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| -rw-r--r-- | Documentation/sysfs-rules.txt | 230 |
1 files changed, 119 insertions, 111 deletions
diff --git a/Documentation/sysfs-rules.txt b/Documentation/sysfs-rules.txt index ce60ffa94d2d..04bdd52cba1d 100644 --- a/Documentation/sysfs-rules.txt +++ b/Documentation/sysfs-rules.txt @@ -1,4 +1,5 @@ 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 @@ -18,36 +19,38 @@ 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. + 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) + 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 + ``/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. @@ -56,17 +59,20 @@ versions of the sysfs interface. - 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, ...) + - 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 + - applications need to handle spaces and characters like ``!`` in the name - o subsystem (block, tty, pci, ...) + - 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) + - 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 @@ -75,110 +81,112 @@ versions of the sysfs interface. driver; copying the driver value in a child device context is a bug in the application - o attributes + - 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. + 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. + 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. + 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. + 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. + 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. + 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. + 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. + 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: + 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. + ``-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 + ``-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. + 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. + 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. |