diff options
author | Kay Sievers <kay.sievers@vrfy.org> | 2007-06-08 16:36:37 -0400 |
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committer | Greg Kroah-Hartman <gregkh@suse.de> | 2007-07-11 19:09:00 -0400 |
commit | 46336009b5009e9fab3bd623a3beb9c7421545ac (patch) | |
tree | afb86b1d6a0c774b6e8a911e018f8743d9504d24 /Documentation/sysfs-rules.txt | |
parent | 4eb6bf6bfb580afaf1e1a1d30cba17a078530cf4 (diff) |
Rules on how to use sysfs in userspace programs
Here's a document to help clear things up.
Signed-off-by: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Diffstat (limited to 'Documentation/sysfs-rules.txt')
-rw-r--r-- | Documentation/sysfs-rules.txt | 166 |
1 files changed, 166 insertions, 0 deletions
diff --git a/Documentation/sysfs-rules.txt b/Documentation/sysfs-rules.txt new file mode 100644 index 00000000000..42861bb0bc9 --- /dev/null +++ b/Documentation/sysfs-rules.txt | |||
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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. As sysfs is a direct export of kernel internal | ||
7 | structures, the sysfs interface can not provide a stable interface eighter, | ||
8 | it may always change along with internal kernel changes. | ||
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 | ||
12 | of sysfs must follow some rules to use an as abstract-as-possible way to | ||
13 | access this filesystem. The current udev and HAL programs already | ||
14 | implement this and users are encouraged to plug, if possible, into the | ||
15 | abstractions these programs provide instead of accessing sysfs | ||
16 | directly. | ||
17 | |||
18 | But if you really do want or need to access sysfs directly, please follow | ||
19 | the following rules and then your programs should work with future | ||
20 | versions of the sysfs interface. | ||
21 | |||
22 | - Do not use libsysfs | ||
23 | It makes assumptions about sysfs which are not true. Its API does not | ||
24 | offer any abstraction, it exposes all the kernel driver-core | ||
25 | implementation details in its own API. Therefore it is not better than | ||
26 | reading directories and opening the files yourself. | ||
27 | Also, it is not actively maintained, in the sense of reflecting the | ||
28 | current kernel-development. The goal of providing a stable interface | ||
29 | to sysfs has failed, it causes more problems, than it solves. It | ||
30 | violates many of the rules in this document. | ||
31 | |||
32 | - sysfs is always at /sys | ||
33 | Parsing /proc/mounts is a waste of time. Other mount points are a | ||
34 | system configuration bug you should not try to solve. For test cases, | ||
35 | possibly support a SYSFS_PATH environment variable to overwrite the | ||
36 | applications behavior, but never try to search for sysfs. Never try | ||
37 | to mount it, if you are not an early boot script. | ||
38 | |||
39 | - devices are only "devices" | ||
40 | There is no such thing like class-, bus-, physical devices, | ||
41 | interfaces, and such that you can rely on in userspace. Everything is | ||
42 | just simply a "device". Class-, bus-, physical, ... types are just | ||
43 | kernel implementation details, which should not be expected by | ||
44 | applications that look for devices in sysfs. | ||
45 | |||
46 | The properties of a device are: | ||
47 | o devpath (/devices/pci0000:00/0000:00:1d.1/usb2/2-2/2-2:1.0) | ||
48 | - identical to the DEVPATH value in the event sent from the kernel | ||
49 | at device creation and removal | ||
50 | - the unique key to the device at that point in time | ||
51 | - the kernels path to the device-directory without the leading | ||
52 | /sys, and always starting with with a slash | ||
53 | - all elements of a devpath must be real directories. Symlinks | ||
54 | pointing to /sys/devices must always be resolved to their real | ||
55 | target, and the target path must be used to access the device. | ||
56 | That way the devpath to the device matches the devpath of the | ||
57 | kernel used at event time. | ||
58 | - using or exposing symlink values as elements in a devpath string | ||
59 | is a bug in the application | ||
60 | |||
61 | o kernel name (sda, tty, 0000:00:1f.2, ...) | ||
62 | - a directory name, identical to the last element of the devpath | ||
63 | - applications need to handle spaces and characters like '!' in | ||
64 | the name | ||
65 | |||
66 | o subsystem (block, tty, pci, ...) | ||
67 | - simple string, never a path or a link | ||
68 | - retrieved by reading the "subsystem"-link and using only the | ||
69 | last element of the target path | ||
70 | |||
71 | o driver (tg3, ata_piix, uhci_hcd) | ||
72 | - a simple string, which may contain spaces, never a path or a | ||
73 | link | ||
74 | - it is retrieved by reading the "driver"-link and using only the | ||
75 | last element of the target path | ||
76 | - devices which do not have "driver"-link, just do not have a | ||
77 | driver; copying the driver value in a child device context, is a | ||
78 | bug in the application | ||
79 | |||
80 | o attributes | ||
81 | - the files in the device directory or files below a subdirectories | ||
