driven PIO message
+transfers. The driver defaults to PIO mode and DMA transfers must be enabled
+by setting the "enable_dma" flag in the "pxa2xx_spi_master" structure. The DMA
+mode supports both coherent and stream based DMA mappings.
+
+The following logic is used to determine the type of I/O to be used on
+a per "spi_transfer" basis:
+
+if !enable_dma then
+ always use PIO transfers
+
+if spi_message.len > 8191 then
+ print "rate limited" warning
+ use PIO transfers
+
+if spi_message.is_dma_mapped and rx_dma_buf != 0 and tx_dma_buf != 0 then
+ use coherent DMA mode
+
+if rx_buf and tx_buf are aligned on 8 byte boundary then
+ use streaming DMA mode
+
+otherwise
+ use PIO transfer
+
+THANKS TO
+---------
+
+David Brownell and others for mentoring the development of this driver.
+
diff --git a/Documentation/spi/spi-lm70llp b/Documentation/spi/spi-lm70llp
new file mode 100644
index 0000000..463f6d0
--- /dev/null
+++ b/Documentation/spi/spi-lm70llp
@@ -0,0 +1,79 @@
+spi_lm70llp : LM70-LLP parport-to-SPI adapter
+==============================================
+
+Supported board/chip:
+ * National Semiconductor LM70 LLP evaluation board
+ Datasheet: http://www.national.com/pf/LM/LM70.html
+
+Author:
+ Kaiwan N Billimoria <kaiwan@designergraphix.com>
+
+Description
+-----------
+This driver provides glue code connecting a National Semiconductor LM70 LLP
+temperature sensor evaluation board to the kernel's SPI core subsystem.
+
+This is a SPI master controller driver. It can be used in conjunction with
+(layered under) the LM70 logical driver (a "SPI protocol driver").
+In effect, this driver turns the parallel port interface on the eval board
+into a SPI bus with a single device, which will be driven by the generic
+LM70 driver (drivers/hwmon/lm70.c).
+
+
+Hardware Interfacing
+--------------------
+The schematic for this particular board (the LM70EVAL-LLP) is
+available (on page 4) here:
+
+ http://www.national.com/appinfo/tempsensors/files/LM70LLPEVALmanual.pdf
+
+The hardware interfacing on the LM70 LLP eval board is as follows:
+
+ Parallel LM70 LLP
+ Port Direction JP2 Header
+ ----------- --------- ----------------
+ D0 2 - -
+ D1 3 --> V+ 5
+ D2 4 --> V+ 5
+ D3 5 --> V+ 5
+ D4 6 --> V+ 5
+ D5 7 --> nCS 8
+ D6 8 --> SCLK 3
+ D7 9 --> SI/O 5
+ GND 25 - GND 7
+ Select 13 <-- SI/O 1
+ ----------- --------- ----------------
+
+Note that since the LM70 uses a "3-wire" variant of SPI, the SI/SO pin
+is connected to both pin D7 (as Master Out) and Select (as Master In)
+using an arrangement that lets either the parport or the LM70 pull the
+pin low. This can't be shared with true SPI devices, but other 3-wire
+devices might share the same SI/SO pin.
+
+The bitbanger routine in this driver (lm70_txrx) is called back from
+the bound "hwmon/lm70" protocol driver through its sysfs hook, using a
+spi_write_then_read() call. It performs Mode 0 (SPI/Microwire) bitbanging.
+The lm70 driver then inteprets the resulting digital temperature value
+and exports it through sysfs.
+
+A "gotcha": National Semiconductor's LM70 LLP eval board circuit schematic
+shows that the SI/O line from the LM70 chip is connected to the base of a
+transistor Q1 (and also a pullup, and a zener diode to D7); while the
+collector is tied to VCC.
+
+Interpreting this circuit, when the LM70 SI/O line is High (or tristate
+and not grounded by the host via D7), the transistor conducts and switches
+the collector to zero, which is reflected on pin 13 of the DB25 parport
+connector. When SI/O is Low (driven by the LM70 or the host) on the other
+hand, the transistor is cut off and the voltage tied to it's collector is
+reflected on pin 13 as a High level.
+
+So: the getmiso inline routine in this driver takes this fact into account,
+inverting the value read at pin 13.
+
+
+Thanks to
+---------
+o David Brownell for mentoring the SPI-side driver development.
+o Dr.Craig Hollabaugh for the (early) "manual" bitbanging driver version.
