v4.19.13 snapshot.
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+Accessing PCI device resources through sysfs
+--------------------------------------------
+
+sysfs, usually mounted at /sys, provides access to PCI resources on platforms
+that support it. For example, a given bus might look like this:
+
+ /sys/devices/pci0000:17
+ |-- 0000:17:00.0
+ | |-- class
+ | |-- config
+ | |-- device
+ | |-- enable
+ | |-- irq
+ | |-- local_cpus
+ | |-- remove
+ | |-- resource
+ | |-- resource0
+ | |-- resource1
+ | |-- resource2
+ | |-- revision
+ | |-- rom
+ | |-- subsystem_device
+ | |-- subsystem_vendor
+ | `-- vendor
+ `-- ...
+
+The topmost element describes the PCI domain and bus number. In this case,
+the domain number is 0000 and the bus number is 17 (both values are in hex).
+This bus contains a single function device in slot 0. The domain and bus
+numbers are reproduced for convenience. Under the device directory are several
+files, each with their own function.
+
+ file function
+ ---- --------
+ class PCI class (ascii, ro)
+ config PCI config space (binary, rw)
+ device PCI device (ascii, ro)
+ enable Whether the device is enabled (ascii, rw)
+ irq IRQ number (ascii, ro)
+ local_cpus nearby CPU mask (cpumask, ro)
+ remove remove device from kernel's list (ascii, wo)
+ resource PCI resource host addresses (ascii, ro)
+ resource0..N PCI resource N, if present (binary, mmap, rw[1])
+ resource0_wc..N_wc PCI WC map resource N, if prefetchable (binary, mmap)
+ revision PCI revision (ascii, ro)
+ rom PCI ROM resource, if present (binary, ro)
+ subsystem_device PCI subsystem device (ascii, ro)
+ subsystem_vendor PCI subsystem vendor (ascii, ro)
+ vendor PCI vendor (ascii, ro)
+
+ ro - read only file
+ rw - file is readable and writable
+ wo - write only file
+ mmap - file is mmapable
+ ascii - file contains ascii text
+ binary - file contains binary data
+ cpumask - file contains a cpumask type
+
+[1] rw for RESOURCE_IO (I/O port) regions only
+
+The read only files are informational, writes to them will be ignored, with
+the exception of the 'rom' file. Writable files can be used to perform
+actions on the device (e.g. changing config space, detaching a device).
+mmapable files are available via an mmap of the file at offset 0 and can be
+used to do actual device programming from userspace. Note that some platforms
+don't support mmapping of certain resources, so be sure to check the return
+value from any attempted mmap. The most notable of these are I/O port
+resources, which also provide read/write access.
+
+The 'enable' file provides a counter that indicates how many times the device
+has been enabled. If the 'enable' file currently returns '4', and a '1' is
+echoed into it, it will then return '5'. Echoing a '0' into it will decrease
+the count. Even when it returns to 0, though, some of the initialisation
+may not be reversed.
+
+The 'rom' file is special in that it provides read-only access to the device's
+ROM file, if available. It's disabled by default, however, so applications
+should write the string "1" to the file to enable it before attempting a read
+call, and disable it following the access by writing "0" to the file. Note
+that the device must be enabled for a rom read to return data successfully.
+In the event a driver is not bound to the device, it can be enabled using the
+'enable' file, documented above.
+
+The 'remove' file is used to remove the PCI device, by writing a non-zero
+integer to the file. This does not involve any kind of hot-plug functionality,
+e.g. powering off the device. The device is removed from the kernel's list of
+PCI devices, the sysfs directory for it is removed, and the device will be
+removed from any drivers attached to it. Removal of PCI root buses is
+disallowed.
+
+Accessing legacy resources through sysfs
+----------------------------------------
+
+Legacy I/O port and ISA memory resources are also provided in sysfs if the
+underlying platform supports them. They're located in the PCI class hierarchy,
+e.g.
+
+ /sys/class/pci_bus/0000:17/
+ |-- bridge -> ../../../devices/pci0000:17
+ |-- cpuaffinity
+ |-- legacy_io
+ `-- legacy_mem
+
+The legacy_io file is a read/write file that can be used by applications to
+do legacy port I/O. The application should open the file, seek to the desired
+port (e.g. 0x3e8) and do a read or a write of 1, 2 or 4 bytes. The legacy_mem
+file should be mmapped with an offset corresponding to the memory offset
+desired, e.g. 0xa0000 for the VGA frame buffer. The application can then
+simply dereference the returned pointer (after checking for errors of course)
+to access legacy memory space.
+
+Supporting PCI access on new platforms
+--------------------------------------
+
+In order to support PCI resource mapping as described above, Linux platform
+code should ideally define ARCH_GENERIC_PCI_MMAP_RESOURCE and use the generic
+implementation of that functionality. To support the historical interface of
+mmap() through files in /proc/bus/pci, platforms may also set HAVE_PCI_MMAP.
+
+Alternatively, platforms which set HAVE_PCI_MMAP may provide their own
+implementation of pci_mmap_page_range() instead of defining
+ARCH_GENERIC_PCI_MMAP_RESOURCE.
+
+Platforms which support write-combining maps of PCI resources must define
+arch_can_pci_mmap_wc() which shall evaluate to non-zero at runtime when
+write-combining is permitted. Platforms which support maps of I/O resources
+define arch_can_pci_mmap_io() similarly.
+
+Legacy resources are protected by the HAVE_PCI_LEGACY define. Platforms
+wishing to support legacy functionality should define it and provide
+pci_legacy_read, pci_legacy_write and pci_mmap_legacy_page_range functions.