1Rules on how to access information in sysfs
2===========================================
3
4The kernel-exported sysfs exports internal kernel implementation details
5and depends on internal kernel structures and layout. It is agreed upon
6by the kernel developers that the Linux kernel does not provide a stable
7internal API. Therefore, there are aspects of the sysfs interface that
8may not be stable across kernel releases.
9
10To minimize the risk of breaking users of sysfs, which are in most cases
11low-level userspace applications, with a new kernel release, the users
12of sysfs must follow some rules to use an as-abstract-as-possible way to
13access this filesystem. The current udev and HAL programs already
14implement this and users are encouraged to plug, if possible, into the
15abstractions these programs provide instead of accessing sysfs directly.
16
17But if you really do want or need to access sysfs directly, please follow
18the following rules and then your programs should work with future
19versions of the sysfs interface.
20
21- Do not use libsysfs
22    It makes assumptions about sysfs which are not true. Its API does not
23    offer any abstraction, it exposes all the kernel driver-core
24    implementation details in its own API. Therefore it is not better than
25    reading directories and opening the files yourself.
26    Also, it is not actively maintained, in the sense of reflecting the
27    current kernel development. The goal of providing a stable interface
28    to sysfs has failed; it causes more problems than it solves. It
29    violates many of the rules in this document.
30
31- sysfs is always at ``/sys``
32    Parsing ``/proc/mounts`` is a waste of time. Other mount points are a
33    system configuration bug you should not try to solve. For test cases,
34    possibly support a ``SYSFS_PATH`` environment variable to overwrite the
35    application's behavior, but never try to search for sysfs. Never try
36    to mount it, if you are not an early boot script.
37
38- devices are only "devices"
39    There is no such thing like class-, bus-, physical devices,
40    interfaces, and such that you can rely on in userspace. Everything is
41    just simply a "device". Class-, bus-, physical, ... types are just
42    kernel implementation details which should not be expected by
43    applications that look for devices in sysfs.
44
45    The properties of a device are:
46
47    - devpath (``/devices/pci0000:00/0000:00:1d.1/usb2/2-2/2-2:1.0``)
48
49      - identical to the DEVPATH value in the event sent from the kernel
50        at device creation and removal
51      - the unique key to the device at that point in time
52      - the kernel's path to the device directory without the leading
53        ``/sys``, and always starting with a slash
54      - all elements of a devpath must be real directories. Symlinks
55        pointing to /sys/devices must always be resolved to their real
56        target and the target path must be used to access the device.
57        That way the devpath to the device matches the devpath of the
58        kernel used at event time.
59      - using or exposing symlink values as elements in a devpath string
60        is a bug in the application
61
62    - kernel name (``sda``, ``tty``, ``0000:00:1f.2``, ...)
63
64      - a directory name, identical to the last element of the devpath
65      - applications need to handle spaces and characters like ``!`` in
66        the name
67
68    - subsystem (``block``, ``tty``, ``pci``, ...)
69
70      - simple string, never a path or a link
71      - retrieved by reading the "subsystem"-link and using only the
72        last element of the target path
73
74    - driver (``tg3``, ``ata_piix``, ``uhci_hcd``)
75
76      - a simple string, which may contain spaces, never a path or a
77        link
78      - it is retrieved by reading the "driver"-link and using only the
79        last element of the target path
80      - devices which do not have "driver"-link just do not have a
81        driver; copying the driver value in a child device context is a
82        bug in the application
83
84    - attributes
85
86      - the files in the device directory or files below subdirectories
87        of the same device directory
88      - accessing attributes reached by a symlink pointing to another device,
89        like the "device"-link, is a bug in the application
90
91    Everything else is just a kernel driver-core implementation detail
92    that should not be assumed to be stable across kernel releases.
93
94- Properties of parent devices never belong into a child device.
95    Always look at the parent devices themselves for determining device
96    context properties. If the device ``eth0`` or ``sda`` does not have a
97    "driver"-link, then this device does not have a driver. Its value is empty.
98    Never copy any property of the parent-device into a child-device. Parent
99    device properties may change dynamically without any notice to the
100    child device.
