linux/Documentation/stable_api_nonsense.txt
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   1The Linux Kernel Driver Interface
   2(all of your questions answered and then some)
   3
   4Greg Kroah-Hartman <greg@kroah.com>
   5
   6This is being written to try to explain why Linux does not have a binary
   7kernel interface, nor does it have a stable kernel interface.  Please
   8realize that this article describes the _in kernel_ interfaces, not the
   9kernel to userspace interfaces.  The kernel to userspace interface is
  10the one that application programs use, the syscall interface.  That
  11interface is _very_ stable over time, and will not break.  I have old
  12programs that were built on a pre 0.9something kernel that still work
  13just fine on the latest 2.6 kernel release.  That interface is the one
  14that users and application programmers can count on being stable.
  15
  16
  17Executive Summary
  18-----------------
  19You think you want a stable kernel interface, but you really do not, and
  20you don't even know it.  What you want is a stable running driver, and
  21you get that only if your driver is in the main kernel tree.  You also
  22get lots of other good benefits if your driver is in the main kernel
  23tree, all of which has made Linux into such a strong, stable, and mature
  24operating system which is the reason you are using it in the first
  25place.
  26
  27
  28Intro
  29-----
  30
  31It's only the odd person who wants to write a kernel driver that needs
  32to worry about the in-kernel interfaces changing.  For the majority of
  33the world, they neither see this interface, nor do they care about it at
  34all.
  35
  36First off, I'm not going to address _any_ legal issues about closed
  37source, hidden source, binary blobs, source wrappers, or any other term
  38that describes kernel drivers that do not have their source code
  39released under the GPL.  Please consult a lawyer if you have any legal
  40questions, I'm a programmer and hence, I'm just going to be describing
  41the technical issues here (not to make light of the legal issues, they
  42are real, and you do need to be aware of them at all times.)
  43
  44So, there are two main topics here, binary kernel interfaces and stable
  45kernel source interfaces.  They both depend on each other, but we will
  46discuss the binary stuff first to get it out of the way.
  47
  48
  49Binary Kernel Interface
  50-----------------------
  51Assuming that we had a stable kernel source interface for the kernel, a
  52binary interface would naturally happen too, right?  Wrong.  Please
  53consider the following facts about the Linux kernel:
  54  - Depending on the version of the C compiler you use, different kernel
  55    data structures will contain different alignment of structures, and
  56    possibly include different functions in different ways (putting
  57    functions inline or not.)  The individual function organization
  58    isn't that important, but the different data structure padding is
  59    very important.
  60  - Depending on what kernel build options you select, a wide range of
  61    different things can be assumed by the kernel:
  62      - different structures can contain different fields
  63      - Some functions may not be implemented at all, (i.e. some locks
  64        compile away to nothing for non-SMP builds.)
  65      - Memory within the kernel can be aligned in different ways,
  66        depending on the build options.
  67  - Linux runs on a wide range of different processor architectures.
  68    There is no way that binary drivers from one architecture will run
  69    on another architecture properly.
  70
  71Now a number of these issues can be addressed by simply compiling your
  72module for the exact specific kernel configuration, using the same exact
  73C compiler that the kernel was built with.  This is sufficient if you
  74want to provide a module for a specific release version of a specific
  75Linux distribution.  But multiply that single build by the number of
  76different Linux distributions and the number of different supported
  77releases of the Linux distribution and you quickly have a nightmare of
  78different build options on different releases.  Also realize that each
  79Linux distribution release contains a number of different kernels, all
  80tuned to different hardware types (different processor types and
  81different options), so for even a single release you will need to create
  82multiple versions of your module.
  83
  84Trust me, you will go insane over time if you try to support this kind
  85of release, I learned this the hard way a long time ago...
  86
  87
  88Stable Kernel Source Interfaces
  89-------------------------------
  90
  91This is a much more "volatile" topic if you talk to people who try to
  92keep a Linux kernel driver that is not in the main kernel tree up to
  93date over time.
