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![Nathan Lynch](/assets/img/avatar_default.png)
The first paragraph of this document implies that user space developers shouldn't use sysfs at all, but then it goes on to describe rules that developers should follow when accessing sysfs. Not only is this somewhat self-contradictory, it has been shown to discourage developers from using established sysfs interfaces. A note of caution is more appropriate than a blanket "sysfs will never be stable" assertion. Signed-off-by: Nathan Lynch <ntl@pobox.com> Cc: Stephen Rothwell <sfr@canb.auug.org.au> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
163 lines
8.1 KiB
Text
163 lines
8.1 KiB
Text
Rules on how to access information in the Linux kernel sysfs
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The kernel-exported sysfs exports internal kernel implementation details
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and depends on internal kernel structures and layout. It is agreed upon
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by the kernel developers that the Linux kernel does not provide a stable
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internal API. Therefore, there are aspects of the sysfs interface that
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may not be stable across kernel releases.
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To minimize the risk of breaking users of sysfs, which are in most cases
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low-level userspace applications, with a new kernel release, the users
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of sysfs must follow some rules to use an as-abstract-as-possible way to
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access this filesystem. The current udev and HAL programs already
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implement this and users are encouraged to plug, if possible, into the
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abstractions these programs provide instead of accessing sysfs directly.
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But if you really do want or need to access sysfs directly, please follow
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the following rules and then your programs should work with future
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versions of the sysfs interface.
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- Do not use libsysfs
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It makes assumptions about sysfs which are not true. Its API does not
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offer any abstraction, it exposes all the kernel driver-core
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implementation details in its own API. Therefore it is not better than
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reading directories and opening the files yourself.
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Also, it is not actively maintained, in the sense of reflecting the
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current kernel development. The goal of providing a stable interface
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to sysfs has failed; it causes more problems than it solves. It
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violates many of the rules in this document.
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- sysfs is always at /sys
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Parsing /proc/mounts is a waste of time. Other mount points are a
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system configuration bug you should not try to solve. For test cases,
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possibly support a SYSFS_PATH environment variable to overwrite the
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application's behavior, but never try to search for sysfs. Never try
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to mount it, if you are not an early boot script.
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- devices are only "devices"
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There is no such thing like class-, bus-, physical devices,
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interfaces, and such that you can rely on in userspace. Everything is
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just simply a "device". Class-, bus-, physical, ... types are just
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kernel implementation details which should not be expected by
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applications that look for devices in sysfs.
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The properties of a device are:
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o devpath (/devices/pci0000:00/0000:00:1d.1/usb2/2-2/2-2:1.0)
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- identical to the DEVPATH value in the event sent from the kernel
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at device creation and removal
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- the unique key to the device at that point in time
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- the kernel's path to the device directory without the leading
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/sys, and always starting with with a slash
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- all elements of a devpath must be real directories. Symlinks
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pointing to /sys/devices must always be resolved to their real
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target and the target path must be used to access the device.
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That way the devpath to the device matches the devpath of the
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kernel used at event time.
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- using or exposing symlink values as elements in a devpath string
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is a bug in the application
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o kernel name (sda, tty, 0000:00:1f.2, ...)
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- a directory name, identical to the last element of the devpath
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- applications need to handle spaces and characters like '!' in
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the name
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o subsystem (block, tty, pci, ...)
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- simple string, never a path or a link
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- retrieved by reading the "subsystem"-link and using only the
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last element of the target path
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o driver (tg3, ata_piix, uhci_hcd)
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- a simple string, which may contain spaces, never a path or a
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link
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- it is retrieved by reading the "driver"-link and using only the
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last element of the target path
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- devices which do not have "driver"-link just do not have a
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driver; copying the driver value in a child device context is a
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bug in the application
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o attributes
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- the files in the device directory or files below subdirectories
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of the same device directory
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- accessing attributes reached by a symlink pointing to another device,
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like the "device"-link, is a bug in the application
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Everything else is just a kernel driver-core implementation detail
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that should not be assumed to be stable across kernel releases.
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- Properties of parent devices never belong into a child device.
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Always look at the parent devices themselves for determining device
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context properties. If the device 'eth0' or 'sda' does not have a
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"driver"-link, then this device does not have a driver. Its value is empty.
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Never copy any property of the parent-device into a child-device. Parent
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device properties may change dynamically without any notice to the
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child device.
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- Hierarchy in a single device tree
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There is only one valid place in sysfs where hierarchy can be examined
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and this is below: /sys/devices.
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It is planned that all device directories will end up in the tree
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below this directory.
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- Classification by subsystem
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There are currently three places for classification of devices:
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/sys/block, /sys/class and /sys/bus. It is planned that these will
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not contain any device directories themselves, but only flat lists of
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symlinks pointing to the unified /sys/devices tree.
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All three places have completely different rules on how to access
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device information. It is planned to merge all three
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classification directories into one place at /sys/subsystem,
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following the layout of the bus directories. All buses and
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classes, including the converted block subsystem, will show up
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there.
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The devices belonging to a subsystem will create a symlink in the
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"devices" directory at /sys/subsystem/<name>/devices.
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If /sys/subsystem exists, /sys/bus, /sys/class and /sys/block can be
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ignored. If it does not exist, you have always to scan all three
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places, as the kernel is free to move a subsystem from one place to
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the other, as long as the devices are still reachable by the same
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subsystem name.
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Assuming /sys/class/<subsystem> and /sys/bus/<subsystem>, or
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/sys/block and /sys/class/block are not interchangeable is a bug in
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the application.
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- Block
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The converted block subsystem at /sys/class/block or
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/sys/subsystem/block will contain the links for disks and partitions
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at the same level, never in a hierarchy. Assuming the block subsytem to
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contain only disks and not partition devices in the same flat list is
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a bug in the application.
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- "device"-link and <subsystem>:<kernel name>-links
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Never depend on the "device"-link. The "device"-link is a workaround
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for the old layout, where class devices are not created in
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/sys/devices/ like the bus devices. If the link-resolving of a
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device directory does not end in /sys/devices/, you can use the
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"device"-link to find the parent devices in /sys/devices/. That is the
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single valid use of the "device"-link; it must never appear in any
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path as an element. Assuming the existence of the "device"-link for
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a device in /sys/devices/ is a bug in the application.
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Accessing /sys/class/net/eth0/device is a bug in the application.
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Never depend on the class-specific links back to the /sys/class
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directory. These links are also a workaround for the design mistake
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that class devices are not created in /sys/devices. If a device
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directory does not contain directories for child devices, these links
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may be used to find the child devices in /sys/class. That is the single
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valid use of these links; they must never appear in any path as an
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element. Assuming the existence of these links for devices which are
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real child device directories in the /sys/devices tree is a bug in
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the application.
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It is planned to remove all these links when all class device
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directories live in /sys/devices.
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- Position of devices along device chain can change.
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Never depend on a specific parent device position in the devpath,
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or the chain of parent devices. The kernel is free to insert devices into
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the chain. You must always request the parent device you are looking for
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by its subsystem value. You need to walk up the chain until you find
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the device that matches the expected subsystem. Depending on a specific
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position of a parent device or exposing relative paths using "../" to
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access the chain of parents is a bug in the application.
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