FreeBSD/Linux Kernel Cross Reference
sys/Documentation/stable_api_nonsense.txt

     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.

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