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Documentation / sysfs-rules.txt

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Based on kernel version 4.9. Page generated on 2016-12-21 14:37 EST.

1	Rules on how to access information in the Linux kernel sysfs
3	The kernel-exported sysfs exports internal kernel implementation details
4	and depends on internal kernel structures and layout. It is agreed upon
5	by the kernel developers that the Linux kernel does not provide a stable
6	internal API. Therefore, there are aspects of the sysfs interface that
7	may not be stable across kernel releases.
9	To minimize the risk of breaking users of sysfs, which are in most cases
10	low-level userspace applications, with a new kernel release, the users
11	of sysfs must follow some rules to use an as-abstract-as-possible way to
12	access this filesystem. The current udev and HAL programs already
13	implement this and users are encouraged to plug, if possible, into the
14	abstractions these programs provide instead of accessing sysfs directly.
16	But if you really do want or need to access sysfs directly, please follow
17	the following rules and then your programs should work with future
18	versions of the sysfs interface.
20	- Do not use libsysfs
21	  It makes assumptions about sysfs which are not true. Its API does not
22	  offer any abstraction, it exposes all the kernel driver-core
23	  implementation details in its own API. Therefore it is not better than
24	  reading directories and opening the files yourself.
25	  Also, it is not actively maintained, in the sense of reflecting the
26	  current kernel development. The goal of providing a stable interface
27	  to sysfs has failed; it causes more problems than it solves. It
28	  violates many of the rules in this document.
30	- sysfs is always at /sys
31	  Parsing /proc/mounts is a waste of time. Other mount points are a
32	  system configuration bug you should not try to solve. For test cases,
33	  possibly support a SYSFS_PATH environment variable to overwrite the
34	  application's behavior, but never try to search for sysfs. Never try
35	  to mount it, if you are not an early boot script.
37	- devices are only "devices"
38	  There is no such thing like class-, bus-, physical devices,
39	  interfaces, and such that you can rely on in userspace. Everything is
40	  just simply a "device". Class-, bus-, physical, ... types are just
41	  kernel implementation details which should not be expected by
42	  applications that look for devices in sysfs.
44	  The properties of a device are:
45	    o devpath (/devices/pci0000:00/0000:00:1d.1/usb2/2-2/2-2:1.0)
46	      - identical to the DEVPATH value in the event sent from the kernel
47	        at device creation and removal
48	      - the unique key to the device at that point in time
49	      - the kernel's path to the device directory without the leading
50	        /sys, and always starting with a slash
51	      - all elements of a devpath must be real directories. Symlinks
52	        pointing to /sys/devices must always be resolved to their real
53	        target and the target path must be used to access the device.
54	        That way the devpath to the device matches the devpath of the
55	        kernel used at event time.
56	      - using or exposing symlink values as elements in a devpath string
57	        is a bug in the application
59	    o kernel name (sda, tty, 0000:00:1f.2, ...)
60	      - a directory name, identical to the last element of the devpath
61	      - applications need to handle spaces and characters like '!' in
62	        the name
64	    o subsystem (block, tty, pci, ...)
65	      - simple string, never a path or a link
66	      - retrieved by reading the "subsystem"-link and using only the
67	        last element of the target path
69	    o driver (tg3, ata_piix, uhci_hcd)
70	      - a simple string, which may contain spaces, never a path or a
71	        link
72	      - it is retrieved by reading the "driver"-link and using only the
73	        last element of the target path
74	      - devices which do not have "driver"-link just do not have a
75	        driver; copying the driver value in a child device context is a
76	        bug in the application
78	    o attributes
79	      - the files in the device directory or files below subdirectories
80	        of the same device directory
81	      - accessing attributes reached by a symlink pointing to another device,
82	        like the "device"-link, is a bug in the application
84	  Everything else is just a kernel driver-core implementation detail
85	  that should not be assumed to be stable across kernel releases.
87	- Properties of parent devices never belong into a child device.
88	  Always look at the parent devices themselves for determining device
89	  context properties. If the device 'eth0' or 'sda' does not have a
90	  "driver"-link, then this device does not have a driver. Its value is empty.
