Adding upstream version 4.19.249.upstream/4.19.249upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 192 insertions, 0 deletions

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+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.