+o Nadir Billimoria for help interpreting the circuit schematic.
diff --git a/Documentation/spi/spi-sc18is602 b/Documentation/spi/spi-sc18is602
new file mode 100644
index 0000000..a457028
--- /dev/null
+++ b/Documentation/spi/spi-sc18is602
@@ -0,0 +1,36 @@
+Kernel driver spi-sc18is602
+===========================
+
+Supported chips:
+ * NXP SI18IS602/602B/603
+ Datasheet: http://www.nxp.com/documents/data_sheet/SC18IS602_602B_603.pdf
+
+Author:
+ Guenter Roeck <linux@roeck-us.net>
+
+
+Description
+-----------
+
+This driver provides connects a NXP SC18IS602/603 I2C-bus to SPI bridge to the
+kernel's SPI core subsystem.
+
+The driver does not probe for supported chips, since the SI18IS602/603 does not
+support Chip ID registers. You will have to instantiate the devices explicitly.
+Please see Documentation/i2c/instantiating-devices for details.
+
+
+Usage Notes
+-----------
+
+This driver requires the I2C adapter driver to support raw I2C messages. I2C
+adapter drivers which can only handle the SMBus protocol are not supported.
+
+The maximum SPI message size supported by SC18IS602/603 is 200 bytes. Attempts
+to initiate longer transfers will fail with -EINVAL. EEPROM read operations and
+similar large accesses have to be split into multiple chunks of no more than
+200 bytes per SPI message (128 bytes of data per message is recommended). This
+means that programs such as "cp" or "od", which automatically use large block
+sizes to access a device, can not be used directly to read data from EEPROM.
+Programs such as dd, where the block size can be specified, should be used
+instead.
diff --git a/Documentation/spi/spi-summary b/Documentation/spi/spi-summary
new file mode 100644
index 0000000..1721c1b
--- /dev/null
+++ b/Documentation/spi/spi-summary
@@ -0,0 +1,625 @@
+Overview of Linux kernel SPI support
+====================================
+
+02-Feb-2012
+
+What is SPI?
+------------
+The "Serial Peripheral Interface" (SPI) is a synchronous four wire serial
+link used to connect microcontrollers to sensors, memory, and peripherals.
+It's a simple "de facto" standard, not complicated enough to acquire a
+standardization body. SPI uses a master/slave configuration.
+
+The three signal wires hold a clock (SCK, often on the order of 10 MHz),
+and parallel data lines with "Master Out, Slave In" (MOSI) or "Master In,
+Slave Out" (MISO) signals. (Other names are also used.) There are four
+clocking modes through which data is exchanged; mode-0 and mode-3 are most
+commonly used. Each clock cycle shifts data out and data in; the clock
+doesn't cycle except when there is a data bit to shift. Not all data bits
+are used though; not every protocol uses those full duplex capabilities.
+
+SPI masters use a fourth "chip select" line to activate a given SPI slave
+device, so those three signal wires may be connected to several chips
+in parallel. All SPI slaves support chipselects; they are usually active
+low signals, labeled nCSx for slave 'x' (e.g. nCS0). Some devices have
+other signals, often including an interrupt to the master.
+
+Unlike serial busses like USB or SMBus, even low level protocols for
+SPI slave functions are usually not interoperable between vendors
+(except for commodities like SPI memory chips).
+
+ - SPI may be used for request/response style device protocols, as with
+ touchscreen sensors and memory chips.
+
+ - It may also be used to stream data in either direction (half duplex),
+ or both of them at the same time (full duplex).
+
+ - Some devices may use eight bit words. Others may use different word
+ lengths, such as streams of 12-bit or 20-bit digital samples.
+
+ - Words are usually sent with their most significant bit (MSB) first,
+ but sometimes the least significant bit (LSB) goes first instead.
+
+ - Sometimes SPI is used to daisy-chain devices, like shift registers.
+
+In the same way, SPI slaves will only rarely support any kind of automatic
+discovery/enumeration protocol. The tree of slave devices accessible from
+a given SPI master will normally be set up manually, with configuration
+tables.
+
+SPI is only one of the names used by such four-wire protocols, and
+most controllers have no problem handling "MicroWire" (think of it as
+half-duplex SPI, for request/response protocols), SSP ("Synchronous
+Serial Protocol"), PSP ("Programmable Serial Protocol"), and other
+related protocols.
+
+Some chips eliminate a signal line by combining MOSI and MISO, and
+limiting themselves to half-duplex at the hardware level. In fact
+some SPI chips have this signal mode as a strapping option. These
+can be accessed using the same programming interface as SPI, but of
+course they won't handle full duplex transfers. You may find such
+chips described as using "three wire" signaling: SCK, data, nCSx.
+(That data line is sometimes called MOMI or SISO.)
+
+Microcontrollers often support both master and slave sides of the SPI
+protocol. This document (and Linux) supports both the master and slave
+sides of SPI interactions.
+
+
+Who uses it? On what kinds of systems?
+---------------------------------------
+Linux developers using SPI are probably writing device drivers for embedded
+systems boards. SPI is used to control external chips, and it is also a
+protocol supported by every MMC or SD memory card. (The older "DataFlash"
+cards, predating MMC cards but using the same connectors and card shape,
+support only SPI.) Some PC hardware uses SPI flash for BIOS code.
