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-rw-r--r--Documentation/sysfs-rules.txt72
1 files changed, 35 insertions, 37 deletions
diff --git a/Documentation/sysfs-rules.txt b/Documentation/sysfs-rules.txt
index 42861bb0bc9b..80ef562160bb 100644
--- a/Documentation/sysfs-rules.txt
+++ b/Documentation/sysfs-rules.txt
@@ -1,19 +1,18 @@
1Rules on how to access information in the Linux kernel sysfs 1Rules on how to access information in the Linux kernel sysfs
2 2
3The kernel exported sysfs exports internal kernel implementation-details 3The kernel-exported sysfs exports internal kernel implementation details
4and depends on internal kernel structures and layout. It is agreed upon 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 5by the kernel developers that the Linux kernel does not provide a stable
6internal API. As sysfs is a direct export of kernel internal 6internal API. As sysfs is a direct export of kernel internal
7structures, the sysfs interface can not provide a stable interface eighter, 7structures, the sysfs interface cannot provide a stable interface either;
8it may always change along with internal kernel changes. 8it may always change along with internal kernel changes.
9 9
10To minimize the risk of breaking users of sysfs, which are in most cases 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 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 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 13access this filesystem. The current udev and HAL programs already
14implement this and users are encouraged to plug, if possible, into the 14implement this and users are encouraged to plug, if possible, into the
15abstractions these programs provide instead of accessing sysfs 15abstractions these programs provide instead of accessing sysfs directly.
16directly.
17 16
18But if you really do want or need to access sysfs directly, please follow 17But if you really do want or need to access sysfs directly, please follow
19the following rules and then your programs should work with future 18the following rules and then your programs should work with future
@@ -25,22 +24,22 @@ versions of the sysfs interface.
25 implementation details in its own API. Therefore it is not better than 24 implementation details in its own API. Therefore it is not better than
26 reading directories and opening the files yourself. 25 reading directories and opening the files yourself.
27 Also, it is not actively maintained, in the sense of reflecting the 26 Also, it is not actively maintained, in the sense of reflecting the
28 current kernel-development. The goal of providing a stable interface 27 current kernel development. The goal of providing a stable interface
29 to sysfs has failed, it causes more problems, than it solves. It 28 to sysfs has failed; it causes more problems than it solves. It
30 violates many of the rules in this document. 29 violates many of the rules in this document.
31 30
32- sysfs is always at /sys 31- sysfs is always at /sys
33 Parsing /proc/mounts is a waste of time. Other mount points are a 32 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, 33 system configuration bug you should not try to solve. For test cases,
35 possibly support a SYSFS_PATH environment variable to overwrite the 34 possibly support a SYSFS_PATH environment variable to overwrite the
36 applications behavior, but never try to search for sysfs. Never try 35 application's behavior, but never try to search for sysfs. Never try
37 to mount it, if you are not an early boot script. 36 to mount it, if you are not an early boot script.
38 37
39- devices are only "devices" 38- devices are only "devices"
40 There is no such thing like class-, bus-, physical devices, 39 There is no such thing like class-, bus-, physical devices,
41 interfaces, and such that you can rely on in userspace. Everything is 40 interfaces, and such that you can rely on in userspace. Everything is
42 just simply a "device". Class-, bus-, physical, ... types are just 41 just simply a "device". Class-, bus-, physical, ... types are just
43 kernel implementation details, which should not be expected by 42 kernel implementation details which should not be expected by
44 applications that look for devices in sysfs. 43 applications that look for devices in sysfs.
45 44
46 The properties of a device are: 45 The properties of a device are:
@@ -48,11 +47,11 @@ versions of the sysfs interface.
48 - identical to the DEVPATH value in the event sent from the kernel 47 - identical to the DEVPATH value in the event sent from the kernel
49 at device creation and removal 48 at device creation and removal
50 - the unique key to the device at that point in time 49 - the unique key to the device at that point in time
51 - the kernels path to the device-directory without the leading 50 - the kernel's path to the device directory without the leading
52 /sys, and always starting with with a slash 51 /sys, and always starting with with a slash
53 - all elements of a devpath must be real directories. Symlinks 52 - all elements of a devpath must be real directories. Symlinks
54 pointing to /sys/devices must always be resolved to their real 53 pointing to /sys/devices must always be resolved to their real
55 target, and the target path must be used to access the device. 54 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 55 That way the devpath to the device matches the devpath of the
57 kernel used at event time. 56 kernel used at event time.
58 - using or exposing symlink values as elements in a devpath string 57 - using or exposing symlink values as elements in a devpath string
@@ -73,17 +72,17 @@ versions of the sysfs interface.
73 link 72 link
74 - it is retrieved by reading the "driver"-link and using only the 73 - it is retrieved by reading the "driver"-link and using only the
75 last element of the target path 74 last element of the target path
76 - devices which do not have "driver"-link, just do not have a 75 - 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 76 driver; copying the driver value in a child device context is a
78 bug in the application 77 bug in the application
79 78
80 o attributes 79 o attributes
81 - the files in the device directory or files below a subdirectories 80 - the files in the device directory or files below subdirectories
82 of the same device directory 81 of the same device directory
83 - accessing attributes reached by a symlink pointing to another device, 82 - accessing attributes reached by a symlink pointing to another device,
84 like the "device"-link, is a bug in the application 83 like the "device"-link, is a bug in the application
85 84
86 Everything else is just a kernel driver-core implementation detail, 85 Everything else is just a kernel driver-core implementation detail
87 that should not be assumed to be stable across kernel releases. 86 that should not be assumed to be stable across kernel releases.
