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
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.
+ 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:
- o devpath (/devices/pci0000:00/0000:00:1d.1/usb2/2-2/2-2:1.0)
+ 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
+ ``/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,
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.
+ 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.
+ 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.
+ 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.
+ 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.
+ 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.
+ 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.
+ 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.
+ 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:
+ 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.
+ ``-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
- the offending change will be reverted.
+ 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.
+ 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.
projects / GitHub / moto-9609 / android_kernel_motorola_exynos9610.git / commitdiff