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Documentation / filesystems / sysfs-pci.txt

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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:55 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	     |   |-- revision
21	     |   |-- rom
22	     |   |-- subsystem_device
23	     |   |-- subsystem_vendor
24	     |   `-- vendor
25	     `-- ...
27	The topmost element describes the PCI domain and bus number.  In this case,
28	the domain number is 0000 and the bus number is 17 (both values are in hex).
29	This bus contains a single function device in slot 0.  The domain and bus
30	numbers are reproduced for convenience.  Under the device directory are several
31	files, each with their own function.
33	       file		   function
34	       ----		   --------
35	       class		   PCI class (ascii, ro)
36	       config		   PCI config space (binary, rw)
37	       device		   PCI device (ascii, ro)
38	       enable	           Whether the device is enabled (ascii, rw)
39	       irq		   IRQ number (ascii, ro)
40	       local_cpus	   nearby CPU mask (cpumask, ro)
41	       remove		   remove device from kernel's list (ascii, wo)
42	       resource		   PCI resource host addresses (ascii, ro)
43	       resource0..N	   PCI resource N, if present (binary, mmap, rw[1])
44	       resource0_wc..N_wc  PCI WC map resource N, if prefetchable (binary, mmap)
45	       revision		   PCI revision (ascii, ro)
46	       rom		   PCI ROM resource, if present (binary, ro)
47	       subsystem_device	   PCI subsystem device (ascii, ro)
48	       subsystem_vendor	   PCI subsystem vendor (ascii, ro)
49	       vendor		   PCI vendor (ascii, ro)
51	  ro - read only file
52	  rw - file is readable and writable
53	  wo - write only file
54	  mmap - file is mmapable
55	  ascii - file contains ascii text
56	  binary - file contains binary data
57	  cpumask - file contains a cpumask type
59	[1] rw for RESOURCE_IO (I/O port) regions only
61	The read only files are informational, writes to them will be ignored, with
62	the exception of the 'rom' file.  Writable files can be used to perform
63	actions on the device (e.g. changing config space, detaching a device).
64	mmapable files are available via an mmap of the file at offset 0 and can be
65	used to do actual device programming from userspace.  Note that some platforms
66	don't support mmapping of certain resources, so be sure to check the return
67	value from any attempted mmap.  The most notable of these are I/O port
68	resources, which also provide read/write access.
70	The 'enable' file provides a counter that indicates how many times the device 
71	has been enabled.  If the 'enable' file currently returns '4', and a '1' is
72	echoed into it, it will then return '5'.  Echoing a '0' into it will decrease
73	the count.  Even when it returns to 0, though, some of the initialisation
74	may not be reversed.  
76	The 'rom' file is special in that it provides read-only access to the device's
77	ROM file, if available.  It's disabled by default, however, so applications
78	should write the string "1" to the file to enable it before attempting a read
79	call, and disable it following the access by writing "0" to the file.  Note
80	that the device must be enabled for a rom read to return data successfully.
81	In the event a driver is not bound to the device, it can be enabled using the
82	'enable' file, documented above.
84	The 'remove' file is used to remove the PCI device, by writing a non-zero
85	integer to the file.  This does not involve any kind of hot-plug functionality,
86	e.g. powering off the device.  The device is removed from the kernel's list of
87	PCI devices, the sysfs directory for it is removed, and the device will be
88	removed from any drivers attached to it. Removal of PCI root buses is
89	disallowed.
91	Accessing legacy resources through sysfs
92	----------------------------------------
94	Legacy I/O port and ISA memory resources are also provided in sysfs if the
95	underlying platform supports them.  They're located in the PCI class hierarchy,
96	e.g.
98		/sys/class/pci_bus/0000:17/
99		|-- bridge -> ../../../devices/pci0000:17
100		|-- cpuaffinity
101		|-- legacy_io
102		`-- legacy_mem
104	The legacy_io file is a read/write file that can be used by applications to
105	do legacy port I/O.  The application should open the file, seek to the desired
106	port (e.g. 0x3e8) and do a read or a write of 1, 2 or 4 bytes.  The legacy_mem
107	file should be mmapped with an offset corresponding to the memory offset
108	desired, e.g. 0xa0000 for the VGA frame buffer.  The application can then
109	simply dereference the returned pointer (after checking for errors of course)
110	to access legacy memory space.
112	Supporting PCI access on new platforms
113	--------------------------------------
115	In order to support PCI resource mapping as described above, Linux platform
116	code should ideally define ARCH_GENERIC_PCI_MMAP_RESOURCE and use the generic
117	implementation of that functionality. To support the historical interface of
118	mmap() through files in /proc/bus/pci, platforms may also set HAVE_PCI_MMAP.
120	Alternatively, platforms which set HAVE_PCI_MMAP may provide their own
121	implementation of pci_mmap_page_range() instead of defining
124	Platforms which support write-combining maps of PCI resources must define
125	arch_can_pci_mmap_wc() which shall evaluate to non-zero at runtime when
126	write-combining is permitted. Platforms which support maps of I/O resources
127	define arch_can_pci_mmap_io() similarly.
129	Legacy resources are protected by the HAVE_PCI_LEGACY define.  Platforms
130	wishing to support legacy functionality should define it and provide
131	pci_legacy_read, pci_legacy_write and pci_mmap_legacy_page_range functions.
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