88 87
89- Properties of parent devices never belong into a child device. 88- Properties of parent devices never belong into a child device.
@@ -91,25 +90,25 @@ versions of the sysfs interface.
91 context properties. If the device 'eth0' or 'sda' does not have a 90 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. 91 "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 92 Never copy any property of the parent-device into a child-device. Parent
94 device-properties may change dynamically without any notice to the 93 device properties may change dynamically without any notice to the
95 child device. 94 child device.
96 95
97- Hierarchy in a single device-tree 96- Hierarchy in a single device tree
98 There is only one valid place in sysfs where hierarchy can be examined 97 There is only one valid place in sysfs where hierarchy can be examined
99 and this is below: /sys/devices. 98 and this is below: /sys/devices.
100 It is planned, that all device directories will end up in the tree 99 It is planned that all device directories will end up in the tree
101 below this directory. 100 below this directory.
102 101
103- Classification by subsystem 102- Classification by subsystem
104 There are currently three places for classification of devices: 103 There are currently three places for classification of devices:
105 /sys/block, /sys/class and /sys/bus. It is planned that these will 104 /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 105 not contain any device directories themselves, but only flat lists of
107 symlinks pointing to the unified /sys/devices tree. 106 symlinks pointing to the unified /sys/devices tree.
108 All three places have completely different rules on how to access 107 All three places have completely different rules on how to access
109 device information. It is planned to merge all three 108 device information. It is planned to merge all three
110 classification-directories into one place at /sys/subsystem, 109 classification directories into one place at /sys/subsystem,
111 following the layout of the bus-directories. All buses and 110 following the layout of the bus directories. All buses and
112 classes, including the converted block-subsystem, will show up 111 classes, including the converted block subsystem, will show up
113 there. 112 there.
114 The devices belonging to a subsystem will create a symlink in the 113 The devices belonging to a subsystem will create a symlink in the
115 "devices" directory at /sys/subsystem/<name>/devices. 114 "devices" directory at /sys/subsystem/<name>/devices.
@@ -121,38 +120,38 @@ versions of the sysfs interface.
121 subsystem name. 120 subsystem name.
122 121
123 Assuming /sys/class/<subsystem> and /sys/bus/<subsystem>, or 122 Assuming /sys/class/<subsystem> and /sys/bus/<subsystem>, or
124 /sys/block and /sys/class/block are not interchangeable, is a bug in 123 /sys/block and /sys/class/block are not interchangeable is a bug in
125 the application. 124 the application.
126 125
127- Block 126- Block
128 The converted block-subsystem at /sys/class/block, or 127 The converted block subsystem at /sys/class/block or
129 /sys/subsystem/block will contain the links for disks and partitions 128 /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 129 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 130 contain only disks and not partition devices in the same flat list is
132 a bug in the application. 131 a bug in the application.
133 132
134- "device"-link and <subsystem>:<kernel name>-links 133- "device"-link and <subsystem>:<kernel name>-links
135 Never depend on the "device"-link. The "device"-link is a workaround 134 Never depend on the "device"-link. The "device"-link is a workaround
136 for the old layout, where class-devices are not created in 135 for the old layout, where class devices are not created in
137 /sys/devices/ like the bus-devices. If the link-resolving of a 136 /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 137 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 138 "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 139 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 140 path as an element. Assuming the existence of the "device"-link for
142 a device in /sys/devices/ is a bug in the application. 141 a device in /sys/devices/ is a bug in the application.
143 Accessing /sys/class/net/eth0/device is a bug in the application. 142 Accessing /sys/class/net/eth0/device is a bug in the application.
144 143
145 Never depend on the class-specific links back to the /sys/class 144 Never depend on the class-specific links back to the /sys/class
146 directory. These links are also a workaround for the design mistake 145 directory. These links are also a workaround for the design mistake
147 that class-devices are not created in /sys/devices. If a device 146 that class devices are not created in /sys/devices. If a device
148 directory does not contain directories for child devices, these links 147 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 148 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 149 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 150 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 151 real child device directories in the /sys/devices tree is a bug in
153 the application. 152 the application.
154 153
155 It is planned to remove all these links when when all class-device 154 It is planned to remove all these links when all class device
156 directories live in /sys/devices. 155 directories live in /sys/devices.
157 156
158- Position of devices along device chain can change. 157- Position of devices along device chain can change.
@@ -161,6 +160,5 @@ versions of the sysfs interface.
161 the chain. You must always request the parent device you are looking for 160 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 161 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 162 the device that matches the expected subsystem. Depending on a specific
164 position of a parent device, or exposing relative paths, using "../" to 163 position of a parent device or exposing relative paths using "../" to
165 access the chain of parents, is a bug in the application. 164 access the chain of parents is a bug in the application.
166