Root/Documentation/filesystems/sysfs-pci.txt

1Accessing PCI device resources through sysfs
2--------------------------------------------
3
4sysfs, usually mounted at /sys, provides access to PCI resources on platforms
5that support it. For example, a given bus might look like this:
6
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     `-- ...
25
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.
31
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)
48
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
56
57[1] rw for RESOURCE_IO (I/O port) regions only
58
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.
67
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.
73
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.
81
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
87disallowed.
88
89Accessing legacy resources through sysfs
90----------------------------------------
91
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,
94e.g.
95
96    /sys/class/pci_bus/0000:17/
97    |-- bridge -> ../../../devices/pci0000:17
98    |-- cpuaffinity
99    |-- legacy_io
100    `-- legacy_mem
101
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.
109
110Supporting PCI access on new platforms
111--------------------------------------
112
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.
117
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.
121

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