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| -rw-r--r-- | Documentation/sysfs-rules.txt | 166 |
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diff --git a/Documentation/sysfs-rules.txt b/Documentation/sysfs-rules.txt new file mode 100644 index 000000000000..42861bb0bc9b --- /dev/null +++ b/Documentation/sysfs-rules.txt @@ -0,0 +1,166 @@ +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. As sysfs is a direct export of kernel internal +structures, the sysfs interface can not provide a stable interface eighter, +it may always change along with internal kernel changes. + +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 + applications 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 kernels path to the device-directory without the leading + /sys, and always starting with 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 a 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 have always 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-subsytem 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 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. + |