Rules on how to use sysfs in userspace programs

[Greg KH]

Over time there have been a number of problems when sysfs has changed in
"unexpected" ways.  Here's a document that Kay wrote a while ago that
I'd like to add to the kernel Documentation directory to help userspace
programmers out.

Any comments or critique of this is greatly appreciated.

thanks,

greg k-h

------------------------------------

Rules to access device-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
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't provide a stable interface too, 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 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 both programs provide instead of accessing sysfs
directly.

But if you really do want to access sysfs, please follow the following
rules and then your programs should work for all future versions of
sysfs.

- 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 SYSFS_PATH 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 handle devices.

  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
      - the "bus"-link is identical to the "subsystem"-link, it is
	planned to remove the "bus"-link

    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 don't have "driver"-link, just don't 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 subdirectory
	of the same device directory
      - exposing attributes reached by a symlink of 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 it does not have a driver. It's value is empty.
  Never copy the value 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, the 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 to access the
  information.  It is planned to merge all three
  classification-directories into one place at /sys/subsystem/,
  following the current layout of the bus-directories. All buses and
  classes, including the converted block-subsystemm, will show up
  there.
  The devices of 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 doesn't 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 /sys/block to be
  there, is a bug in the application.  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's 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. Exposing /sys/class/net/eth0/device is a bug in the
  application. The "device"-link must be resolved to the real
  device-path it points to, if you look for the parent.

  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's 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 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.  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.
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