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authorMauro Carvalho Chehab <mchehab@s-opensource.com>2016-09-21 09:51:11 -0300
committerMauro Carvalho Chehab <mchehab@s-opensource.com>2016-10-24 08:12:35 -0200
commit9d85025b0418163fae079c9ba8f8445212de8568 (patch)
tree4629e2dedf4a9ed45a6855c129101f9b52138372 /Documentation/sysfs-rules.txt
parent186128f75392f8478ad1b32a675627d738881ca4 (diff)
docs-rst: create an user's manual book
Place README, REPORTING-BUGS, SecurityBugs and kernel-parameters on an user's manual book. As we'll be numbering the user's manual, remove the manual numbering from SecurityBugs. Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
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-Rules on how to access 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
-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.

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