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+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.