82 | of the same device directory | ||
83 | - accessing attributes reached by a symlink pointing to another device, | ||
84 | like the "device"-link, is a bug in the application | ||
85 | |||
86 | Everything else is just a kernel driver-core implementation detail, | ||
87 | that should not be assumed to be stable across kernel releases. | ||
88 | |||
89 | - Properties of parent devices never belong into a child device. | ||
90 | Always look at the parent devices themselves for determining device | ||
91 | context properties. If the device 'eth0' or 'sda' does not have a | ||
92 | "driver"-link, then this device does not have a driver. Its value is empty. | ||
93 | Never copy any property of the parent-device into a child-device. Parent | ||
94 | device-properties may change dynamically without any notice to the | ||
95 | child device. | ||
96 | |||
97 | - Hierarchy in a single device-tree | ||
98 | There is only one valid place in sysfs where hierarchy can be examined | ||
99 | and this is below: /sys/devices. | ||
100 | It is planned, that all device directories will end up in the tree | ||
101 | below this directory. | ||
102 | |||
103 | - Classification by subsystem | ||
104 | There are currently three places for classification of devices: | ||
105 | /sys/block, /sys/class and /sys/bus. It is planned that these will | ||
106 | not contain any device-directories themselves, but only flat lists of | ||
107 | symlinks pointing to the unified /sys/devices tree. | ||
108 | All three places have completely different rules on how to access | ||
109 | device information. It is planned to merge all three | ||
110 | classification-directories into one place at /sys/subsystem, | ||
111 | following the layout of the bus-directories. All buses and | ||
112 | classes, including the converted block-subsystem, will show up | ||
113 | there. | ||
114 | The devices belonging to a subsystem will create a symlink in the | ||
115 | "devices" directory at /sys/subsystem/<name>/devices. | ||
116 | |||
117 | If /sys/subsystem exists, /sys/bus, /sys/class and /sys/block can be | ||
118 | ignored. If it does not exist, you have always to scan all three | ||
119 | places, as the kernel is free to move a subsystem from one place to | ||
120 | the other, as long as the devices are still reachable by the same | ||
121 | subsystem name. | ||
122 | |||
123 | Assuming /sys/class/<subsystem> and /sys/bus/<subsystem>, or | ||
124 | /sys/block and /sys/class/block are not interchangeable, is a bug in | ||
125 | the application. | ||
126 | |||
127 | - Block | ||
128 | The converted block-subsystem at /sys/class/block, or | ||
129 | /sys/subsystem/block will contain the links for disks and partitions | ||
130 | at the same level, never in a hierarchy. Assuming the block-subsytem to | ||
131 | contain only disks and not partition-devices in the same flat list is | ||
132 | a bug in the application. | ||
133 | |||
134 | - "device"-link and <subsystem>:<kernel name>-links | ||
135 | Never depend on the "device"-link. The "device"-link is a workaround | ||
136 | for the old layout, where class-devices are not created in | ||
137 | /sys/devices/ like the bus-devices. If the link-resolving of a | ||
138 | device-directory does not end in /sys/devices/, you can use the | ||
139 | "device"-link to find the parent devices in /sys/devices/. That is the | ||
140 | single valid use of the "device"-link, it must never appear in any | ||
141 | path as an element. Assuming the existence of the "device"-link for | ||
142 | a device in /sys/devices/ is a bug in the application. | ||
143 | Accessing /sys/class/net/eth0/device is a bug in the application. | ||
144 | |||
145 | Never depend on the class-specific links back to the /sys/class | ||
146 | directory. These links are also a workaround for the design mistake | ||
147 | that class-devices are not created in /sys/devices. If a device | ||
148 | directory does not contain directories for child devices, these links | ||
149 | may be used to find the child devices in /sys/class. That is the single | ||
150 | valid use of these links, they must never appear in any path as an | ||
151 | element. Assuming the existence of these links for devices which are | ||
152 | real child device directories in the /sys/devices tree, is a bug in | ||
153 | the application. | ||
154 | |||
155 | It is planned to remove all these links when when all class-device | ||
156 | directories live in /sys/devices. | ||
157 | |||
158 | - Position of devices along device chain can change. | ||
159 | Never depend on a specific parent device position in the devpath, | ||
160 | or the chain of parent devices. The kernel is free to insert devices into | ||
161 | the chain. You must always request the parent device you are looking for | ||
162 | by its subsystem value. You need to walk up the chain until you find | ||
163 | the device that matches the expected subsystem. Depending on a specific | ||
164 | position of a parent device, or exposing relative paths, using "../" to | ||
165 | access the chain of parents, is a bug in the application. | ||
166 | |||