+
+SPI slave chips range from digital/analog converters used for analog
+sensors and codecs, to memory, to peripherals like USB controllers
+or Ethernet adapters; and more.
+
+Most systems using SPI will integrate a few devices on a mainboard.
+Some provide SPI links on expansion connectors; in cases where no
+dedicated SPI controller exists, GPIO pins can be used to create a
+low speed "bitbanging" adapter. Very few systems will "hotplug" an SPI
+controller; the reasons to use SPI focus on low cost and simple operation,
+and if dynamic reconfiguration is important, USB will often be a more
+appropriate low-pincount peripheral bus.
+
+Many microcontrollers that can run Linux integrate one or more I/O
+interfaces with SPI modes. Given SPI support, they could use MMC or SD
+cards without needing a special purpose MMC/SD/SDIO controller.
+
+
+I'm confused. What are these four SPI "clock modes"?
+-----------------------------------------------------
+It's easy to be confused here, and the vendor documentation you'll
+find isn't necessarily helpful. The four modes combine two mode bits:
+
+ - CPOL indicates the initial clock polarity. CPOL=0 means the
+ clock starts low, so the first (leading) edge is rising, and
+ the second (trailing) edge is falling. CPOL=1 means the clock
+ starts high, so the first (leading) edge is falling.
+
+ - CPHA indicates the clock phase used to sample data; CPHA=0 says
+ sample on the leading edge, CPHA=1 means the trailing edge.
+
+ Since the signal needs to stablize before it's sampled, CPHA=0
+ implies that its data is written half a clock before the first
+ clock edge. The chipselect may have made it become available.
+
+Chip specs won't always say "uses SPI mode X" in as many words,
+but their timing diagrams will make the CPOL and CPHA modes clear.
+
+In the SPI mode number, CPOL is the high order bit and CPHA is the
+low order bit. So when a chip's timing diagram shows the clock
+starting low (CPOL=0) and data stabilized for sampling during the
+trailing clock edge (CPHA=1), that's SPI mode 1.
+
+Note that the clock mode is relevant as soon as the chipselect goes
+active. So the master must set the clock to inactive before selecting
+a slave, and the slave can tell the chosen polarity by sampling the
+clock level when its select line goes active. That's why many devices
+support for example both modes 0 and 3: they don't care about polarity,
+and always clock data in/out on rising clock edges.
+
+
+How do these driver programming interfaces work?
+------------------------------------------------
+The <linux/spi/spi.h> header file includes kerneldoc, as does the
+main source code, and you should certainly read that chapter of the
+kernel API document. This is just an overview, so you get the big
+picture before those details.
+
+SPI requests always go into I/O queues. Requests for a given SPI device
+are always executed in FIFO order, and complete asynchronously through
+completion callbacks. There are also some simple synchronous wrappers
+for those calls, including ones for common transaction types like writing
+a command and then reading its response.
+
+There are two types of SPI driver, here called:
+
+ Controller drivers ... controllers may be built into System-On-Chip
+ processors, and often support both Master and Slave roles.
+ These drivers touch hardware registers and may use DMA.
+ Or they can be PIO bitbangers, needing just GPIO pins.
+
+ Protocol drivers ... these pass messages through the controller
+ driver to communicate with a Slave or Master device on the
+ other side of an SPI link.
+
+So for example one protocol driver might talk to the MTD layer to export
+data to filesystems stored on SPI flash like DataFlash; and others might
+control audio interfaces, present touchscreen sensors as input interfaces,
+or monitor temperature and voltage levels during industrial processing.
+And those might all be sharing the same controller driver.
+
+A "struct spi_device" encapsulates the controller-side interface between
+those two types of drivers.
+
+There is a minimal core of SPI programming interfaces, focussing on
+using the driver model to connect controller and protocol drivers using
+device tables provided by board specific initialization code. SPI
+shows up in sysfs in several locations:
+
+ /sys/devices/.../CTLR ... physical node for a given SPI controller
+
+ /sys/devices/.../CTLR/spiB.C ... spi_device on bus "B",
+ chipselect C, accessed through CTLR.
+
+ /sys/bus/spi/devices/spiB.C ... symlink to that physical
+ .../CTLR/spiB.C device
+
+ /sys/devices/.../CTLR/spiB.C/modalias ... identifies the driver
+ that should be used with this device (for hotplug/coldplug)
+
+ /sys/bus/spi/drivers/D ... driver for one or more spi*.* devices
+
+ /sys/class/spi_master/spiB ... symlink (or actual device node) to
+ a logical node which could hold class related state for the SPI
+ master controller managing bus "B". All spiB.* devices share one
+ physical SPI bus segment, with SCLK, MOSI, and MISO.