  94
  95Linux kernel development is continuous and at a rapid pace, never
  96stopping to slow down.  As such, the kernel developers find bugs in
  97current interfaces, or figure out a better way to do things.  If they do
  98that, they then fix the current interfaces to work better.  When they do
  99so, function names may change, structures may grow or shrink, and
 100function parameters may be reworked.  If this happens, all of the
 101instances of where this interface is used within the kernel are fixed up
 102at the same time, ensuring that everything continues to work properly.
 103
 104As a specific examples of this, the in-kernel USB interfaces have
 105undergone at least three different reworks over the lifetime of this
 106subsystem.  These reworks were done to address a number of different
 107issues:
 108  - A change from a synchronous model of data streams to an asynchronous
 109    one.  This reduced the complexity of a number of drivers and
 110    increased the throughput of all USB drivers such that we are now
 111    running almost all USB devices at their maximum speed possible.
 112  - A change was made in the way data packets were allocated from the
 113    USB core by USB drivers so that all drivers now needed to provide
 114    more information to the USB core to fix a number of documented
 115    deadlocks.
 116
 117This is in stark contrast to a number of closed source operating systems
 118which have had to maintain their older USB interfaces over time.  This
 119provides the ability for new developers to accidentally use the old
 120interfaces and do things in improper ways, causing the stability of the
 121operating system to suffer.
 122
 123In both of these instances, all developers agreed that these were
 124important changes that needed to be made, and they were made, with
 125relatively little pain.  If Linux had to ensure that it preserve a
 126stable source interface, a new interface would have been created, and
 127the older, broken one would have had to be maintained over time, leading
 128to extra work for the USB developers.  Since all Linux USB developers do
 129their work on their own time, asking programmers to do extra work for no
 130gain, for free, is not a possibility.
 131
 132Security issues are also very important for Linux.  When a
 133security issue is found, it is fixed in a very short amount of time.  A
 134number of times this has caused internal kernel interfaces to be
 135reworked to prevent the security problem from occurring.  When this
 136happens, all drivers that use the interfaces were also fixed at the
 137same time, ensuring that the security problem was fixed and could not
 138come back at some future time accidentally.  If the internal interfaces
 139were not allowed to change, fixing this kind of security problem and
 140insuring that it could not happen again would not be possible.
 141
 142Kernel interfaces are cleaned up over time.  If there is no one using a
 143current interface, it is deleted.  This ensures that the kernel remains
 144as small as possible, and that all potential interfaces are tested as
 145well as they can be (unused interfaces are pretty much impossible to
 146test for validity.)
 147
 148
 149What to do
 150----------
 151
 152So, if you have a Linux kernel driver that is not in the main kernel
 153tree, what are you, a developer, supposed to do?  Releasing a binary
 154driver for every different kernel version for every distribution is a
 155nightmare, and trying to keep up with an ever changing kernel interface
 156is also a rough job.
 157
 158Simple, get your kernel driver into the main kernel tree (remember we
 159are talking about GPL released drivers here, if your code doesn't fall
 160under this category, good luck, you are on your own here, you leech
 161<insert link to leech comment from Andrew and Linus here>.)  If your
 162driver is in the tree, and a kernel interface changes, it will be fixed
 163up by the person who did the kernel change in the first place.  This
 164ensures that your driver is always buildable, and works over time, with
 165very little effort on your part.
 166
 167The very good side effects of having your driver in the main kernel tree
 168are:
 169  - The quality of the driver will rise as the maintenance costs (to the
 170    original developer) will decrease.
 171  - Other developers will add features to your driver.
 172  - Other people will find and fix bugs in your driver.
 173  - Other people will find tuning opportunities in your driver.
 174  - Other people will update the driver for you when external interface
 175    changes require it.
 176  - The driver automatically gets shipped in all Linux distributions
 177    without having to ask the distros to add it.
 178    
 179As Linux supports a larger number of different devices "out of the box"
 180than any other operating system, and it supports these devices on more
 181different processor architectures than any other operating system, this
 182proven type of development model must be doing something right :)
 183
 184
 185
 186------
 187
 188Thanks to Randy Dunlap, Andrew Morton, David Brownell, Hanna Linder,
 189Robert Love, and Nishanth Aravamudan for their review and comments on
 190early drafts of this paper.
 191
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