101
102- Hierarchy in a single device tree
103    There is only one valid place in sysfs where hierarchy can be examined
104    and this is below: ``/sys/devices.``
105    It is planned that all device directories will end up in the tree
106    below this directory.
107
108- Classification by subsystem
109    There are currently three places for classification of devices:
110    ``/sys/block,`` ``/sys/class`` and ``/sys/bus.`` It is planned that these will
111    not contain any device directories themselves, but only flat lists of
112    symlinks pointing to the unified ``/sys/devices`` tree.
113    All three places have completely different rules on how to access
114    device information. It is planned to merge all three
115    classification directories into one place at ``/sys/subsystem``,
116    following the layout of the bus directories. All buses and
117    classes, including the converted block subsystem, will show up
118    there.
119    The devices belonging to a subsystem will create a symlink in the
120    "devices" directory at ``/sys/subsystem/<name>/devices``,
121
122    If ``/sys/subsystem`` exists, ``/sys/bus``, ``/sys/class`` and ``/sys/block``
123    can be ignored. If it does not exist, you always have to scan all three
124    places, as the kernel is free to move a subsystem from one place to
125    the other, as long as the devices are still reachable by the same
126    subsystem name.
127
128    Assuming ``/sys/class/<subsystem>`` and ``/sys/bus/<subsystem>``, or
129    ``/sys/block`` and ``/sys/class/block`` are not interchangeable is a bug in
130    the application.
131
132- Block
133    The converted block subsystem at ``/sys/class/block`` or
134    ``/sys/subsystem/block`` will contain the links for disks and partitions
135    at the same level, never in a hierarchy. Assuming the block subsystem to
136    contain only disks and not partition devices in the same flat list is
137    a bug in the application.
138
139- "device"-link and <subsystem>:<kernel name>-links
140    Never depend on the "device"-link. The "device"-link is a workaround
141    for the old layout, where class devices are not created in
142    ``/sys/devices/`` like the bus devices. If the link-resolving of a
143    device directory does not end in ``/sys/devices/``, you can use the
144    "device"-link to find the parent devices in ``/sys/devices/``, That is the
145    single valid use of the "device"-link; it must never appear in any
146    path as an element. Assuming the existence of the "device"-link for
147    a device in ``/sys/devices/`` is a bug in the application.
148    Accessing ``/sys/class/net/eth0/device`` is a bug in the application.
149
150    Never depend on the class-specific links back to the ``/sys/class``
151    directory.  These links are also a workaround for the design mistake
152    that class devices are not created in ``/sys/devices.`` If a device
153    directory does not contain directories for child devices, these links
154    may be used to find the child devices in ``/sys/class.`` That is the single
155    valid use of these links; they must never appear in any path as an
156    element. Assuming the existence of these links for devices which are
157    real child device directories in the ``/sys/devices`` tree is a bug in
158    the application.
159
160    It is planned to remove all these links when all class device
161    directories live in ``/sys/devices.``
162
163- Position of devices along device chain can change.
164    Never depend on a specific parent device position in the devpath,
165    or the chain of parent devices. The kernel is free to insert devices into
166    the chain. You must always request the parent device you are looking for
167    by its subsystem value. You need to walk up the chain until you find
168    the device that matches the expected subsystem. Depending on a specific
169    position of a parent device or exposing relative paths using ``../`` to
170    access the chain of parents is a bug in the application.
171
172- When reading and writing sysfs device attribute files, avoid dependency
173    on specific error codes wherever possible. This minimizes coupling to
174    the error handling implementation within the kernel.
175
176    In general, failures to read or write sysfs device attributes shall
177    propagate errors wherever possible. Common errors include, but are not
178    limited to:
179
180	``-EIO``: The read or store operation is not supported, typically
181	returned by the sysfs system itself if the read or store pointer
182	is ``NULL``.
183
184	``-ENXIO``: The read or store operation failed
185
186    Error codes will not be changed without good reason, and should a change
187    to error codes result in user-space breakage, it will be fixed, or the
188    the offending change will be reverted.
189
190    Userspace applications can, however, expect the format and contents of
191    the attribute files to remain consistent in the absence of a version
192    attribute change in the context of a given attribute.
193