+
+ /sys/devices/.../CTLR/slave ... virtual file for (un)registering the
+ slave device for an SPI slave controller.
+ Writing the driver name of an SPI slave handler to this file
+ registers the slave device; writing "(null)" unregisters the slave
+ device.
+ Reading from this file shows the name of the slave device ("(null)"
+ if not registered).
+
+ /sys/class/spi_slave/spiB ... symlink (or actual device node) to
+ a logical node which could hold class related state for the SPI
+ slave controller on bus "B". When registered, a single spiB.*
+ device is present here, possible sharing the physical SPI bus
+ segment with other SPI slave devices.
+
+Note that the actual location of the controller's class state depends
+on whether you enabled CONFIG_SYSFS_DEPRECATED or not. At this time,
+the only class-specific state is the bus number ("B" in "spiB"), so
+those /sys/class entries are only useful to quickly identify busses.
+
+
+How does board-specific init code declare SPI devices?
+------------------------------------------------------
+Linux needs several kinds of information to properly configure SPI devices.
+That information is normally provided by board-specific code, even for
+chips that do support some of automated discovery/enumeration.
+
+DECLARE CONTROLLERS
+
+The first kind of information is a list of what SPI controllers exist.
+For System-on-Chip (SOC) based boards, these will usually be platform
+devices, and the controller may need some platform_data in order to
+operate properly. The "struct platform_device" will include resources
+like the physical address of the controller's first register and its IRQ.
+
+Platforms will often abstract the "register SPI controller" operation,
+maybe coupling it with code to initialize pin configurations, so that
+the arch/.../mach-*/board-*.c files for several boards can all share the
+same basic controller setup code. This is because most SOCs have several
+SPI-capable controllers, and only the ones actually usable on a given
+board should normally be set up and registered.
+
+So for example arch/.../mach-*/board-*.c files might have code like:
+
+ #include <mach/spi.h> /* for mysoc_spi_data */
+
+ /* if your mach-* infrastructure doesn't support kernels that can
+ * run on multiple boards, pdata wouldn't benefit from "__init".
+ */
+ static struct mysoc_spi_data pdata __initdata = { ... };
+
+ static __init board_init(void)
+ {
+ ...
+ /* this board only uses SPI controller #2 */
+ mysoc_register_spi(2, &pdata);
+ ...
+ }
+
+And SOC-specific utility code might look something like:
+
+ #include <mach/spi.h>
+
+ static struct platform_device spi2 = { ... };
+
+ void mysoc_register_spi(unsigned n, struct mysoc_spi_data *pdata)
+ {
+ struct mysoc_spi_data *pdata2;
+
+ pdata2 = kmalloc(sizeof *pdata2, GFP_KERNEL);
+ *pdata2 = pdata;
+ ...
+ if (n == 2) {
+ spi2->dev.platform_data = pdata2;
+ register_platform_device(&spi2);
+
+ /* also: set up pin modes so the spi2 signals are
+ * visible on the relevant pins ... bootloaders on
+ * production boards may already have done this, but
+ * developer boards will often need Linux to do it.
+ */
+ }
+ ...
+ }
+
+Notice how the platform_data for boards may be different, even if the
+same SOC controller is used. For example, on one board SPI might use
+an external clock, where another derives the SPI clock from current
+settings of some master clock.
+
+
+DECLARE SLAVE DEVICES
+
+The second kind of information is a list of what SPI slave devices exist
+on the target board, often with some board-specific data needed for the
+driver to work correctly.
+
+Normally your arch/.../mach-*/board-*.c files would provide a small table
+listing the SPI devices on each board. (This would typically be only a
+small handful.) That might look like:
+
+ static struct ads7846_platform_data ads_info = {
+ .vref_delay_usecs = 100,
+ .x_plate_ohms = 580,
+ .y_plate_ohms = 410,
+ };
+
+ static struct spi_board_info spi_board_info[] __initdata = {
+ {
+ .modalias = "ads7846",
+ .platform_data = &ads_info,
+ .mode = SPI_MODE_0,
+ .irq = GPIO_IRQ(31),
+ .max_speed_hz = 120000 /* max sample rate at 3V */ * 16,
+ .bus_num = 1,
+ .chip_select = 0,
+ },
+ };
+
+Again, notice how board-specific information is provided; each chip may need
+several types. This example shows generic constraints like the fastest SPI
+clock to allow (a function of board voltage in this case) or how an IRQ pin
+is wired, plus chip-specific constraints like an important delay that's
+changed by the capacitance at one pin.
+
+(There's also "controller_data", information that may be useful to the
+controller driver. An example would be peripheral-specific DMA tuning
+data or chipselect callbacks. This is stored in spi_device later.)
