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1Rules on how to access information in the Linux kernel sysfs
2
30b1b280 3The kernel-exported sysfs exports internal kernel implementation details
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4and depends on internal kernel structures and layout. It is agreed upon
5by the kernel developers that the Linux kernel does not provide a stable
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6internal API. Therefore, there are aspects of the sysfs interface that
7may not be stable across kernel releases.
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8
9To minimize the risk of breaking users of sysfs, which are in most cases
10low-level userspace applications, with a new kernel release, the users
30b1b280 11of sysfs must follow some rules to use an as-abstract-as-possible way to
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12access this filesystem. The current udev and HAL programs already
13implement this and users are encouraged to plug, if possible, into the
30b1b280 14abstractions these programs provide instead of accessing sysfs directly.
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15
16But if you really do want or need to access sysfs directly, please follow
17the following rules and then your programs should work with future
18versions 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
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26 current kernel development. The goal of providing a stable interface
27 to sysfs has failed; it causes more problems than it solves. It
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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
30b1b280 34 application's behavior, but never try to search for sysfs. Never try
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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
30b1b280 41 kernel implementation details which should not be expected by
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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
30b1b280 49 - the kernel's path to the device directory without the leading
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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
30b1b280 53 target and the target path must be used to access the device.
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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
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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
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76 bug in the application
77
78 o attributes
30b1b280 79 - the files in the device directory or files below subdirectories
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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
30b1b280 84 Everything else is just a kernel driver-core implementation detail
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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
30b1b280 92 device properties may change dynamically without any notice to the
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93 child device.
94
30b1b280 95- Hierarchy in a single device tree
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96 There is only one valid place in sysfs where hierarchy can be examined
97 and this is below: /sys/devices.
30b1b280 98 It is planned that all device directories will end up in the tree
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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
30b1b280 104 not contain any device directories themselves, but only flat lists of
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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
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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
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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
441ee4cb 116 ignored. If it does not exist, you always have to scan all three
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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
30b1b280 122 /sys/block and /sys/class/block are not interchangeable is a bug in
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123 the application.
124
125- Block
30b1b280 126 The converted block subsystem at /sys/class/block or
46336009 127 /sys/subsystem/block will contain the links for disks and partitions
88393161 128 at the same level, never in a hierarchy. Assuming the block subsystem to
30b1b280 129 contain only disks and not partition devices in the same flat list is
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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
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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
46336009 137 "device"-link to find the parent devices in /sys/devices/. That is the
30b1b280 138 single valid use of the "device"-link; it must never appear in any
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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
30b1b280 145 that class devices are not created in /sys/devices. If a device
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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
30b1b280 148 valid use of these links; they must never appear in any path as an
46336009 149 element. Assuming the existence of these links for devices which are
30b1b280 150 real child device directories in the /sys/devices tree is a bug in
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151 the application.
152
30b1b280 153 It is planned to remove all these links when all class device
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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
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162 position of a parent device or exposing relative paths using "../" to
163 access the chain of parents is a bug in the application.