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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 | |-- irq
13 | |-- local_cpus
14 | |-- resource
15 | |-- resource0
16 | |-- resource1
17 | |-- resource2
18 | |-- rom
19 | |-- subsystem_device
20 | |-- subsystem_vendor
21 | `-- vendor
22 `-- ...
23
24The topmost element describes the PCI domain and bus number. In this case,
25the domain number is 0000 and the bus number is 17 (both values are in hex).
26This bus contains a single function device in slot 0. The domain and bus
27numbers are reproduced for convenience. Under the device directory are several
28files, each with their own function.
29
30 file function
31 ---- --------
32 class PCI class (ascii, ro)
33 config PCI config space (binary, rw)
34 device PCI device (ascii, ro)
35 irq IRQ number (ascii, ro)
36 local_cpus nearby CPU mask (cpumask, ro)
37 resource PCI resource host addresses (ascii, ro)
38 resource0..N PCI resource N, if present (binary, mmap)
39 rom PCI ROM resource, if present (binary, ro)
40 subsystem_device PCI subsystem device (ascii, ro)
41 subsystem_vendor PCI subsystem vendor (ascii, ro)
42 vendor PCI vendor (ascii, ro)
43
44 ro - read only file
45 rw - file is readable and writable
46 mmap - file is mmapable
47 ascii - file contains ascii text
48 binary - file contains binary data
49 cpumask - file contains a cpumask type
50
51The read only files are informational, writes to them will be ignored, with
52the exception of the 'rom' file. Writable files can be used to perform
53actions on the device (e.g. changing config space, detaching a device).
54mmapable files are available via an mmap of the file at offset 0 and can be
55used to do actual device programming from userspace. Note that some platforms
56don't support mmapping of certain resources, so be sure to check the return
57value from any attempted mmap.
58
59The 'rom' file is special in that it provides read-only access to the device's
60ROM file, if available. It's disabled by default, however, so applications
61should write the string "1" to the file to enable it before attempting a read
62call, and disable it following the access by writing "0" to the file.
63
64Accessing legacy resources through sysfs
65----------------------------------------
66
67Legacy I/O port and ISA memory resources are also provided in sysfs if the
68underlying platform supports them. They're located in the PCI class hierarchy,
69e.g.
70
71 /sys/class/pci_bus/0000:17/
72 |-- bridge -> ../../../devices/pci0000:17
73 |-- cpuaffinity
74 |-- legacy_io
75 `-- legacy_mem
76
77The legacy_io file is a read/write file that can be used by applications to
78do legacy port I/O. The application should open the file, seek to the desired
79port (e.g. 0x3e8) and do a read or a write of 1, 2 or 4 bytes. The legacy_mem
80file should be mmapped with an offset corresponding to the memory offset
81desired, e.g. 0xa0000 for the VGA frame buffer. The application can then
82simply dereference the returned pointer (after checking for errors of course)
83to access legacy memory space.
84
85Supporting PCI access on new platforms
86--------------------------------------
87
88In order to support PCI resource mapping as described above, Linux platform
89code must define HAVE_PCI_MMAP and provide a pci_mmap_page_range function.
90Platforms are free to only support subsets of the mmap functionality, but
91useful return codes should be provided.
92
93Legacy resources are protected by the HAVE_PCI_LEGACY define. Platforms
94wishing to support legacy functionality should define it and provide
95pci_legacy_read, pci_legacy_write and pci_mmap_legacy_page_range functions.