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Commit 46336009b5009e9fab3bd623a3beb9c7421545ac

Authored by Kay Sievers 2007-06-09 04:36:37 +0800

Committed by Greg Kroah-Hartman 2007-07-12 07:09:00 +0800

Exists in master and in 7 other branches

Rules on how to use sysfs in userspace programs

Here's a document to help clear things up.

Signed-off-by: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

Showing 1 changed file with 166 additions and 0 deletions Side-by-side Diff

Documentation/sysfs-rules.txt

Documentation/sysfs-rules.txt

Diff comments View file @ 4633600

	1	+Rules on how to access information in the Linux kernel sysfs
	2	+
	3	+The kernel exported sysfs exports internal kernel implementation-details
	4	+and depends on internal kernel structures and layout. It is agreed upon
	5	+by the kernel developers that the Linux kernel does not provide a stable
	6	+internal API. As sysfs is a direct export of kernel internal
	7	+structures, the sysfs interface can not provide a stable interface eighter,
	8	+it may always change along with internal kernel changes.
	9	+
	10	+To minimize the risk of breaking users of sysfs, which are in most cases
	11	+low-level userspace applications, with a new kernel release, the users
	12	+of sysfs must follow some rules to use an as abstract-as-possible way to
	13	+access this filesystem. The current udev and HAL programs already
	14	+implement this and users are encouraged to plug, if possible, into the
	15	+abstractions these programs provide instead of accessing sysfs
	16	+directly.
	17	+
	18	+But if you really do want or need to access sysfs directly, please follow
	19	+the following rules and then your programs should work with future
	20	+versions of the sysfs interface.
	21	+
	22	+- Do not use libsysfs
	23	+ It makes assumptions about sysfs which are not true. Its API does not
	24	+ offer any abstraction, it exposes all the kernel driver-core
	25	+ implementation details in its own API. Therefore it is not better than
	26	+ reading directories and opening the files yourself.
	27	+ Also, it is not actively maintained, in the sense of reflecting the
	28	+ current kernel-development. The goal of providing a stable interface
	29	+ to sysfs has failed, it causes more problems, than it solves. It
	30	+ violates many of the rules in this document.
	31	+
	32	+- sysfs is always at /sys
	33	+ Parsing /proc/mounts is a waste of time. Other mount points are a
	34	+ system configuration bug you should not try to solve. For test cases,
	35	+ possibly support a SYSFS_PATH environment variable to overwrite the
	36	+ applications behavior, but never try to search for sysfs. Never try
	37	+ to mount it, if you are not an early boot script.
	38	+
	39	+- devices are only "devices"
	40	+ There is no such thing like class-, bus-, physical devices,
	41	+ interfaces, and such that you can rely on in userspace. Everything is
	42	+ just simply a "device". Class-, bus-, physical, ... types are just
	43	+ kernel implementation details, which should not be expected by
	44	+ applications that look for devices in sysfs.
	45	+
	46	+ The properties of a device are:
	47	+ o devpath (/devices/pci0000:00/0000:00:1d.1/usb2/2-2/2-2:1.0)
	48	+ - identical to the DEVPATH value in the event sent from the kernel
	49	+ at device creation and removal
	50	+ - the unique key to the device at that point in time
	51	+ - the kernels path to the device-directory without the leading
	52	+ /sys, and always starting with with a slash
	53	+ - all elements of a devpath must be real directories. Symlinks
	54	+ pointing to /sys/devices must always be resolved to their real
	55	+ target, and the target path must be used to access the device.
	56	+ That way the devpath to the device matches the devpath of the
	57	+ kernel used at event time.
	58	+ - using or exposing symlink values as elements in a devpath string
	59	+ is a bug in the application
	60	+
	61	+ o kernel name (sda, tty, 0000:00:1f.2, ...)
	62	+ - a directory name, identical to the last element of the devpath
	63	+ - applications need to handle spaces and characters like '!' in
	64	+ the name
	65	+
	66	+ o subsystem (block, tty, pci, ...)
	67	+ - simple string, never a path or a link
	68	+ - retrieved by reading the "subsystem"-link and using only the
	69	+ last element of the target path
	70	+
	71	+ o driver (tg3, ata_piix, uhci_hcd)
	72	+ - a simple string, which may contain spaces, never a path or a
	73	+ link
	74	+ - it is retrieved by reading the "driver"-link and using only the
	75	+ last element of the target path
	76	+ - devices which do not have "driver"-link, just do not have a
	77	+ driver; copying the driver value in a child device context, is a
	78	+ bug in the application
	79	+
	80	+ o attributes
	81	+ - the files in the device directory or files below a subdirectories