+
+The board_info should provide enough information to let the system work
+without the chip's driver being loaded. The most troublesome aspect of
+that is likely the SPI_CS_HIGH bit in the spi_device.mode field, since
+sharing a bus with a device that interprets chipselect "backwards" is
+not possible until the infrastructure knows how to deselect it.
+
+Then your board initialization code would register that table with the SPI
+infrastructure, so that it's available later when the SPI master controller
+driver is registered:
+
+ spi_register_board_info(spi_board_info, ARRAY_SIZE(spi_board_info));
+
+Like with other static board-specific setup, you won't unregister those.
+
+The widely used "card" style computers bundle memory, cpu, and little else
+onto a card that's maybe just thirty square centimeters. On such systems,
+your arch/.../mach-.../board-*.c file would primarily provide information
+about the devices on the mainboard into which such a card is plugged. That
+certainly includes SPI devices hooked up through the card connectors!
+
+
+NON-STATIC CONFIGURATIONS
+
+Developer boards often play by different rules than product boards, and one
+example is the potential need to hotplug SPI devices and/or controllers.
+
+For those cases you might need to use spi_busnum_to_master() to look
+up the spi bus master, and will likely need spi_new_device() to provide the
+board info based on the board that was hotplugged. Of course, you'd later
+call at least spi_unregister_device() when that board is removed.
+
+When Linux includes support for MMC/SD/SDIO/DataFlash cards through SPI, those
+configurations will also be dynamic. Fortunately, such devices all support
+basic device identification probes, so they should hotplug normally.
+
+
+How do I write an "SPI Protocol Driver"?
+----------------------------------------
+Most SPI drivers are currently kernel drivers, but there's also support
+for userspace drivers. Here we talk only about kernel drivers.
+
+SPI protocol drivers somewhat resemble platform device drivers:
+
+ static struct spi_driver CHIP_driver = {
+ .driver = {
+ .name = "CHIP",
+ .owner = THIS_MODULE,
+ .pm = &CHIP_pm_ops,
+ },
+
+ .probe = CHIP_probe,
+ .remove = CHIP_remove,
+ };
+
+The driver core will automatically attempt to bind this driver to any SPI
+device whose board_info gave a modalias of "CHIP". Your probe() code
+might look like this unless you're creating a device which is managing
+a bus (appearing under /sys/class/spi_master).
+
+ static int CHIP_probe(struct spi_device *spi)
+ {
+ struct CHIP *chip;
+ struct CHIP_platform_data *pdata;
+
+ /* assuming the driver requires board-specific data: */
+ pdata = &spi->dev.platform_data;
+ if (!pdata)
+ return -ENODEV;
+
+ /* get memory for driver's per-chip state */
+ chip = kzalloc(sizeof *chip, GFP_KERNEL);
+ if (!chip)
+ return -ENOMEM;
+ spi_set_drvdata(spi, chip);
+
+ ... etc
+ return 0;
+ }
+
+As soon as it enters probe(), the driver may issue I/O requests to
+the SPI device using "struct spi_message". When remove() returns,
+or after probe() fails, the driver guarantees that it won't submit
+any more such messages.
+
+ - An spi_message is a sequence of protocol operations, executed
+ as one atomic sequence. SPI driver controls include:
+
+ + when bidirectional reads and writes start ... by how its
+ sequence of spi_transfer requests is arranged;
+
+ + which I/O buffers are used ... each spi_transfer wraps a
+ buffer for each transfer direction, supporting full duplex
+ (two pointers, maybe the same one in both cases) and half
+ duplex (one pointer is NULL) transfers;
+
+ + optionally defining short delays after transfers ... using
+ the spi_transfer.delay_usecs setting (this delay can be the
+ only protocol effect, if the buffer length is zero);
+
+ + whether the chipselect becomes inactive after a transfer and
+ any delay ... by using the spi_transfer.cs_change flag;
+
+ + hinting whether the next message is likely to go to this same
+ device ... using the spi_transfer.cs_change flag on the last
+ transfer in that atomic group, and potentially saving costs
+ for chip deselect and select operations.
+
+ - Follow standard kernel rules, and provide DMA-safe buffers in
+ your messages. That way controller drivers using DMA aren't forced
+ to make extra copies unless the hardware requires it (e.g. working
+ around hardware errata that force the use of bounce buffering).
+
+ If standard dma_map_single() handling of these buffers is inappropriate,
+ you can use spi_message.is_dma_mapped to tell the controller driver
+ that you've already provided the relevant DMA addresses.
+
+ - The basic I/O primitive is spi_async(). Async requests may be
+ issued in any context (irq handler, task, etc) and completion
+ is reported using a callback provided with the message.
+ After any detected error, the chip is deselected and processing
+ of that spi_message is aborted.
+
+ - There are also synchronous wrappers like spi_sync(), and wrappers
+ like spi_read(), spi_write(), and spi_write_then_read(). These
+ may be issued only in contexts that may sleep, and they're all
+ clean (and small, and "optional") layers over spi_async().
