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diff --git a/Documentation/admin-guide/sysfs-rules.rst b/Documentation/admin-guide/sysfs-rules.rst new file mode 100644 index 000000000..abad33526 --- /dev/null +++ b/Documentation/admin-guide/sysfs-rules.rst @@ -0,0 +1,192 @@ +Rules on how to access information in 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. Therefore, there are aspects of the sysfs interface that +may not be stable across kernel releases. + +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 + application's 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: + + - 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 kernel's path to the device directory without the leading + ``/sys``, and always starting 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 + + - 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 + + - 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 + + - 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 + + - attributes + + - the files in the device directory or files below 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 always have 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 subsystem 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 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. + +- When reading and writing sysfs device attribute files, avoid dependency + on specific error codes wherever possible. This minimizes coupling to + the error handling implementation within the kernel. + + In general, failures to read or write sysfs device attributes shall + propagate errors wherever possible. Common errors include, but are not + limited to: + + ``-EIO``: The read or store operation is not supported, typically + returned by the sysfs system itself if the read or store pointer + is ``NULL``. + + ``-ENXIO``: The read or store operation failed + + Error codes will not be changed without good reason, and should a change + to error codes result in user-space breakage, it will be fixed, or the + the offending change will be reverted. + + Userspace applications can, however, expect the format and contents of + the attribute files to remain consistent in the absence of a version + attribute change in the context of a given attribute. |