Rules on how to use sysfs in userspace programs

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

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

  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.