Documentation/sysfs-rules.txt

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