91	  Never copy any property of the parent-device into a child-device. Parent
92	  device properties may change dynamically without any notice to the
93	  child device.
95	- Hierarchy in a single device tree
96	  There is only one valid place in sysfs where hierarchy can be examined
97	  and this is below: /sys/devices.
98	  It is planned that all device directories will end up in the tree
99	  below this directory.
101	- Classification by subsystem
102	  There are currently three places for classification of devices:
103	  /sys/block, /sys/class and /sys/bus. It is planned that these will
104	  not contain any device directories themselves, but only flat lists of
105	  symlinks pointing to the unified /sys/devices tree.
106	  All three places have completely different rules on how to access
107	  device information. It is planned to merge all three
108	  classification directories into one place at /sys/subsystem,
109	  following the layout of the bus directories. All buses and
110	  classes, including the converted block subsystem, will show up
111	  there.
112	  The devices belonging to a subsystem will create a symlink in the
113	  "devices" directory at /sys/subsystem/<name>/devices.
115	  If /sys/subsystem exists, /sys/bus, /sys/class and /sys/block can be
116	  ignored. If it does not exist, you always have to scan all three
117	  places, as the kernel is free to move a subsystem from one place to
118	  the other, as long as the devices are still reachable by the same
119	  subsystem name.
121	  Assuming /sys/class/<subsystem> and /sys/bus/<subsystem>, or
122	  /sys/block and /sys/class/block are not interchangeable is a bug in
123	  the application.
125	- Block
126	  The converted block subsystem at /sys/class/block or
127	  /sys/subsystem/block will contain the links for disks and partitions
128	  at the same level, never in a hierarchy. Assuming the block subsystem to
129	  contain only disks and not partition devices in the same flat list is
130	  a bug in the application.
132	- "device"-link and <subsystem>:<kernel name>-links
133	  Never depend on the "device"-link. The "device"-link is a workaround
134	  for the old layout, where class devices are not created in
135	  /sys/devices/ like the bus devices. If the link-resolving of a
136	  device directory does not end in /sys/devices/, you can use the
137	  "device"-link to find the parent devices in /sys/devices/. That is the
138	  single valid use of the "device"-link; it must never appear in any
139	  path as an element. Assuming the existence of the "device"-link for
140	  a device in /sys/devices/ is a bug in the application.
141	  Accessing /sys/class/net/eth0/device is a bug in the application.
143	  Never depend on the class-specific links back to the /sys/class
144	  directory.  These links are also a workaround for the design mistake
145	  that class devices are not created in /sys/devices. If a device
146	  directory does not contain directories for child devices, these links
147	  may be used to find the child devices in /sys/class. That is the single
148	  valid use of these links; they must never appear in any path as an
149	  element. Assuming the existence of these links for devices which are
150	  real child device directories in the /sys/devices tree is a bug in
151	  the application.
153	  It is planned to remove all these links when all class device
154	  directories live in /sys/devices.
156	- Position of devices along device chain can change.
157	  Never depend on a specific parent device position in the devpath,
158	  or the chain of parent devices. The kernel is free to insert devices into
159	  the chain. You must always request the parent device you are looking for
160	  by its subsystem value. You need to walk up the chain until you find
161	  the device that matches the expected subsystem. Depending on a specific
162	  position of a parent device or exposing relative paths using "../" to
163	  access the chain of parents is a bug in the application.
165	- When reading and writing sysfs device attribute files, avoid dependency
166	  on specific error codes wherever possible. This minimizes coupling to
167	  the error handling implementation within the kernel.
169	  In general, failures to read or write sysfs device attributes shall
170	  propagate errors wherever possible. Common errors include, but are not
171	  limited to:
173	  -EIO: The read or store operation is not supported, typically returned by
174	        the sysfs system itself if the read or store pointer is NULL.
176	  -ENXIO: The read or store operation failed
178	  Error codes will not be changed without good reason, and should a change
179	  to error codes result in user-space breakage, it will be fixed, or the
180	  the offending change will be reverted.
182	  Userspace applications can, however, expect the format and contents of
183	  the attribute files to remain consistent in the absence of a version
184	  attribute change in the context of a given attribute.
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