The Linux Kernel Driver Interface
  (all of your questions answered and then some)
  
  Greg Kroah-Hartman <greg@kroah.com>
  
  This is being written to try to explain why Linux does not have a binary
  kernel interface, nor does it have a stable kernel interface.  Please
  realize that this article describes the _in kernel_ interfaces, not the
  kernel to userspace interfaces.  The kernel to userspace interface is
  the one that application programs use, the syscall interface.  That
  interface is _very_ stable over time, and will not break.  I have old
  programs that were built on a pre 0.9something kernel that still work
  just fine on the latest 2.6 kernel release.  That interface is the one
  that users and application programmers can count on being stable.
  
  
  Executive Summary
  -----------------
  You think you want a stable kernel interface, but you really do not, and
  you don't even know it.  What you want is a stable running driver, and
  you get that only if your driver is in the main kernel tree.  You also
  get lots of other good benefits if your driver is in the main kernel
  tree, all of which has made Linux into such a strong, stable, and mature
  operating system which is the reason you are using it in the first
  place.
  
  
  Intro
  -----
  
  It's only the odd person who wants to write a kernel driver that needs
  to worry about the in-kernel interfaces changing.  For the majority of
  the world, they neither see this interface, nor do they care about it at
  all.
  
  First off, I'm not going to address _any_ legal issues about closed
  source, hidden source, binary blobs, source wrappers, or any other term
  that describes kernel drivers that do not have their source code
  released under the GPL.  Please consult a lawyer if you have any legal
  questions, I'm a programmer and hence, I'm just going to be describing
  the technical issues here (not to make light of the legal issues, they
  are real, and you do need to be aware of them at all times.)
  
  So, there are two main topics here, binary kernel interfaces and stable
  kernel source interfaces.  They both depend on each other, but we will
  discuss the binary stuff first to get it out of the way.
  
  
  Binary Kernel Interface
  -----------------------
  Assuming that we had a stable kernel source interface for the kernel, a
  binary interface would naturally happen too, right?  Wrong.  Please
  consider the following facts about the Linux kernel:
    - Depending on the version of the C compiler you use, different kernel
      data structures will contain different alignment of structures, and
      possibly include different functions in different ways (putting
      functions inline or not.)  The individual function organization
      isn't that important, but the different data structure padding is
      very important.
    - Depending on what kernel build options you select, a wide range of
      different things can be assumed by the kernel:
        - different structures can contain different fields
        - Some functions may not be implemented at all, (i.e. some locks
  	compile away to nothing for non-SMP builds.)
        - Memory within the kernel can be aligned in different ways,
  	depending on the build options.
    - Linux runs on a wide range of different processor architectures.
      There is no way that binary drivers from one architecture will run
      on another architecture properly.
  
  Now a number of these issues can be addressed by simply compiling your
  module for the exact specific kernel configuration, using the same exact
  C compiler that the kernel was built with.  This is sufficient if you
  want to provide a module for a specific release version of a specific
  Linux distribution.  But multiply that single build by the number of
  different Linux distributions and the number of different supported
  releases of the Linux distribution and you quickly have a nightmare of
  different build options on different releases.  Also realize that each
  Linux distribution release contains a number of different kernels, all
  tuned to different hardware types (different processor types and
  different options), so for even a single release you will need to create
  multiple versions of your module.
  
  Trust me, you will go insane over time if you try to support this kind
  of release, I learned this the hard way a long time ago...
  
  
  Stable Kernel Source Interfaces
  -------------------------------
  
  This is a much more "volatile" topic if you talk to people who try to
  keep a Linux kernel driver that is not in the main kernel tree up to
  date over time.
  
  Linux kernel development is continuous and at a rapid pace, never
  stopping to slow down.  As such, the kernel developers find bugs in
  current interfaces, or figure out a better way to do things.  If they do
  that, they then fix the current interfaces to work better.  When they do
  so, function names may change, structures may grow or shrink, and
  function parameters may be reworked.  If this happens, all of the
  instances of where this interface is used within the kernel are fixed up
  at the same time, ensuring that everything continues to work properly.
  
  As a specific examples of this, the in-kernel USB interfaces have
  undergone at least three different reworks over the lifetime of this
  subsystem.  These reworks were done to address a number of different
  issues:
    - A change from a synchronous model of data streams to an asynchronous
      one.  This reduced the complexity of a number of drivers and
      increased the throughput of all USB drivers such that we are now
      running almost all USB devices at their maximum speed possible.
    - A change was made in the way data packets were allocated from the
      USB core by USB drivers so that all drivers now needed to provide
      more information to the USB core to fix a number of documented
      deadlocks.
  
  This is in stark contrast to a number of closed source operating systems
  which have had to maintain their older USB interfaces over time.  This
  provides the ability for new developers to accidentally use the old
  interfaces and do things in improper ways, causing the stability of the
  operating system to suffer.
  
  In both of these instances, all developers agreed that these were
  important changes that needed to be made, and they were made, with
  relatively little pain.  If Linux had to ensure that it will preserve a
  stable source interface, a new interface would have been created, and
  the older, broken one would have had to be maintained over time, leading
  to extra work for the USB developers.  Since all Linux USB developers do
  their work on their own time, asking programmers to do extra work for no
  gain, for free, is not a possibility.
  
  Security issues are also very important for Linux.  When a
  security issue is found, it is fixed in a very short amount of time.  A
  number of times this has caused internal kernel interfaces to be
  reworked to prevent the security problem from occurring.  When this
  happens, all drivers that use the interfaces were also fixed at the
  same time, ensuring that the security problem was fixed and could not
  come back at some future time accidentally.  If the internal interfaces
  were not allowed to change, fixing this kind of security problem and
  insuring that it could not happen again would not be possible.
  
  Kernel interfaces are cleaned up over time.  If there is no one using a
  current interface, it is deleted.  This ensures that the kernel remains
  as small as possible, and that all potential interfaces are tested as
  well as they can be (unused interfaces are pretty much impossible to
  test for validity.)
  
  
  What to do
  ----------
  
  So, if you have a Linux kernel driver that is not in the main kernel
  tree, what are you, a developer, supposed to do?  Releasing a binary
  driver for every different kernel version for every distribution is a
  nightmare, and trying to keep up with an ever changing kernel interface
  is also a rough job.
  
  Simple, get your kernel driver into the main kernel tree (remember we
  are talking about GPL released drivers here, if your code doesn't fall
  under this category, good luck, you are on your own here, you leech
  <insert link to leech comment from Andrew and Linus here>.)  If your
  driver is in the tree, and a kernel interface changes, it will be fixed
  up by the person who did the kernel change in the first place.  This
  ensures that your driver is always buildable, and works over time, with
  very little effort on your part.
  
  The very good side effects of having your driver in the main kernel tree
  are:
    - The quality of the driver will rise as the maintenance costs (to the
      original developer) will decrease.
    - Other developers will add features to your driver.
    - Other people will find and fix bugs in your driver.
    - Other people will find tuning opportunities in your driver.
    - Other people will update the driver for you when external interface
      changes require it.
    - The driver automatically gets shipped in all Linux distributions
      without having to ask the distros to add it.
      
  As Linux supports a larger number of different devices "out of the box"
  than any other operating system, and it supports these devices on more
  different processor architectures than any other operating system, this
  proven type of development model must be doing something right :)
  
  
  
  ------
  
  Thanks to Randy Dunlap, Andrew Morton, David Brownell, Hanna Linder,
  Robert Love, and Nishanth Aravamudan for their review and comments on
  early drafts of this paper.