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The Linux Kernel Driver Interface

(all of your questions answered and then some)

Greg Kroah-Hartman 

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 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

.)  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.