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