|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. Therefore, there are aspects of the sysfs interface that
|may not be stable across kernel releases.
|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 directly.
|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
| application's 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 kernel's 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
| - 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 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
| 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 always have 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.
| 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 subsystem 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 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.