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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:
- o 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 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
-
- o 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
-
- o 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)
- - 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
-
- o 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.