+
+ - The spi_write_then_read() call, and convenience wrappers around
+ it, should only be used with small amounts of data where the
+ cost of an extra copy may be ignored. It's designed to support
+ common RPC-style requests, such as writing an eight bit command
+ and reading a sixteen bit response -- spi_w8r16() being one its
+ wrappers, doing exactly that.
+
+Some drivers may need to modify spi_device characteristics like the
+transfer mode, wordsize, or clock rate. This is done with spi_setup(),
+which would normally be called from probe() before the first I/O is
+done to the device. However, that can also be called at any time
+that no message is pending for that device.
+
+While "spi_device" would be the bottom boundary of the driver, the
+upper boundaries might include sysfs (especially for sensor readings),
+the input layer, ALSA, networking, MTD, the character device framework,
+or other Linux subsystems.
+
+Note that there are two types of memory your driver must manage as part
+of interacting with SPI devices.
+
+ - I/O buffers use the usual Linux rules, and must be DMA-safe.
+ You'd normally allocate them from the heap or free page pool.
+ Don't use the stack, or anything that's declared "static".
+
+ - The spi_message and spi_transfer metadata used to glue those
+ I/O buffers into a group of protocol transactions. These can
+ be allocated anywhere it's convenient, including as part of
+ other allocate-once driver data structures. Zero-init these.
+
+If you like, spi_message_alloc() and spi_message_free() convenience
+routines are available to allocate and zero-initialize an spi_message
+with several transfers.
+
+
+How do I write an "SPI Master Controller Driver"?
+-------------------------------------------------
+An SPI controller will probably be registered on the platform_bus; write
+a driver to bind to the device, whichever bus is involved.
+
+The main task of this type of driver is to provide an "spi_master".
+Use spi_alloc_master() to allocate the master, and spi_master_get_devdata()
+to get the driver-private data allocated for that device.
+
+ struct spi_master *master;
+ struct CONTROLLER *c;
+
+ master = spi_alloc_master(dev, sizeof *c);
+ if (!master)
+ return -ENODEV;
+
+ c = spi_master_get_devdata(master);
+
+The driver will initialize the fields of that spi_master, including the
+bus number (maybe the same as the platform device ID) and three methods
+used to interact with the SPI core and SPI protocol drivers. It will
+also initialize its own internal state. (See below about bus numbering
+and those methods.)
+
+After you initialize the spi_master, then use spi_register_master() to
+publish it to the rest of the system. At that time, device nodes for the
+controller and any predeclared spi devices will be made available, and
+the driver model core will take care of binding them to drivers.
+
+If you need to remove your SPI controller driver, spi_unregister_master()
+will reverse the effect of spi_register_master().
+
+
+BUS NUMBERING
+
+Bus numbering is important, since that's how Linux identifies a given
+SPI bus (shared SCK, MOSI, MISO). Valid bus numbers start at zero. On
+SOC systems, the bus numbers should match the numbers defined by the chip
+manufacturer. For example, hardware controller SPI2 would be bus number 2,
+and spi_board_info for devices connected to it would use that number.
+
+If you don't have such hardware-assigned bus number, and for some reason
+you can't just assign them, then provide a negative bus number. That will
+then be replaced by a dynamically assigned number. You'd then need to treat
+this as a non-static configuration (see above).
+
+
+SPI MASTER METHODS
+
+ master->setup(struct spi_device *spi)
+ This sets up the device clock rate, SPI mode, and word sizes.
+ Drivers may change the defaults provided by board_info, and then
+ call spi_setup(spi) to invoke this routine. It may sleep.
+
+ Unless each SPI slave has its own configuration registers, don't
+ change them right away ... otherwise drivers could corrupt I/O
+ that's in progress for other SPI devices.
+
+ ** BUG ALERT: for some reason the first version of
+ ** many spi_master drivers seems to get this wrong.
+ ** When you code setup(), ASSUME that the controller
+ ** is actively processing transfers for another device.
+
+ master->cleanup(struct spi_device *spi)
+ Your controller driver may use spi_device.controller_state to hold
+ state it dynamically associates with that device. If you do that,
+ be sure to provide the cleanup() method to free that state.
+
+ master->prepare_transfer_hardware(struct spi_master *master)
+ This will be called by the queue mechanism to signal to the driver
+ that a message is coming in soon, so the subsystem requests the
+ driver to prepare the transfer hardware by issuing this call.
+ This may sleep.
+
+ master->unprepare_transfer_hardware(struct spi_master *master)
+ This will be called by the queue mechanism to signal to the driver
+ that there are no more messages pending in the queue and it may
+ relax the hardware (e.g. by power management calls). This may sleep.
