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  	why it is obsolete and when it can be expected to be removed.

  
    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.

  
  
  Notable bits of non-ABI, which should not under any circumstances be considered
  stable:
  
  - Kconfig.  Userspace should not rely on the presence or absence of any
    particular Kconfig symbol, in /proc/config.gz, in the copy of .config
    commonly installed to /boot, or in any invocation of the kernel build
    process.
  
  - Kernel-internal symbols.  Do not rely on the presence, absence, location, or
    type of any kernel symbol, either in System.map files or the kernel binary
    itself.  See Documentation/stable_api_nonsense.txt.