src/kernel/linux/v4.19/Documentation/filesystems/sysfs-pci.txt

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.