+
+ master->transfer_one_message(struct spi_master *master,
+ struct spi_message *mesg)
+ The subsystem calls the driver to transfer a single message while
+ queuing transfers that arrive in the meantime. When the driver is
+ finished with this message, it must call
+ spi_finalize_current_message() so the subsystem can issue the next
+ message. This may sleep.
+
+ master->transfer_one(struct spi_master *master, struct spi_device *spi,
+ struct spi_transfer *transfer)
+ The subsystem calls the driver to transfer a single transfer while
+ queuing transfers that arrive in the meantime. When the driver is
+ finished with this transfer, it must call
+ spi_finalize_current_transfer() so the subsystem can issue the next
+ transfer. This may sleep. Note: transfer_one and transfer_one_message
+ are mutually exclusive; when both are set, the generic subsystem does
+ not call your transfer_one callback.
+
+ Return values:
+ negative errno: error
+ 0: transfer is finished
+ 1: transfer is still in progress
+
+ DEPRECATED METHODS
+
+ master->transfer(struct spi_device *spi, struct spi_message *message)
+ This must not sleep. Its responsibility is to arrange that the
+ transfer happens and its complete() callback is issued. The two
+ will normally happen later, after other transfers complete, and
+ if the controller is idle it will need to be kickstarted. This
+ method is not used on queued controllers and must be NULL if
+ transfer_one_message() and (un)prepare_transfer_hardware() are
+ implemented.
+
+
+SPI MESSAGE QUEUE
+
+If you are happy with the standard queueing mechanism provided by the
+SPI subsystem, just implement the queued methods specified above. Using
+the message queue has the upside of centralizing a lot of code and
+providing pure process-context execution of methods. The message queue
+can also be elevated to realtime priority on high-priority SPI traffic.
+
+Unless the queueing mechanism in the SPI subsystem is selected, the bulk
+of the driver will be managing the I/O queue fed by the now deprecated
+function transfer().
+
+That queue could be purely conceptual. For example, a driver used only
+for low-frequency sensor access might be fine using synchronous PIO.
+
+But the queue will probably be very real, using message->queue, PIO,
+often DMA (especially if the root filesystem is in SPI flash), and
+execution contexts like IRQ handlers, tasklets, or workqueues (such
+as keventd). Your driver can be as fancy, or as simple, as you need.
+Such a transfer() method would normally just add the message to a
+queue, and then start some asynchronous transfer engine (unless it's
+already running).
+
+
+THANKS TO
+---------
+Contributors to Linux-SPI discussions include (in alphabetical order,
+by last name):
+
+Mark Brown
+David Brownell
+Russell King
+Grant Likely
+Dmitry Pervushin
+Stephen Street
+Mark Underwood
+Andrew Victor
+Linus Walleij
+Vitaly Wool
diff --git a/Documentation/spi/spidev b/Documentation/spi/spidev
new file mode 100644
index 0000000..3d14035
--- /dev/null
+++ b/Documentation/spi/spidev
@@ -0,0 +1,149 @@
+SPI devices have a limited userspace API, supporting basic half-duplex
+read() and write() access to SPI slave devices. Using ioctl() requests,
+full duplex transfers and device I/O configuration are also available.
+
+ #include <fcntl.h>
+ #include <unistd.h>
+ #include <sys/ioctl.h>
+ #include <linux/types.h>
+ #include <linux/spi/spidev.h>
+
+Some reasons you might want to use this programming interface include:
+
+ * Prototyping in an environment that's not crash-prone; stray pointers
+ in userspace won't normally bring down any Linux system.
+
+ * Developing simple protocols used to talk to microcontrollers acting
+ as SPI slaves, which you may need to change quite often.
+
+Of course there are drivers that can never be written in userspace, because
+they need to access kernel interfaces (such as IRQ handlers or other layers
+of the driver stack) that are not accessible to userspace.
+
+
+DEVICE CREATION, DRIVER BINDING
+===============================
+The simplest way to arrange to use this driver is to just list it in the
+spi_board_info for a device as the driver it should use: the "modalias"
+entry is "spidev", matching the name of the driver exposing this API.
+Set up the other device characteristics (bits per word, SPI clocking,
+chipselect polarity, etc) as usual, so you won't always need to override
+them later.
+
+(Sysfs also supports userspace driven binding/unbinding of drivers to
+devices. That mechanism might be supported here in the future.)
+
+When you do that, the sysfs node for the SPI device will include a child
+device node with a "dev" attribute that will be understood by udev or mdev.
+(Larger systems will have "udev". Smaller ones may configure "mdev" into
+busybox; it's less featureful, but often enough.) For a SPI device with
+chipselect C on bus B, you should see:
+
+ /dev/spidevB.C ... character special device, major number 153 with
+ a dynamically chosen minor device number. This is the node
+ that userspace programs will open, created by "udev" or "mdev".
+
+ /sys/devices/.../spiB.C ... as usual, the SPI device node will
+ be a child of its SPI master controller.
