1Rules on how to access information in sysfs 2=========================================== 3 4The kernel-exported sysfs exports internal kernel implementation details 5and depends on internal kernel structures and layout. It is agreed upon 6by the kernel developers that the Linux kernel does not provide a stable 7internal API. Therefore, there are aspects of the sysfs interface that 8may not be stable across kernel releases. 9 10To minimize the risk of breaking users of sysfs, which are in most cases 11low-level userspace applications, with a new kernel release, the users 12of sysfs must follow some rules to use an as-abstract-as-possible way to 13access this filesystem. The current udev and HAL programs already 14implement this and users are encouraged to plug, if possible, into the 15abstractions these programs provide instead of accessing sysfs directly. 16 17But if you really do want or need to access sysfs directly, please follow 18the following rules and then your programs should work with future 19versions of the sysfs interface. 20 21- Do not use libsysfs 22 It makes assumptions about sysfs which are not true. Its API does not 23 offer any abstraction, it exposes all the kernel driver-core 24 implementation details in its own API. Therefore it is not better than 25 reading directories and opening the files yourself. 26 Also, it is not actively maintained, in the sense of reflecting the 27 current kernel development. The goal of providing a stable interface 28 to sysfs has failed; it causes more problems than it solves. It 29 violates many of the rules in this document. 30 31- sysfs is always at ``/sys`` 32 Parsing ``/proc/mounts`` is a waste of time. Other mount points are a 33 system configuration bug you should not try to solve. For test cases, 34 possibly support a ``SYSFS_PATH`` environment variable to overwrite the 35 application's behavior, but never try to search for sysfs. Never try 36 to mount it, if you are not an early boot script. 37 38- devices are only "devices" 39 There is no such thing like class-, bus-, physical devices, 40 interfaces, and such that you can rely on in userspace. Everything is 41 just simply a "device". Class-, bus-, physical, ... types are just 42 kernel implementation details which should not be expected by 43 applications that look for devices in sysfs. 44 45 The properties of a device are: 46 47 - devpath (``/devices/pci0000:00/0000:00:1d.1/usb2/2-2/2-2:1.0``) 48 49 - identical to the DEVPATH value in the event sent from the kernel 50 at device creation and removal 51 - the unique key to the device at that point in time 52 - the kernel's path to the device directory without the leading 53 ``/sys``, and always starting with a slash 54 - all elements of a devpath must be real directories. Symlinks 55 pointing to /sys/devices must always be resolved to their real 56 target and the target path must be used to access the device. 57 That way the devpath to the device matches the devpath of the 58 kernel used at event time. 59 - using or exposing symlink values as elements in a devpath string 60 is a bug in the application 61 62 - kernel name (``sda``, ``tty``, ``0000:00:1f.2``, ...) 63 64 - a directory name, identical to the last element of the devpath 65 - applications need to handle spaces and characters like ``!`` in 66 the name 67 68 - subsystem (``block``, ``tty``, ``pci``, ...) 69 70 - simple string, never a path or a link 71 - retrieved by reading the "subsystem"-link and using only the 72 last element of the target path 73 74 - driver (``tg3``, ``ata_piix``, ``uhci_hcd``) 75 76 - a simple string, which may contain spaces, never a path or a 77 link 78 - it is retrieved by reading the "driver"-link and using only the 79 last element of the target path 80 - devices which do not have "driver"-link just do not have a 81 driver; copying the driver value in a child device context is a 82 bug in the application 83 84 - attributes 85 86 - the files in the device directory or files below subdirectories 87 of the same device directory 88 - accessing attributes reached by a symlink pointing to another device, 89 like the "device"-link, is a bug in the application 90 91 Everything else is just a kernel driver-core implementation detail 92 that should not be assumed to be stable across kernel releases. 93 94- Properties of parent devices never belong into a child device. 95 Always look at the parent devices themselves for determining device 96 context properties. If the device ``eth0`` or ``sda`` does not have a 97 "driver"-link, then this device does not have a driver. Its value is empty. 98 Never copy any property of the parent-device into a child-device. Parent 99 device properties may change dynamically without any notice to the 100 child device. 