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Documentation / filesystems / sysfs-pci.txt

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Based on kernel version 4.9. Page generated on 2016-12-21 14:34 EST.

1	Accessing PCI device resources through sysfs
2	--------------------------------------------
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:
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	     `-- ...
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.
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)
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
57	[1] rw for RESOURCE_IO (I/O port) regions only
59	The read only files are informational, writes to them will be ignored, with
60	the exception of the 'rom' file.  Writable files can be used to perform
61	actions on the device (e.g. changing config space, detaching a device).
62	mmapable files are available via an mmap of the file at offset 0 and can be
63	used to do actual device programming from userspace.  Note that some platforms
64	don't support mmapping of certain resources, so be sure to check the return
65	value from any attempted mmap.  The most notable of these are I/O port
66	resources, which also provide read/write access.
68	The 'enable' file provides a counter that indicates how many times the device 
69	has been enabled.  If the 'enable' file currently returns '4', and a '1' is
70	echoed into it, it will then return '5'.  Echoing a '0' into it will decrease
71	the count.  Even when it returns to 0, though, some of the initialisation
72	may not be reversed.  
74	The 'rom' file is special in that it provides read-only access to the device's
75	ROM file, if available.  It's disabled by default, however, so applications
76	should write the string "1" to the file to enable it before attempting a read
77	call, and disable it following the access by writing "0" to the file.  Note
78	that the device must be enabled for a rom read to return data successfully.
79	In the event a driver is not bound to the device, it can be enabled using the
80	'enable' file, documented above.
82	The 'remove' file is used to remove the PCI device, by writing a non-zero
83	integer to the file.  This does not involve any kind of hot-plug functionality,
84	e.g. powering off the device.  The device is removed from the kernel's list of
85	PCI devices, the sysfs directory for it is removed, and the device will be
86	removed from any drivers attached to it. Removal of PCI root buses is
87	disallowed.
89	Accessing legacy resources through sysfs
90	----------------------------------------
92	Legacy I/O port and ISA memory resources are also provided in sysfs if the
93	underlying platform supports them.  They're located in the PCI class hierarchy,
94	e.g.
96		/sys/class/pci_bus/0000:17/
97		|-- bridge -> ../../../devices/pci0000:17
98		|-- cpuaffinity
99		|-- legacy_io
100		`-- legacy_mem
102	The legacy_io file is a read/write file that can be used by applications to
103	do legacy port I/O.  The application should open the file, seek to the desired
104	port (e.g. 0x3e8) and do a read or a write of 1, 2 or 4 bytes.  The legacy_mem
105	file should be mmapped with an offset corresponding to the memory offset
106	desired, e.g. 0xa0000 for the VGA frame buffer.  The application can then
107	simply dereference the returned pointer (after checking for errors of course)
108	to access legacy memory space.
110	Supporting PCI access on new platforms
111	--------------------------------------
113	In order to support PCI resource mapping as described above, Linux platform
114	code must define HAVE_PCI_MMAP and provide a pci_mmap_page_range function.
115	Platforms are free to only support subsets of the mmap functionality, but
116	useful return codes should be provided.
118	Legacy resources are protected by the HAVE_PCI_LEGACY define.  Platforms
119	wishing to support legacy functionality should define it and provide
120	pci_legacy_read, pci_legacy_write and pci_mmap_legacy_page_range functions.
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