sysfs-pci.txt 5.68 KB
Newer Older
Linus Torvalds's avatar
Linus Torvalds committed
1
Accessing PCI device resources through sysfs
2
--------------------------------------------
Linus Torvalds's avatar
Linus Torvalds committed
3 4 5 6 7 8 9 10 11

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
12
     |   |-- enable
Linus Torvalds's avatar
Linus Torvalds committed
13 14
     |   |-- irq
     |   |-- local_cpus
15
     |   |-- remove
Linus Torvalds's avatar
Linus Torvalds committed
16 17 18 19
     |   |-- resource
     |   |-- resource0
     |   |-- resource1
     |   |-- resource2
20
     |   |-- revision
Linus Torvalds's avatar
Linus Torvalds committed
21 22 23 24
     |   |-- rom
     |   |-- subsystem_device
     |   |-- subsystem_vendor
     |   `-- vendor
25
     `-- ...
Linus Torvalds's avatar
Linus Torvalds committed
26 27 28 29 30 31 32 33 34 35 36 37

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)
38
       enable	           Whether the device is enabled (ascii, rw)
Linus Torvalds's avatar
Linus Torvalds committed
39 40
       irq		   IRQ number (ascii, ro)
       local_cpus	   nearby CPU mask (cpumask, ro)
41
       remove		   remove device from kernel's list (ascii, wo)
Linus Torvalds's avatar
Linus Torvalds committed
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)
Linus Torvalds's avatar
Linus Torvalds committed
46 47 48 49 50 51 52
       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
53
  wo - write only file
Linus Torvalds's avatar
Linus Torvalds committed
54 55 56 57 58
  mmap - file is mmapable
  ascii - file contains ascii text
  binary - file contains binary data
  cpumask - file contains a cpumask type

59 60
[1] rw for RESOURCE_IO (I/O port) regions only

61 62 63 64 65 66
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
67 68
value from any attempted mmap.  The most notable of these are I/O port
resources, which also provide read/write access.
69

70 71 72 73 74 75
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.  

76 77 78
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
79
call, and disable it following the access by writing "0" to the file.  Note
80
that the device must be enabled for a rom read to return data successfully.
81 82
In the event a driver is not bound to the device, it can be enabled using the
'enable' file, documented above.
Linus Torvalds's avatar
Linus Torvalds committed
83

84 85 86 87 88 89 90
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.

Linus Torvalds's avatar
Linus Torvalds committed
91
Accessing legacy resources through sysfs
92
----------------------------------------
Linus Torvalds's avatar
Linus Torvalds committed
93 94

Legacy I/O port and ISA memory resources are also provided in sysfs if the
95
underlying platform supports them.  They're located in the PCI class hierarchy,
Linus Torvalds's avatar
Linus Torvalds committed
96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112
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
113
--------------------------------------
Linus Torvalds's avatar
Linus Torvalds committed
114 115

In order to support PCI resource mapping as described above, Linux platform
116 117 118 119 120 121 122
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.
Linus Torvalds's avatar
Linus Torvalds committed
123

124 125
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
126 127
write-combining is permitted. Platforms which support maps of I/O resources
define arch_can_pci_mmap_io() similarly.
128

Linus Torvalds's avatar
Linus Torvalds committed
129 130
Legacy resources are protected by the HAVE_PCI_LEGACY define.  Platforms
wishing to support legacy functionality should define it and provide
131
pci_legacy_read, pci_legacy_write and pci_mmap_legacy_page_range functions.