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Documentation / filesystems / sysfs-pci.txt

Based on kernel version 2.6.26. Page generated on 2008-07-16 21:12 EST.

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 hierarchy,
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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