linux/Documentation/sysfs-rules.txt
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   1Rules on how to access information in the Linux kernel sysfs
   2
   3The kernel-exported sysfs exports internal kernel implementation details
   4and depends on internal kernel structures and layout. It is agreed upon
   5by the kernel developers that the Linux kernel does not provide a stable
   6internal API. Therefore, there are aspects of the sysfs interface that
   7may not be stable across kernel releases.
   8
   9To minimize the risk of breaking users of sysfs, which are in most cases
  10low-level userspace applications, with a new kernel release, the users
  11of sysfs must follow some rules to use an as-abstract-as-possible way to
  12access this filesystem. The current udev and HAL programs already
  13implement this and users are encouraged to plug, if possible, into the
  14abstractions these programs provide instead of accessing sysfs directly.
  15
  16But if you really do want or need to access sysfs directly, please follow
  17the following rules and then your programs should work with future
  18versions 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 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 subsytem 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.
 164
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