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1Rules on how to access information in sysfs2===========================================3 4The kernel-exported sysfs exports internal kernel implementation details5and depends on internal kernel structures and layout. It is agreed upon6by the kernel developers that the Linux kernel does not provide a stable7internal API. Therefore, there are aspects of the sysfs interface that8may not be stable across kernel releases.9 10To minimize the risk of breaking users of sysfs, which are in most cases11low-level userspace applications, with a new kernel release, the users12of sysfs must follow some rules to use an as-abstract-as-possible way to13access this filesystem. The current udev and HAL programs already14implement this and users are encouraged to plug, if possible, into the15abstractions these programs provide instead of accessing sysfs directly.16 17But if you really do want or need to access sysfs directly, please follow18the following rules and then your programs should work with future19versions of the sysfs interface.20 21- Do not use libsysfs22 It makes assumptions about sysfs which are not true. Its API does not23 offer any abstraction, it exposes all the kernel driver-core24 implementation details in its own API. Therefore it is not better than25 reading directories and opening the files yourself.26 Also, it is not actively maintained, in the sense of reflecting the27 current kernel development. The goal of providing a stable interface28 to sysfs has failed; it causes more problems than it solves. It29 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 a33 system configuration bug you should not try to solve. For test cases,34 possibly support a ``SYSFS_PATH`` environment variable to overwrite the35 application's behavior, but never try to search for sysfs. Never try36 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 is41 just simply a "device". Class-, bus-, physical, ... types are just42 kernel implementation details which should not be expected by43 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 kernel50 at device creation and removal51 - the unique key to the device at that point in time52 - the kernel's path to the device directory without the leading53 ``/sys``, and always starting with a slash54 - all elements of a devpath must be real directories. Symlinks55 pointing to /sys/devices must always be resolved to their real56 target and the target path must be used to access the device.57 That way the devpath to the device matches the devpath of the58 kernel used at event time.59 - using or exposing symlink values as elements in a devpath string60 is a bug in the application61 62 - kernel name (``sda``, ``tty``, ``0000:00:1f.2``, ...)63 64 - a directory name, identical to the last element of the devpath65 - applications need to handle spaces and characters like ``!`` in66 the name67 68 - subsystem (``block``, ``tty``, ``pci``, ...)69 70 - simple string, never a path or a link71 - retrieved by reading the "subsystem"-link and using only the72 last element of the target path73 74 - driver (``tg3``, ``ata_piix``, ``uhci_hcd``)75 76 - a simple string, which may contain spaces, never a path or a77 link78 - it is retrieved by reading the "driver"-link and using only the79 last element of the target path80 - devices which do not have "driver"-link just do not have a81 driver; copying the driver value in a child device context is a82 bug in the application83 84 - attributes85 86 - the files in the device directory or files below subdirectories87 of the same device directory88 - accessing attributes reached by a symlink pointing to another device,89 like the "device"-link, is a bug in the application90 91 Everything else is just a kernel driver-core implementation detail92 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 device96 context properties. If the device ``eth0`` or ``sda`` does not have a97 "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. Parent99 device properties may change dynamically without any notice to the100 child device.101 102- Hierarchy in a single device tree103 There is only one valid place in sysfs where hierarchy can be examined104 and this is below: ``/sys/devices.``105 It is planned that all device directories will end up in the tree106 below this directory.107 108- Classification by subsystem109 There are currently three places for classification of devices:110 ``/sys/block,`` ``/sys/class`` and ``/sys/bus.`` It is planned that these will111 not contain any device directories themselves, but only flat lists of112 symlinks pointing to the unified ``/sys/devices`` tree.113 All three places have completely different rules on how to access114 device information. It is planned to merge all three115 classification directories into one place at ``/sys/subsystem``,116 following the layout of the bus directories. All buses and117 classes, including the converted block subsystem, will show up118 there.119 The devices belonging to a subsystem will create a symlink in the120 "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 three124 places, as the kernel is free to move a subsystem from one place to125 the other, as long as the devices are still reachable by the same126 subsystem name.127 128 Assuming ``/sys/class/<subsystem>`` and ``/sys/bus/<subsystem>``, or129 ``/sys/block`` and ``/sys/class/block`` are not interchangeable is a bug in130 the application.131 132- Block133 The converted block subsystem at ``/sys/class/block`` or134 ``/sys/subsystem/block`` will contain the links for disks and partitions135 at the same level, never in a hierarchy. Assuming the block subsystem to136 contain only disks and not partition devices in the same flat list is137 a bug in the application.138 139- "device"-link and <subsystem>:<kernel name>-links140 Never depend on the "device"-link. The "device"-link is a workaround141 for the old layout, where class devices are not created in142 ``/sys/devices/`` like the bus devices. If the link-resolving of a143 device directory does not end in ``/sys/devices/``, you can use the144 "device"-link to find the parent devices in ``/sys/devices/``, That is the145 single valid use of the "device"-link; it must never appear in any146 path as an element. Assuming the existence of the "device"-link for147 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 mistake152 that class devices are not created in ``/sys/devices.`` If a device153 directory does not contain directories for child devices, these links154 may be used to find the child devices in ``/sys/class.`` That is the single155 valid use of these links; they must never appear in any path as an156 element. Assuming the existence of these links for devices which are157 real child device directories in the ``/sys/devices`` tree is a bug in158 the application.159 160 It is planned to remove all these links when all class device161 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 into166 the chain. You must always request the parent device you are looking for167 by its subsystem value. You need to walk up the chain until you find168 the device that matches the expected subsystem. Depending on a specific169 position of a parent device or exposing relative paths using ``../`` to170 access the chain of parents is a bug in the application.171 172- When reading and writing sysfs device attribute files, avoid dependency173 on specific error codes wherever possible. This minimizes coupling to174 the error handling implementation within the kernel.175 176 In general, failures to read or write sysfs device attributes shall177 propagate errors wherever possible. Common errors include, but are not178 limited to:179 180 ``-EIO``: The read or store operation is not supported, typically181 returned by the sysfs system itself if the read or store pointer182 is ``NULL``.183 184 ``-ENXIO``: The read or store operation failed185 186 Error codes will not be changed without good reason, and should a change187 to error codes result in user-space breakage, it will be fixed, or the188 the offending change will be reverted.189 190 Userspace applications can, however, expect the format and contents of191 the attribute files to remain consistent in the absence of a version192 attribute change in the context of a given attribute.193