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Based on kernel version 2.6.33. Page generated on 2010-02-24 15:36 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	     |   |-- 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	
26	The topmost element describes the PCI domain and bus number.  In this case,
27	the domain number is 0000 and the bus number is 17 (both values are in hex).
28	This bus contains a single function device in slot 0.  The domain and bus
29	numbers are reproduced for convenience.  Under the device directory are several
30	files, 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)
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	The read only files are informational, writes to them will be ignored, with
58	the exception of the 'rom' file.  Writable files can be used to perform
59	actions on the device (e.g. changing config space, detaching a device).
60	mmapable files are available via an mmap of the file at offset 0 and can be
61	used to do actual device programming from userspace.  Note that some platforms
62	don't support mmapping of certain resources, so be sure to check the return
63	value from any attempted mmap.
64	
65	The 'enable' file provides a counter that indicates how many times the device 
66	has been enabled.  If the 'enable' file currently returns '4', and a '1' is
67	echoed into it, it will then return '5'.  Echoing a '0' into it will decrease
68	the count.  Even when it returns to 0, though, some of the initialisation
69	may not be reversed.  
70	
71	The 'rom' file is special in that it provides read-only access to the device's
72	ROM file, if available.  It's disabled by default, however, so applications
73	should write the string "1" to the file to enable it before attempting a read
74	call, and disable it following the access by writing "0" to the file.  Note
75	that the device must be enabled for a rom read to return data successfully.
76	In the event a driver is not bound to the device, it can be enabled using the
77	'enable' file, documented above.
78	
79	The 'remove' file is used to remove the PCI device, by writing a non-zero
80	integer to the file.  This does not involve any kind of hot-plug functionality,
81	e.g. powering off the device.  The device is removed from the kernel's list of
82	PCI devices, the sysfs directory for it is removed, and the device will be
83	removed from any drivers attached to it. Removal of PCI root buses is
84	disallowed.
85	
86	Accessing legacy resources through sysfs
87	----------------------------------------
88	
89	Legacy I/O port and ISA memory resources are also provided in sysfs if the
90	underlying platform supports them.  They're located in the PCI class hierarchy,
91	e.g.
92	
93		/sys/class/pci_bus/0000:17/
94		|-- bridge -> ../../../devices/pci0000:17
95		|-- cpuaffinity
96		|-- legacy_io
97		`-- legacy_mem
98	
99	The legacy_io file is a read/write file that can be used by applications to
100	do legacy port I/O.  The application should open the file, seek to the desired
101	port (e.g. 0x3e8) and do a read or a write of 1, 2 or 4 bytes.  The legacy_mem
102	file should be mmapped with an offset corresponding to the memory offset
103	desired, e.g. 0xa0000 for the VGA frame buffer.  The application can then
104	simply dereference the returned pointer (after checking for errors of course)
105	to access legacy memory space.
106	
107	Supporting PCI access on new platforms
108	--------------------------------------
109	
110	In order to support PCI resource mapping as described above, Linux platform
111	code must define HAVE_PCI_MMAP and provide a pci_mmap_page_range function.
112	Platforms are free to only support subsets of the mmap functionality, but
113	useful return codes should be provided.
114	
115	Legacy resources are protected by the HAVE_PCI_LEGACY define.  Platforms
116	wishing to support legacy functionality should define it and provide
117	pci_legacy_read, pci_legacy_write and pci_mmap_legacy_page_range functions.
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