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authorKay Sievers <kay.sievers@vrfy.org>2007-06-08 16:36:37 -0400
committerGreg Kroah-Hartman <gregkh@suse.de>2007-07-11 19:09:00 -0400
commit46336009b5009e9fab3bd623a3beb9c7421545ac (patch)
treeafb86b1d6a0c774b6e8a911e018f8743d9504d24 /Documentation/sysfs-rules.txt
parent4eb6bf6bfb580afaf1e1a1d30cba17a078530cf4 (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>
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1Rules on how to access information in the Linux kernel sysfs
2
3The kernel exported sysfs exports internal kernel implementation-details
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
6internal API. As sysfs is a direct export of kernel internal
7structures, the sysfs interface can not provide a stable interface eighter,
8it may always change along with internal kernel changes.
9
10To minimize the risk of breaking users of sysfs, which are in most cases
11low-level userspace applications, with a new kernel release, the users
12of sysfs must follow some rules to use an as abstract-as-possible way to
13access this filesystem. The current udev and HAL programs already
14implement this and users are encouraged to plug, if possible, into the
15abstractions these programs provide instead of accessing sysfs
16directly.
17
18But if you really do want or need to access sysfs directly, please follow
19the following rules and then your programs should work with future
20versions 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