Rules on how to access information in sysfs=========================================== The kernel-exported sysfs exports internal kernel implementation detailsand depends on internal kernel structures and layout. It is agreed uponby the kernel developers that the Linux kernel does not provide a stableinternal API. Therefore, there are aspects of the sysfs interface thatmay not be stable across kernel releases. To minimize the risk of breaking users of sysfs, which are in most caseslow-level userspace applications, with a new kernel release, the usersof sysfs must follow some rules to use an as-abstract-as-possible way toaccess this filesystem. The current udev and HAL programs alreadyimplement this and users are encouraged to plug, if possible, into theabstractions these programs provide instead of accessing sysfs directly. But if you really do want or need to access sysfs directly, please followthe following rules and then your programs should work with futureversions 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.