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Commit dbe1ab95 authored by Jeff Garzik's avatar Jeff Garzik
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Merge branch 'master' into upstream

parents 612eff0e d588fcbe
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This directory attempts to document the ABI between the Linux kernel and
userspace, and the relative stability of these interfaces.  Due to the
everchanging nature of Linux, and the differing maturity levels, these
interfaces should be used by userspace programs in different ways.

We have four different levels of ABI stability, as shown by the four
different subdirectories in this location.  Interfaces may change levels
of stability according to the rules described below.

The different levels of stability are:

  stable/
	This directory documents the interfaces that the developer has
	defined to be stable.  Userspace programs are free to use these
	interfaces with no restrictions, and backward compatibility for
	them will be guaranteed for at least 2 years.  Most interfaces
	(like syscalls) are expected to never change and always be
	available.

  testing/
	This directory documents interfaces that are felt to be stable,
	as the main development of this interface has been completed.
	The interface can be changed to add new features, but the
	current interface will not break by doing this, unless grave
	errors or security problems are found in them.  Userspace
	programs can start to rely on these interfaces, but they must be
	aware of changes that can occur before these interfaces move to
	be marked stable.  Programs that use these interfaces are
	strongly encouraged to add their name to the description of
	these interfaces, so that the kernel developers can easily
	notify them if any changes occur (see the description of the
	layout of the files below for details on how to do this.)

  obsolete/
  	This directory documents interfaces that are still remaining in
	the kernel, but are marked to be removed at some later point in
	time.  The description of the interface will document the reason
	why it is obsolete and when it can be expected to be removed.
	The file Documentation/feature-removal-schedule.txt may describe
	some of these interfaces, giving a schedule for when they will
	be removed.

  removed/
	This directory contains a list of the old interfaces that have
	been removed from the kernel.

Every file in these directories will contain the following information:

What:		Short description of the interface
Date:		Date created
KernelVersion:	Kernel version this feature first showed up in.
Contact:	Primary contact for this interface (may be a mailing list)
Description:	Long description of the interface and how to use it.
Users:		All users of this interface who wish to be notified when
		it changes.  This is very important for interfaces in
		the "testing" stage, so that kernel developers can work
		with userspace developers to ensure that things do not
		break in ways that are unacceptable.  It is also
		important to get feedback for these interfaces to make
		sure they are working in a proper way and do not need to
		be changed further.


How things move between levels:

Interfaces in stable may move to obsolete, as long as the proper
notification is given.

Interfaces may be removed from obsolete and the kernel as long as the
documented amount of time has gone by.

Interfaces in the testing state can move to the stable state when the
developers feel they are finished.  They cannot be removed from the
kernel tree without going through the obsolete state first.

It's up to the developer to place their interfaces in the category they
wish for it to start out in.
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What:		devfs
Date:		July 2005
Contact:	Greg Kroah-Hartman <gregkh@suse.de>
Description:
	devfs has been unmaintained for a number of years, has unfixable
	races, contains a naming policy within the kernel that is
	against the LSB, and can be replaced by using udev.
	The files fs/devfs/*, include/linux/devfs_fs*.h will be removed,
	along with the the assorted devfs function calls throughout the
	kernel tree.

Users:
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What:		The kernel syscall interface
Description:
	This interface matches much of the POSIX interface and is based
	on it and other Unix based interfaces.  It will only be added to
	over time, and not have things removed from it.

	Note that this interface is different for every architecture
	that Linux supports.  Please see the architecture-specific
	documentation for details on the syscall numbers that are to be
	mapped to each syscall.
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What:		/sys/module
Description:
	The /sys/module tree consists of the following structure:

	/sys/module/MODULENAME
		The name of the module that is in the kernel.  This
		module name will show up either if the module is built
		directly into the kernel, or if it is loaded as a
		dyanmic module.

	/sys/module/MODULENAME/parameters
		This directory contains individual files that are each
		individual parameters of the module that are able to be
		changed at runtime.  See the individual module
		documentation as to the contents of these parameters and
		what they accomplish.

		Note: The individual parameter names and values are not
		considered stable, only the fact that they will be
		placed in this location within sysfs.  See the
		individual driver documentation for details as to the
		stability of the different parameters.

	/sys/module/MODULENAME/refcnt
		If the module is able to be unloaded from the kernel, this file
		will contain the current reference count of the module.

		Note: If the module is built into the kernel, or if the
		CONFIG_MODULE_UNLOAD kernel configuration value is not enabled,
		this file will not be present.
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What:		/sys/class/
Date:		Febuary 2006
Contact:	Greg Kroah-Hartman <gregkh@suse.de>
Description:
		The /sys/class directory will consist of a group of
		subdirectories describing individual classes of devices
		in the kernel.  The individual directories will consist
		of either subdirectories, or symlinks to other
		directories.

		All programs that use this directory tree must be able
		to handle both subdirectories or symlinks in order to
		work properly.

Users:
	udev <linux-hotplug-devel@lists.sourceforge.net>
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