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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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+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. As sysfs is a direct export of kernel internal
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-structures, the sysfs interface can not provide a stable interface eighter,
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+structures, the sysfs interface cannot provide a stable interface either;
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it may always change along with internal kernel changes.
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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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+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
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-directly.
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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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@@ -25,22 +24,22 @@ versions of the sysfs interface.
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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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+ 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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- applications behavior, but never try to search for sysfs. Never try
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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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+ 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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@@ -48,11 +47,11 @@ versions of the sysfs interface.
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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 kernels path to the device-directory without the leading
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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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+ 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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@@ -73,17 +72,17 @@ versions of the sysfs interface.
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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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+ - 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 a subdirectories
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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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+ 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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@@ -91,25 +90,25 @@ versions of the sysfs interface.
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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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+ 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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+- 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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+ 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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+ 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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+ 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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@@ -121,38 +120,38 @@ versions of the sysfs interface.
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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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+ /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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+ 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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+ 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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+ 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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+ 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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+ 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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+ 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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+ 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 when all class-device
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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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@@ -161,6 +160,5 @@ versions of the sysfs interface.
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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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-
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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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