Documentation/sysfs-rules.txt - kernel/quantenna - Git at Google

 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 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.

 - 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.
	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 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.

	- 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.