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     |   |-- revision
  21     |   |-- rom
  22     |   |-- subsystem_device
  23     |   |-- subsystem_vendor
  24     |   `-- vendor
  25     `-- ...
  27The topmost element describes the PCI domain and bus number.  In this case,
  28the domain number is 0000 and the bus number is 17 (both values are in hex).
  29This bus contains a single function device in slot 0.  The domain and bus
  30numbers are reproduced for convenience.  Under the device directory are several
  31files, each with their own function.
  33       file                function
  34       ----                --------
  35       class               PCI class (ascii, ro)
  36       config              PCI config space (binary, rw)
  37       device              PCI device (ascii, ro)
  38       enable              Whether the device is enabled (ascii, rw)
  39       irq                 IRQ number (ascii, ro)
  40       local_cpus          nearby CPU mask (cpumask, ro)
  41       remove              remove device from kernel's list (ascii, wo)
  42       resource            PCI resource host addresses (ascii, ro)
  43       resource0..N        PCI resource N, if present (binary, mmap, rw[1])
  44       resource0_wc..N_wc  PCI WC map resource N, if prefetchable (binary, mmap)
  45       revision            PCI revision (ascii, ro)
  46       rom                 PCI ROM resource, if present (binary, ro)
  47       subsystem_device    PCI subsystem device (ascii, ro)
  48       subsystem_vendor    PCI subsystem vendor (ascii, ro)
  49       vendor              PCI vendor (ascii, ro)
  51  ro - read only file
  52  rw - file is readable and writable
  53  wo - write only file
  54  mmap - file is mmapable
  55  ascii - file contains ascii text
  56  binary - file contains binary data
  57  cpumask - file contains a cpumask type
  59[1] rw for RESOURCE_IO (I/O port) regions only
  61The read only files are informational, writes to them will be ignored, with
  62the exception of the 'rom' file.  Writable files can be used to perform
  63actions on the device (e.g. changing config space, detaching a device).
  64mmapable files are available via an mmap of the file at offset 0 and can be
  65used to do actual device programming from userspace.  Note that some platforms
  66don't support mmapping of certain resources, so be sure to check the return
  67value from any attempted mmap.  The most notable of these are I/O port
  68resources, which also provide read/write access.
  70The 'enable' file provides a counter that indicates how many times the device 
  71has been enabled.  If the 'enable' file currently returns '4', and a '1' is
  72echoed into it, it will then return '5'.  Echoing a '0' into it will decrease
  73the count.  Even when it returns to 0, though, some of the initialisation
  74may not be reversed.  
  76The 'rom' file is special in that it provides read-only access to the device's
  77ROM file, if available.  It's disabled by default, however, so applications
  78should write the string "1" to the file to enable it before attempting a read
  79call, and disable it following the access by writing "0" to the file.  Note
  80that the device must be enabled for a rom read to return data successfully.
  81In the event a driver is not bound to the device, it can be enabled using the
  82'enable' file, documented above.
  84The 'remove' file is used to remove the PCI device, by writing a non-zero
  85integer to the file.  This does not involve any kind of hot-plug functionality,
  86e.g. powering off the device.  The device is removed from the kernel's list of
  87PCI devices, the sysfs directory for it is removed, and the device will be
  88removed from any drivers attached to it. Removal of PCI root buses is
  91Accessing legacy resources through sysfs
  94Legacy I/O port and ISA memory resources are also provided in sysfs if the
  95underlying platform supports them.  They're located in the PCI class hierarchy,
  98        /sys/class/pci_bus/0000:17/
  99        |-- bridge -> ../../../devices/pci0000:17
 100        |-- cpuaffinity
 101        |-- legacy_io
 102        `-- legacy_mem
 104The legacy_io file is a read/write file that can be used by applications to
 105do legacy port I/O.  The application should open the file, seek to the desired
 106port (e.g. 0x3e8) and do a read or a write of 1, 2 or 4 bytes.  The legacy_mem
 107file should be mmapped with an offset corresponding to the memory offset
 108desired, e.g. 0xa0000 for the VGA frame buffer.  The application can then
 109simply dereference the returned pointer (after checking for errors of course)
 110to access legacy memory space.
 112Supporting PCI access on new platforms
 115In order to support PCI resource mapping as described above, Linux platform
 116code should ideally define ARCH_GENERIC_PCI_MMAP_RESOURCE and use the generic
 117implementation of that functionality. To support the historical interface of
 118mmap() through files in /proc/bus/pci, platforms may also set HAVE_PCI_MMAP.
 120Alternatively, platforms which set HAVE_PCI_MMAP may provide their own
 121implementation of pci_mmap_page_range() instead of defining
 124Platforms which support write-combining maps of PCI resources must define
 125arch_can_pci_mmap_wc() which shall evaluate to non-zero at runtime when
 126write-combining is permitted. Platforms which support maps of I/O resources
 127define arch_can_pci_mmap_io() similarly.
 129Legacy resources are protected by the HAVE_PCI_LEGACY define.  Platforms
 130wishing to support legacy functionality should define it and provide
 131pci_legacy_read, pci_legacy_write and pci_mmap_legacy_page_range functions.
 132 kindly hosted by Redpill Linpro AS, provider of Linux consulting and operations services since 1995.