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Diffstat (limited to 'Documentation/sysfs-rules.txt')
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diff --git a/Documentation/sysfs-rules.txt b/Documentation/sysfs-rules.txt deleted file mode 100644 index ce60ffa94d2d..000000000000 --- a/Documentation/sysfs-rules.txt +++ /dev/null @@ -1,184 +0,0 @@ -Rules on how to access information in the Linux kernel 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: - o 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 - - o 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 - - o 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 - - o 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 - - o 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. |