Rules on how to access information in 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:
  
      - 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 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

      - 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

      - 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

      - 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

      - 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
      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 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.

  
  - 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 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.

  
  - When reading and writing sysfs device attribute files, avoid dependency

      on specific error codes wherever possible. This minimizes coupling to
      the error handling implementation within the kernel.

      In general, failures to read or write sysfs device attributes shall
      propagate errors wherever possible. Common errors include, but are not
      limited to:

  	``-EIO``: The read or store operation is not supported, typically
  	returned by the sysfs system itself if the read or store pointer
  	is ``NULL``.

  	``-ENXIO``: The read or store operation failed

      Error codes will not be changed without good reason, and should a change
      to error codes result in user-space breakage, it will be fixed, or the
      the offending change will be reverted.

      Userspace applications can, however, expect the format and contents of
      the attribute files to remain consistent in the absence of a version
      attribute change in the context of a given attribute.