+
+ /sys/class/spidev/spidevB.C ... created when the "spidev" driver
+ binds to that device. (Directory or symlink, based on whether
+ or not you enabled the "deprecated sysfs files" Kconfig option.)
+
+Do not try to manage the /dev character device special file nodes by hand.
+That's error prone, and you'd need to pay careful attention to system
+security issues; udev/mdev should already be configured securely.
+
+If you unbind the "spidev" driver from that device, those two "spidev" nodes
+(in sysfs and in /dev) should automatically be removed (respectively by the
+kernel and by udev/mdev). You can unbind by removing the "spidev" driver
+module, which will affect all devices using this driver. You can also unbind
+by having kernel code remove the SPI device, probably by removing the driver
+for its SPI controller (so its spi_master vanishes).
+
+Since this is a standard Linux device driver -- even though it just happens
+to expose a low level API to userspace -- it can be associated with any number
+of devices at a time. Just provide one spi_board_info record for each such
+SPI device, and you'll get a /dev device node for each device.
+
+
+BASIC CHARACTER DEVICE API
+==========================
+Normal open() and close() operations on /dev/spidevB.D files work as you
+would expect.
+
+Standard read() and write() operations are obviously only half-duplex, and
+the chipselect is deactivated between those operations. Full-duplex access,
+and composite operation without chipselect de-activation, is available using
+the SPI_IOC_MESSAGE(N) request.
+
+Several ioctl() requests let your driver read or override the device's current
+settings for data transfer parameters:
+
+ SPI_IOC_RD_MODE, SPI_IOC_WR_MODE ... pass a pointer to a byte which will
+ return (RD) or assign (WR) the SPI transfer mode. Use the constants
+ SPI_MODE_0..SPI_MODE_3; or if you prefer you can combine SPI_CPOL
+ (clock polarity, idle high iff this is set) or SPI_CPHA (clock phase,
+ sample on trailing edge iff this is set) flags.
+ Note that this request is limited to SPI mode flags that fit in a
+ single byte.
+
+ SPI_IOC_RD_MODE32, SPI_IOC_WR_MODE32 ... pass a pointer to a uin32_t
+ which will return (RD) or assign (WR) the full SPI transfer mode,
+ not limited to the bits that fit in one byte.
+
+ SPI_IOC_RD_LSB_FIRST, SPI_IOC_WR_LSB_FIRST ... pass a pointer to a byte
+ which will return (RD) or assign (WR) the bit justification used to
+ transfer SPI words. Zero indicates MSB-first; other values indicate
+ the less common LSB-first encoding. In both cases the specified value
+ is right-justified in each word, so that unused (TX) or undefined (RX)
+ bits are in the MSBs.
+
+ SPI_IOC_RD_BITS_PER_WORD, SPI_IOC_WR_BITS_PER_WORD ... pass a pointer to
+ a byte which will return (RD) or assign (WR) the number of bits in
+ each SPI transfer word. The value zero signifies eight bits.
+
+ SPI_IOC_RD_MAX_SPEED_HZ, SPI_IOC_WR_MAX_SPEED_HZ ... pass a pointer to a
+ u32 which will return (RD) or assign (WR) the maximum SPI transfer
+ speed, in Hz. The controller can't necessarily assign that specific
+ clock speed.
+
+NOTES:
+
+ - At this time there is no async I/O support; everything is purely
+ synchronous.
+
+ - There's currently no way to report the actual bit rate used to
+ shift data to/from a given device.
+
+ - From userspace, you can't currently change the chip select polarity;
+ that could corrupt transfers to other devices sharing the SPI bus.
+ Each SPI device is deselected when it's not in active use, allowing
+ other drivers to talk to other devices.
+
+ - There's a limit on the number of bytes each I/O request can transfer
+ to the SPI device. It defaults to one page, but that can be changed
+ using a module parameter.
+
+ - Because SPI has no low-level transfer acknowledgement, you usually
+ won't see any I/O errors when talking to a non-existent device.
+
+
+FULL DUPLEX CHARACTER DEVICE API
+================================
+
+See the spidev_fdx.c sample program for one example showing the use of the
+full duplex programming interface. (Although it doesn't perform a full duplex
+transfer.) The model is the same as that used in the kernel spi_sync()
+request; the individual transfers offer the same capabilities as are
+available to kernel drivers (except that it's not asynchronous).
+
+The example shows one half-duplex RPC-style request and response message.
+These requests commonly require that the chip not be deselected between
+the request and response. Several such requests could be chained into
+a single kernel request, even allowing the chip to be deselected after
+each response. (Other protocol options include changing the word size
+and bitrate for each transfer segment.)
+
+To make a full duplex request, provide both rx_buf and tx_buf for the
+same transfer. It's even OK if those are the same buffer.