[RFC] HOWTO do Linux kernel development - take 2

Greg KH
2005-11-15T21:20:32Z

Here's an updated version of the "HOTO do Linux kernel development"
document that I've been working on.

For a description of why I started this, please see:
http://thread.gmane.org/gmane.linux.kernel/348689

I've addressed all of the comments that I have received, and flushed out
the the TODO sections. I'd appreciate any comments on this version, as
I think it's looking pretty good and finished for now. If no one
objects, I'll send it in a patch for inclusion in the main tree soon.

thanks,

greg k-h

------------------------------

HOWTO do Linux kernel development
---------------------------------

This is the be-all, end-all document on this topic. It contains
instructions on how to become a Linux kernel developer and how to learn
to work with the Linux kernel development community.

If anything in this document becomes out of date, please send in patches
to the maintainer of this file, who is listed at the bottom of the
document.

Introduction
------------

So, you want to learn how to become a Linux kernel developer? Or you
have been told by your manager, "Go write a Linux driver for this
device." This document's goal is to teach you everything you need to
know to achieve this by describing the process you need to go through,
and hints on how to work with the community. It will also try to
explain some of the reasons why the community works like it does.

The kernel is written mostly in C, with some architecture-dependent
parts written in assembly. A good understanding of C is required for
kernel development. Assembly (any architecture) is not required unless
you plan to do low-level development for that architecture. Though they
are not a good substitute for a solid C education and/or years of
experience, the following books are good for, if anything, reference:
- "The C Programming Language" by Kernighan and Ritchie [Prentice Hall]
- "Practical C Programming" by Steve Oualline [O'Reilly]
- "Programming the 80386" by Crawford and Gelsinger [Sybek]
- "UNIX Systems for Modern Architectures" by Curt Schimmel [Addison Wesley]

The kernel is written using GNU C and the GNU toolchain. While it
adheres to the ISO C89 standard, it uses a number of extensions that are
not featured in the standard. The kernel is a freestanding C
environment, with no reliance on the standard C library, so some
portions of the C standard are not supported. Arbitrary long long
divisions and floating point are not allowed. It can sometimes be
difficult to understand the assumptions the kernel has on the toolchain
and the extensions that it uses, and unfortunately there is no
definitive reference for them. Please check the gcc info pages (`info
gcc`) for some information on them.

Please remember that you are trying to learn how to work with the
existing development community. It is a very diverse group of people,
with very high standards for coding, style and procedure. These
standards have been created over time based on what they have found to
work best for such a large and geographically dispersed team. Try to
learn as much as possible about these standards ahead of time, as they
are well documented; do not expect people to adapt to you or your
company's way of doing things.

Legal Issues
------------

The Linux kernel source code is released under the GPL. Please see the
file, COPYING, in the main directory of the source tree, for details on
the license. If you have further questions about the license, please
contact a lawyer, and do not ask on the Linux kernel mailing list. The
people on the mailing lists are not lawyers, and you should not rely on
their statements on legal matters.

Documentation
------------

The Linux kernel source tree has a large range of documents that are
invaluable for learning how to interact with the kernel community. When
new features are added to the kernel, it is recommended that new
documentation files are also added which explain how to use the feature.
Here is a list of files that are in the kernel source tree that are
required reading:
Documentation/CodingStyle
This describes the Linux kernel coding style, and some of the
rationale behind it. All new code is expected to follow the
guidelines in this document. Most maintainers will only accept
patches if these rules are followed, and many people will only
review code if it is in the proper style.

Documentation/SubmittingPatches
Documentation/SubmittingDrivers
These files describe in explicit detail how to successfully create
and send a patch, including (but not limited to):
- Email contents
- Email format
- Who to send it to
Following these rules will not guarantee success (as all patches are
subject to scrutiny for content and style), but not following them
will almost always prevent it.

Other excellent descriptions of how to create patches properly are:
"The Perfect Patch"
http://www.zip.com.au/~akpm/linux/patches/stuff/tpp.txt
"Linux kernel patch submission format"
http://linux.yyz.us/patch-format.html

Documentation/stable_api_nonsense.txt
This file describes the rationale behind the conscious decision to
not have a stable API within the kernel, including for things like:
- Subsystem shim-layers (for compatibility?)
- Driver portability between Operating Systems.
- Mitigating rapid change within the kernel source tree (or
preventing rapid change)
This document is crucial for understand the Linux development
philosophy and is very important for people moving to Linux from
development on other Operating Systems.

Documentation/SecurityBugs
If you feel you have found a security problem in the Linux kernel,
please follow the steps in this document to help notify the kernel
developers, and help solve the issue.

Documentation/ManagementStyle
This document describes how Linux kernel maintainers operate and the
shared ethos behind their methodologies. This is important reading
for anyone new to kernel development (or anyone simply curious about
it), as it resolves a lot of common misconceptions and confusion
about the unique behavior of kernel maintainers.

Documentation/stable_kernel_rules.txt
This file describes the rules on how the stable kernel releases
happen, and what to do if you want to get a change into one of these
releases.

Documentation/kernel-docs.txt
A list of external documentation that pertains to kernel
development. Please consult this list if you do not find what you
are looking for within the in-kernel documentation.