101 102- Hierarchy in a single device tree 103 There is only one valid place in sysfs where hierarchy can be examined 104 and this is below: ``/sys/devices.`` 105 It is planned that all device directories will end up in the tree 106 below this directory. 107 108- Classification by subsystem 109 There are currently three places for classification of devices: 110 ``/sys/block,`` ``/sys/class`` and ``/sys/bus.`` It is planned that these will 111 not contain any device directories themselves, but only flat lists of 112 symlinks pointing to the unified ``/sys/devices`` tree. 113 All three places have completely different rules on how to access 114 device information. It is planned to merge all three 115 classification directories into one place at ``/sys/subsystem``, 116 following the layout of the bus directories. All buses and 117 classes, including the converted block subsystem, will show up 118 there. 119 The devices belonging to a subsystem will create a symlink in the 120 "devices" directory at ``/sys/subsystem/<name>/devices``, 121 122 If ``/sys/subsystem`` exists, ``/sys/bus``, ``/sys/class`` and ``/sys/block`` 123 can be ignored. If it does not exist, you always have to scan all three 124 places, as the kernel is free to move a subsystem from one place to 125 the other, as long as the devices are still reachable by the same 126 subsystem name. 127 128 Assuming ``/sys/class/<subsystem>`` and ``/sys/bus/<subsystem>``, or 129 ``/sys/block`` and ``/sys/class/block`` are not interchangeable is a bug in 130 the application. 131 132- Block 133 The converted block subsystem at ``/sys/class/block`` or 134 ``/sys/subsystem/block`` will contain the links for disks and partitions 135 at the same level, never in a hierarchy. Assuming the block subsystem to 136 contain only disks and not partition devices in the same flat list is 137 a bug in the application. 138 139- "device"-link and <subsystem>:<kernel name>-links 140 Never depend on the "device"-link. The "device"-link is a workaround 141 for the old layout, where class devices are not created in 142 ``/sys/devices/`` like the bus devices. If the link-resolving of a 143 device directory does not end in ``/sys/devices/``, you can use the 144 "device"-link to find the parent devices in ``/sys/devices/``, That is the 145 single valid use of the "device"-link; it must never appear in any 146 path as an element. Assuming the existence of the "device"-link for 147 a device in ``/sys/devices/`` is a bug in the application. 148 Accessing ``/sys/class/net/eth0/device`` is a bug in the application. 149 150 Never depend on the class-specific links back to the ``/sys/class`` 151 directory. These links are also a workaround for the design mistake 152 that class devices are not created in ``/sys/devices.`` If a device 153 directory does not contain directories for child devices, these links 154 may be used to find the child devices in ``/sys/class.`` That is the single 155 valid use of these links; they must never appear in any path as an 156 element. Assuming the existence of these links for devices which are 157 real child device directories in the ``/sys/devices`` tree is a bug in 158 the application. 159 160 It is planned to remove all these links when all class device 161 directories live in ``/sys/devices.`` 162 163- Position of devices along device chain can change. 164 Never depend on a specific parent device position in the devpath, 165 or the chain of parent devices. The kernel is free to insert devices into 166 the chain. You must always request the parent device you are looking for 167 by its subsystem value. You need to walk up the chain until you find 168 the device that matches the expected subsystem. Depending on a specific 169 position of a parent device or exposing relative paths using ``../`` to 170 access the chain of parents is a bug in the application. 171 172- When reading and writing sysfs device attribute files, avoid dependency 173 on specific error codes wherever possible. This minimizes coupling to 174 the error handling implementation within the kernel. 175 176 In general, failures to read or write sysfs device attributes shall 177 propagate errors wherever possible. Common errors include, but are not 178 limited to: 179 180 ``-EIO``: The read or store operation is not supported, typically 181 returned by the sysfs system itself if the read or store pointer 182 is ``NULL``. 183 184 ``-ENXIO``: The read or store operation failed 185 186 Error codes will not be changed without good reason, and should a change 187 to error codes result in user-space breakage, it will be fixed, or the 188 the offending change will be reverted. 189 190 Userspace applications can, however, expect the format and contents of 191 the attribute files to remain consistent in the absence of a version 192 attribute change in the context of a given attribute. 193