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>

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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. As sysfs is a direct export of kernel internal
+structures, the sysfs interface can not provide a stable interface eighter,
+it may always change along with internal kernel changes.
+
+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
+  applications 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 kernels 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 a 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 have always 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-subsytem 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 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.
+