1Accessing PCI device resources through sysfs
   4sysfs, usually mounted at /sys, provides access to PCI resources on platforms
   5that support it.  For example, a given bus might look like this:
   7     /sys/devices/pci0000:17
   8     |-- 0000:17:00.0
   9     |   |-- class
  10     |   |-- config
  11     |   |-- device
  12     |   |-- enable
  13     |   |-- irq
  14     |   |-- local_cpus
  15     |   |-- remove
  16     |   |-- resource
  17     |   |-- resource0
  18     |   |-- resource1
  19     |   |-- resource2
  20     |   |-- rom
  21     |   |-- subsystem_device
  22     |   |-- subsystem_vendor
  23     |   `-- vendor
  24     `-- ...
  26The topmost element describes the PCI domain and bus number.  In this case,
  27the domain number is 0000 and the bus number is 17 (both values are in hex).
  28This bus contains a single function device in slot 0.  The domain and bus
  29numbers are reproduced for convenience.  Under the device directory are several
  30files, each with their own function.
  32       file                function
  33       ----                --------
  34       class               PCI class (ascii, ro)
  35       config              PCI config space (binary, rw)
  36       device              PCI device (ascii, ro)
  37       enable              Whether the device is enabled (ascii, rw)
  38       irq                 IRQ number (ascii, ro)
  39       local_cpus          nearby CPU mask (cpumask, ro)
  40       remove              remove device from kernel's list (ascii, wo)
  41       resource            PCI resource host addresses (ascii, ro)
  42       resource0..N        PCI resource N, if present (binary, mmap, rw[1])
  43       resource0_wc..N_wc  PCI WC map resource N, if prefetchable (binary, mmap)
  44       rom                 PCI ROM resource, if present (binary, ro)
  45       subsystem_device    PCI subsystem device (ascii, ro)
  46       subsystem_vendor    PCI subsystem vendor (ascii, ro)
  47       vendor              PCI vendor (ascii, ro)
  49  ro - read only file
  50  rw - file is readable and writable
  51  wo - write only file
  52  mmap - file is mmapable
  53  ascii - file contains ascii text
  54  binary - file contains binary data
  55  cpumask - file contains a cpumask type
  57[1] rw for RESOURCE_IO (I/O port) regions only
  59The read only files are informational, writes to them will be ignored, with
  60the exception of the 'rom' file.  Writable files can be used to perform
  61actions on the device (e.g. changing config space, detaching a device).
  62mmapable files are available via an mmap of the file at offset 0 and can be
  63used to do actual device programming from userspace.  Note that some platforms
  64don't support mmapping of certain resources, so be sure to check the return
  65value from any attempted mmap.  The most notable of these are I/O port
  66resources, which also provide read/write access.
  68The 'enable' file provides a counter that indicates how many times the device 
  69has been enabled.  If the 'enable' file currently returns '4', and a '1' is
  70echoed into it, it will then return '5'.  Echoing a '0' into it will decrease
  71the count.  Even when it returns to 0, though, some of the initialisation
  72may not be reversed.  
  74The 'rom' file is special in that it provides read-only access to the device's
  75ROM file, if available.  It's disabled by default, however, so applications
  76should write the string "1" to the file to enable it before attempting a read
  77call, and disable it following the access by writing "0" to the file.  Note
  78that the device must be enabled for a rom read to return data successfully.
  79In the event a driver is not bound to the device, it can be enabled using the
  80'enable' file, documented above.
  82The 'remove' file is used to remove the PCI device, by writing a non-zero
  83integer to the file.  This does not involve any kind of hot-plug functionality,
  84e.g. powering off the device.  The device is removed from the kernel's list of
  85PCI devices, the sysfs directory for it is removed, and the device will be
  86removed from any drivers attached to it. Removal of PCI root buses is
  89Accessing legacy resources through sysfs
  92Legacy I/O port and ISA memory resources are also provided in sysfs if the
  93underlying platform supports them.  They're located in the PCI class hierarchy,
  96        /sys/class/pci_bus/0000:17/
  97        |-- bridge -> ../../../devices/pci0000:17
  98        |-- cpuaffinity
  99        |-- legacy_io
 100        `-- legacy_mem
 102The legacy_io file is a read/write file that can be used by applications to
 103do legacy port I/O.  The application should open the file, seek to the desired
 104port (e.g. 0x3e8) and do a read or a write of 1, 2 or 4 bytes.  The legacy_mem
 105file should be mmapped with an offset corresponding to the memory offset
 106desired, e.g. 0xa0000 for the VGA frame buffer.  The application can then
 107simply dereference the returned pointer (after checking for errors of course)
 108to access legacy memory space.
 110Supporting PCI access on new platforms
 113In order to support PCI resource mapping as described above, Linux platform
 114code must define HAVE_PCI_MMAP and provide a pci_mmap_page_range function.
 115Platforms are free to only support subsets of the mmap functionality, but
 116useful return codes should be provided.
 118Legacy resources are protected by the HAVE_PCI_LEGACY define.  Platforms
 119wishing to support legacy functionality should define it and provide
 120pci_legacy_read, pci_legacy_write and pci_mmap_legacy_page_range functions.