1 Accessing PCI device resources through sysfs
2 --------------------------------------------
3
4 sysfs, usually mounted at /sys, provides access to PCI resources on platforms
5 that 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
24 The topmost element describes the PCI domain and bus number. In this case,
25 the domain number is 0000 and the bus number is 17 (both values are in hex).
26 This bus contains a single function device in slot 0. The domain and bus
27 numbers are reproduced for convenience. Under the device directory are several
28 files, 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
51 The read only files are informational, writes to them will be ignored, with
52 the exception of the 'rom' file. Writable files can be used to perform
53 actions on the device (e.g. changing config space, detaching a device).
54 mmapable files are available via an mmap of the file at offset 0 and can be
55 used to do actual device programming from userspace. Note that some platforms
56 don't support mmapping of certain resources, so be sure to check the return
57 value from any attempted mmap.
58
59 The 'rom' file is special in that it provides read-only access to the device's
60 ROM file, if available. It's disabled by default, however, so applications
61 should write the string "1" to the file to enable it before attempting a read
62 call, and disable it following the access by writing "0" to the file.
63
64 Accessing legacy resources through sysfs
65 ----------------------------------------
66
67 Legacy I/O port and ISA memory resources are also provided in sysfs if the
68 underlying platform supports them. They're located in the PCI class heirarchy,
69 e.g.
70
71 /sys/class/pci_bus/0000:17/
72 |-- bridge -> ../../../devices/pci0000:17
73 |-- cpuaffinity
74 |-- legacy_io
75 `-- legacy_mem
76
77 The legacy_io file is a read/write file that can be used by applications to
78 do legacy port I/O. The application should open the file, seek to the desired
79 port (e.g. 0x3e8) and do a read or a write of 1, 2 or 4 bytes. The legacy_mem
80 file should be mmapped with an offset corresponding to the memory offset
81 desired, e.g. 0xa0000 for the VGA frame buffer. The application can then
82 simply dereference the returned pointer (after checking for errors of course)
83 to access legacy memory space.
84
85 Supporting PCI access on new platforms
86 --------------------------------------
87
88 In order to support PCI resource mapping as described above, Linux platform
89 code must define HAVE_PCI_MMAP and provide a pci_mmap_page_range function.
90 Platforms are free to only support subsets of the mmap functionality, but
91 useful return codes should be provided.
92
93 Legacy resources are protected by the HAVE_PCI_LEGACY define. Platforms
94 wishing to support legacy functionality should define it and provide
95 pci_legacy_read, pci_legacy_write and pci_mmap_legacy_page_range functions.
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