	82	+ of the same device directory
	83	+ - accessing attributes reached by a symlink pointing to another device,
	84	+ like the "device"-link, is a bug in the application
	85	+
	86	+ Everything else is just a kernel driver-core implementation detail,
	87	+ that should not be assumed to be stable across kernel releases.
	88	+
	89	+- Properties of parent devices never belong into a child device.
	90	+ Always look at the parent devices themselves for determining device
	91	+ context properties. If the device 'eth0' or 'sda' does not have a
	92	+ "driver"-link, then this device does not have a driver. Its value is empty.
	93	+ Never copy any property of the parent-device into a child-device. Parent
	94	+ device-properties may change dynamically without any notice to the
	95	+ child device.
	96	+
	97	+- Hierarchy in a single device-tree
	98	+ There is only one valid place in sysfs where hierarchy can be examined
	99	+ and this is below: /sys/devices.
	100	+ It is planned, that all device directories will end up in the tree
	101	+ below this directory.
	102	+
	103	+- Classification by subsystem
	104	+ There are currently three places for classification of devices:
	105	+ /sys/block, /sys/class and /sys/bus. It is planned that these will
	106	+ not contain any device-directories themselves, but only flat lists of
	107	+ symlinks pointing to the unified /sys/devices tree.
	108	+ All three places have completely different rules on how to access
	109	+ device information. It is planned to merge all three
	110	+ classification-directories into one place at /sys/subsystem,
	111	+ following the layout of the bus-directories. All buses and
	112	+ classes, including the converted block-subsystem, will show up
	113	+ there.
	114	+ The devices belonging to a subsystem will create a symlink in the
	115	+ "devices" directory at /sys/subsystem/<name>/devices.
	116	+
	117	+ If /sys/subsystem exists, /sys/bus, /sys/class and /sys/block can be
	118	+ ignored. If it does not exist, you have always to scan all three
	119	+ places, as the kernel is free to move a subsystem from one place to
	120	+ the other, as long as the devices are still reachable by the same
	121	+ subsystem name.
	122	+
	123	+ Assuming /sys/class/<subsystem> and /sys/bus/<subsystem>, or
	124	+ /sys/block and /sys/class/block are not interchangeable, is a bug in
	125	+ the application.
	126	+
	127	+- Block
	128	+ The converted block-subsystem at /sys/class/block, or
	129	+ /sys/subsystem/block will contain the links for disks and partitions
	130	+ at the same level, never in a hierarchy. Assuming the block-subsytem to
	131	+ contain only disks and not partition-devices in the same flat list is
	132	+ a bug in the application.
	133	+
	134	+- "device"-link and <subsystem>:<kernel name>-links
	135	+ Never depend on the "device"-link. The "device"-link is a workaround
	136	+ for the old layout, where class-devices are not created in
	137	+ /sys/devices/ like the bus-devices. If the link-resolving of a
	138	+ device-directory does not end in /sys/devices/, you can use the
	139	+ "device"-link to find the parent devices in /sys/devices/. That is the
	140	+ single valid use of the "device"-link, it must never appear in any
	141	+ path as an element. Assuming the existence of the "device"-link for
	142	+ a device in /sys/devices/ is a bug in the application.
	143	+ Accessing /sys/class/net/eth0/device is a bug in the application.
	144	+
	145	+ Never depend on the class-specific links back to the /sys/class
	146	+ directory. These links are also a workaround for the design mistake
	147	+ that class-devices are not created in /sys/devices. If a device
	148	+ directory does not contain directories for child devices, these links
	149	+ may be used to find the child devices in /sys/class. That is the single
	150	+ valid use of these links, they must never appear in any path as an
	151	+ element. Assuming the existence of these links for devices which are
	152	+ real child device directories in the /sys/devices tree, is a bug in
	153	+ the application.
	154	+
	155	+ It is planned to remove all these links when when all class-device
	156	+ directories live in /sys/devices.
	157	+
	158	+- Position of devices along device chain can change.
	159	+ Never depend on a specific parent device position in the devpath,
	160	+ or the chain of parent devices. The kernel is free to insert devices into
	161	+ the chain. You must always request the parent device you are looking for
	162	+ by its subsystem value. You need to walk up the chain until you find
	163	+ the device that matches the expected subsystem. Depending on a specific
	164	+ position of a parent device, or exposing relative paths, using "../" to
	165	+ access the chain of parents, is a bug in the application.