Documentation/sysfs-rules.txt: convert it to ReST markup

Rules on how to access information in the Linux kernel sysfs

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The kernel-exported sysfs exports internal kernel implementation details

and depends on internal kernel structures and layout. It is agreed upon

    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

- 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

    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.

    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)

    - 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

        ``/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.

      - 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, ...)

    - 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

      - applications need to handle spaces and characters like ``!`` in

        the name

    o subsystem (block, tty, pci, ...)

    - 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)

    - 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

        driver; copying the driver value in a child device context is a

        bug in the application

    o attributes

    - 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,

- 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

    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

- 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.

    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

    ``/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.

    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,

    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.

    "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

    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

    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

    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

    ``/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.

    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

    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

    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

    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

    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.

    directories live in ``/sys/devices.``

- Position of devices along device chain can change.

    Never depend on a specific parent device position in the devpath,

    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

    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

    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.

	``-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

	``-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