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1 | Rules on how to access information in the Linux kernel sysfs | |
2 | ||
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. | |
8 | ||
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. | |
15 | ||
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. | |
19 | ||
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. | |
29 | ||
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. | |
36 | ||
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. | |
43 | ||
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 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 | |
58 | ||
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 | |
63 | ||
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 | |
68 | ||
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 | |
77 | ||
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 | |
83 | ||
84 | Everything else is just a kernel driver-core implementation detail | |
85 | that should not be assumed to be stable across kernel releases. | |
86 | ||
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. | |
94 | ||
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. | |
100 | ||
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. | |
114 | ||
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. | |
120 | ||
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. | |
124 | ||
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. | |
131 | ||
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. | |
142 | ||
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. | |
152 | ||
153 | It is planned to remove all these links when all class device | |
154 | directories live in /sys/devices. | |
155 | ||
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. |