path: root/Documentation/sysfs-rules.txt
diff options
authorKay Sievers <>2007-06-08 13:36:37 -0700
committerGreg Kroah-Hartman <>2007-07-11 16:09:00 -0700
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 <> Signed-off-by: Greg Kroah-Hartman <>
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+Rules on how to access information in the Linux kernel sysfs
+The kernel exported sysfs exports internal kernel implementation-details
+and depends on internal kernel structures and layout. It is agreed upon
+by the kernel developers that the Linux kernel does not provide a stable
+internal API. As sysfs is a direct export of kernel internal
+structures, the sysfs interface can not provide a stable interface eighter,
+it may always change along with internal kernel changes.
+To minimize the risk of breaking users of sysfs, which are in most cases
+low-level userspace applications, with a new kernel release, the users
+of sysfs must follow some rules to use an as abstract-as-possible way to
+access this filesystem. The current udev and HAL programs already
+implement this and users are encouraged to plug, if possible, into the
+abstractions these programs provide instead of accessing sysfs
+But if you really do want or need to access sysfs directly, please follow
+the following rules and then your programs should work with future
+versions of the sysfs interface.
+- Do not use libsysfs
+ It makes assumptions about sysfs which are not true. Its API does not
+ offer any abstraction, it exposes all the kernel driver-core
+ implementation details in its own API. Therefore it is not better than
+ reading directories and opening the files yourself.
+ Also, it is not actively maintained, in the sense of reflecting the
+ current kernel-development. The goal of providing a stable interface
+ to sysfs has failed, it causes more problems, than it solves. It
+ violates many of the rules in this document.
+- sysfs is always at /sys
+ Parsing /proc/mounts is a waste of time. Other mount points are a
+ system configuration bug you should not try to solve. For test cases,
+ possibly support a SYSFS_PATH environment variable to overwrite the
+ applications behavior, but never try to search for sysfs. Never try
+ to mount it, if you are not an early boot script.
+- devices are only "devices"
+ There is no such thing like class-, bus-, physical devices,
+ interfaces, and such that you can rely on in userspace. Everything is
+ just simply a "device". Class-, bus-, physical, ... types are just
+ kernel implementation details, which should not be expected by
+ applications that look for devices in sysfs.
+ The properties of a device are:
+ o devpath (/devices/pci0000:00/0000:00:1d.1/usb2/2-2/2-2:1.0)
+ - identical to the DEVPATH value in the event sent from the kernel
+ at device creation and removal
+ - the unique key to the device at that point in time
+ - the kernels path to the device-directory without the leading
+ /sys, and always starting with with a slash
+ - all elements of a devpath must be real directories. Symlinks
+ pointing to /sys/devices must always be resolved to their real
+ target, and the target path must be used to access the device.
+ That way the devpath to the device matches the devpath of the
+ kernel used at event time.
+ - using or exposing symlink values as elements in a devpath string
+ is a bug in the application
+ o kernel name (sda, tty, 0000:00:1f.2, ...)
+ - a directory name, identical to the last element of the devpath
+ - applications need to handle spaces and characters like '!' in
+ the name
+ o subsystem (block, tty, pci, ...)
+ - simple string, never a path or a link
+ - retrieved by reading the "subsystem"-link and using only the
+ last element of the target path
+ o driver (tg3, ata_piix, uhci_hcd)
+ - a simple string, which may contain spaces, never a path or a
+ link
+ - it is retrieved by reading the "driver"-link and using only the
+ last element of the target path
+ - devices which do not have "driver"-link, just do not have a
+ driver; copying the driver value in a child device context, is a
+ bug in the application
+ o attributes
+ - the files in the device directory or files below a subdirectories
+ of the same device directory
+ - accessing attributes reached by a symlink pointing to another device,
+ like the "device"-link, is a bug in the application
+ Everything else is just a kernel driver-core implementation detail,
+ that should not be assumed to be stable across kernel releases.
+- Properties of parent devices never belong into a child device.
+ Always look at the parent devices themselves for determining device
+ context properties. If the device 'eth0' or 'sda' does not have a
+ "driver"-link, then this device does not have a driver. Its value is empty.
+ Never copy any property of the parent-device into a child-device. Parent
+ device-properties may change dynamically without any notice to the
+ child device.
+- Hierarchy in a single device-tree
+ There is only one valid place in sysfs where hierarchy can be examined
+ and this is below: /sys/devices.
+ It is planned, that all device directories will end up in the tree
+ below this directory.
+- Classification by subsystem
+ There are currently three places for classification of devices:
+ /sys/block, /sys/class and /sys/bus. It is planned that these will
+ not contain any device-directories themselves, but only flat lists of
+ symlinks pointing to the unified /sys/devices tree.
+ All three places have completely different rules on how to access
+ device information. It is planned to merge all three
+ classification-directories into one place at /sys/subsystem,
+ following the layout of the bus-directories. All buses and
+ classes, including the converted block-subsystem, will show up
+ there.
+ The devices belonging to a subsystem will create a symlink in the
+ "devices" directory at /sys/subsystem/<name>/devices.
+ If /sys/subsystem exists, /sys/bus, /sys/class and /sys/block can be
+ ignored. If it does not exist, you have always to scan all three
+ places, as the kernel is free to move a subsystem from one place to
+ the other, as long as the devices are still reachable by the same
+ subsystem name.
+ Assuming /sys/class/<subsystem> and /sys/bus/<subsystem>, or
+ /sys/block and /sys/class/block are not interchangeable, is a bug in
+ the application.
+- Block
+ The converted block-subsystem at /sys/class/block, or
+ /sys/subsystem/block will contain the links for disks and partitions
+ at the same level, never in a hierarchy. Assuming the block-subsytem to
+ contain only disks and not partition-devices in the same flat list is
+ a bug in the application.
+- "device"-link and <subsystem>:<kernel name>-links
+ Never depend on the "device"-link. The "device"-link is a workaround
+ for the old layout, where class-devices are not created in
+ /sys/devices/ like the bus-devices. If the link-resolving of a
+ device-directory does not end in /sys/devices/, you can use the
+ "device"-link to find the parent devices in /sys/devices/. That is the
+ single valid use of the "device"-link, it must never appear in any
+ path as an element. Assuming the existence of the "device"-link for
+ a device in /sys/devices/ is a bug in the application.
+ Accessing /sys/class/net/eth0/device is a bug in the application.
+ Never depend on the class-specific links back to the /sys/class
+ directory. These links are also a workaround for the design mistake
+ that class-devices are not created in /sys/devices. If a device
+ directory does not contain directories for child devices, these links
+ may be used to find the child devices in /sys/class. That is the single
+ valid use of these links, they must never appear in any path as an
+ element. Assuming the existence of these links for devices which are
+ real child device directories in the /sys/devices tree, is a bug in
+ the application.
+ It is planned to remove all these links when when all class-device
+ directories live in /sys/devices.
+- Position of devices along device chain can change.
+ Never depend on a specific parent device position in the devpath,
+ or the chain of parent devices. The kernel is free to insert devices into
+ the chain. You must always request the parent device you are looking for
+ by its subsystem value. You need to walk up the chain until you find
+ the device that matches the expected subsystem. Depending on a specific
+ position of a parent device, or exposing relative paths, using "../" to
+ access the chain of parents, is a bug in the application.