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1 | Rules on how to access information in sysfs |
2 | =========================================== | |
46336009 | 3 | |
30b1b280 | 4 | The kernel-exported sysfs exports internal kernel implementation details |
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5 | and depends on internal kernel structures and layout. It is agreed upon |
6 | by the kernel developers that the Linux kernel does not provide a stable | |
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7 | internal API. Therefore, there are aspects of the sysfs interface that |
8 | may not be stable across kernel releases. | |
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9 | |
10 | To minimize the risk of breaking users of sysfs, which are in most cases | |
11 | low-level userspace applications, with a new kernel release, the users | |
30b1b280 | 12 | of sysfs must follow some rules to use an as-abstract-as-possible way to |
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13 | access this filesystem. The current udev and HAL programs already |
14 | implement this and users are encouraged to plug, if possible, into the | |
30b1b280 | 15 | abstractions these programs provide instead of accessing sysfs directly. |
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16 | |
17 | But if you really do want or need to access sysfs directly, please follow | |
18 | the following rules and then your programs should work with future | |
19 | versions of the sysfs interface. | |
20 | ||
21 | - Do not use libsysfs | |
3177ae4a MCC |
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. | |
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37 | |
38 | - devices are only "devices" | |
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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 | ||
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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 | |
30b1b280 | 52 | - the kernel's path to the device directory without the leading |
3177ae4a | 53 | ``/sys``, and always starting with a slash |
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54 | - all elements of a devpath must be real directories. Symlinks |
55 | pointing to /sys/devices must always be resolved to their real | |
30b1b280 | 56 | target and the target path must be used to access the device. |
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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 | ||
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62 | - kernel name (``sda``, ``tty``, ``0000:00:1f.2``, ...) |
63 | ||
46336009 | 64 | - a directory name, identical to the last element of the devpath |
3177ae4a | 65 | - applications need to handle spaces and characters like ``!`` in |
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66 | the name |
67 | ||
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68 | - subsystem (``block``, ``tty``, ``pci``, ...) |
69 | ||
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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 | ||
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74 | - driver (``tg3``, ``ata_piix``, ``uhci_hcd``) |
75 | ||
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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 | |
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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 | |
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82 | bug in the application |
83 | ||
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84 | - attributes |
85 | ||
30b1b280 | 86 | - the files in the device directory or files below subdirectories |
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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 | ||
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91 | Everything else is just a kernel driver-core implementation detail |
92 | that should not be assumed to be stable across kernel releases. | |
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93 | |
94 | - Properties of parent devices never belong into a child device. | |
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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. | |
46336009 | 101 | |
30b1b280 | 102 | - Hierarchy in a single device tree |
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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. | |
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107 | |
108 | - Classification by subsystem | |
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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. | |
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131 | |
132 | - Block | |
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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. | |
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138 | |
139 | - "device"-link and <subsystem>:<kernel name>-links | |
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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.`` | |
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162 | |
163 | - Position of devices along device chain can change. | |
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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. | |
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171 | |
172 | - When reading and writing sysfs device attribute files, avoid dependency | |
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173 | on specific error codes wherever possible. This minimizes coupling to |
174 | the error handling implementation within the kernel. | |
00e262fd | 175 | |
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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: | |
00e262fd | 179 | |
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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``. | |
00e262fd | 183 | |
3177ae4a | 184 | ``-ENXIO``: The read or store operation failed |
00e262fd | 185 | |
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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. | |
00e262fd | 189 | |
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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. |