Documentation/applying-patches.txt
A good introduction describing exactly what a patch is, how to
create it, and how to apply it to the different development branches
of the kernel.

The kernel also has a large number of documents that can be
automatically generated from the source code itself. This includes a
full description of the in-kernel API, and rules on how to handle
locking properly. The documents will be created in the
Documentation/DocBook/ directory and can be generated as PDF,
Postscript, HTML, and man pages by running:
make pdfdocs
make psdocs
make htmldocs
make mandocs
respectively from the main kernel source directory.

Becoming A Kernel Developer
---------------------------

If you do not know anything about Linux kernel development, you should
look at the Linux KernelNewbies project:
http://kernelnewbies.org
It consists of a helpful mailing list where you can ask almost any type
of basic kernel development question (make sure to search the archives
first, before asking something that has already been answered in the
past.) It also has a IRC channel that you can use to ask questions in
real-time, and a lot of helpful documentation that is useful for
learning about Linux kernel development.

The website has basic information about code organization, subsystems,
and current projects (both in-tree and out-of-tree). It also describes
some basic logistical information, like how to compile a kernel and
apply a patch.

If you do not know where you want to start, but you want to look for
some task to start doing to join into the kernel development community,
go to the Linux Kernel Janitor's project:
http://janitor.kernelnewbies.org/
It is a great place to start. It describes a list of relatively simple
problems that need to be cleaned up and fixed within the Linux kernel
source tree. Working with the developers in charge of this project, you
will learn the basics of getting your patch into the Linux kernel tree,
and possibly be pointed in the direction of what to go work on next, if
you do not already have an idea.

If you already have a chunk of code that you want to put into the kernel
tree, but need some help getting it in the proper form, the
kernel-mentors project was created to help you out with this. It is a
mailing list, and can be found at:
http://selenic.com/mailman/listinfo/kernel-mentors

Before making any actual modifications to the Linux kernel code, it is
imperative to understand how the code in question works. For this
purpose, nothing is better than reading through it directly (most tricky
bits are commented well), perhaps even with the help of specialized
tools. One such tool that is particularly recommended is the Linux
Cross-Reference project, which is able to present source code in a
self-referential, indexed webpage format. An excellent up-to-date
repository of the kernel code may be found at:
http://sosdg.org/~coywolf/lxr/

The development process
-----------------------

Linux kernel development process currently consists of a few different
main kernel "branches" and lots of different subsystem-specific kernel
branches. These different branches are:
- main 2.6.x kernel tree
- 2.6.x.y -stable kernel tree
- 2.6.x -git kernel patches
- 2.6.x -mm kernel patches
- subsystem specific kernel trees and patches

2.6.x kernel tree
-----------------
2.6.x kernels are maintained by Linus Torvalds, and can be found on
kernel.org in the pub/linux/kernel/v2.6/ directory. It's development process
is as follows:
- As soon a new kernel is released a two weeks windows is open, during this
period of time maintainers can submit big diffs to Linus, usually the
patched sited in -mm kernels for a few weeks. The preferred way to submit
big changes is using git (more information about git can be found at
http://git.or.cz/) but plain patches are also just fine.
- After two weeks a -rc1 kernel is released and now is possible to push only
patches that do not include new functionalities that could affect the
stability of the whole kernel. Please note that a whole new driver (or
filesystem) might be accepted after -rc1 because there is no risk of
causing regressions with such a change as long as the change is
self-contained and does not affect areas outside of the code that is being
added. git can be used to send patches to Linus after -rc1 is
released, but the patches need to also be sent to a public mailing
list for review.
- A new -rc is released whenever Linus deems the current git (the
kernel's source management) tree to be in a reasonably sane state
adequate for testing. The goal is to release a new -rc kernel every
week.
- Process continues until the kernel is considered "ready", the
process should last around 6 weeks.

It is worth to mention what Andrew Morton wrote on the linux-kernel
mailing list about kernel releases:
"Nobody knows when a kernel will be released, because it's
released according to perceived bug status, not according to a
preconceived timeline."

2.6.x.y -stable kernel tree
---------------------------
Kernels with 4 digit versions are -stable kernels. They contain
relativly small and critical fixes for security problems or significant
regressions discovered in a given 2.6.x kernel.

This is the recommended branch for users who want the most recent stable
kernel and are not interested in helping test development/experimental
versions.

If no 2.6.x.y kernel is available, then the highest numbered 2.6.x
kernel is the current stable kernel.

2.6.x.y are maintained by the "stable" team <stable@kernel.org>, are
released almost every week.

The file Documentation/stable_kernel_rules.txt in the kernel tree
documents what kinds of changes are acceptable for the -stable tree, and
how the release process works.

2.6.x -git patches
------------------
These are daily snapshots of Linus' kernel tree which is managed in a
git repository (hence the name.) These patches are usually released
daily and represent the current state of Linus' tree. They are more
experimental than -rc kernels since they are generated automatically
without even a cursory glance to see if they are sane.

2.6.x -mm kernel patches
------------------------
These are experimental kernel patches released by Andrew Morton. Andrew
takes all of the different subsystem kernel trees and patches and mushes
them together, along with a lot of patches that have been plucked from
the linux-kernel mailing list. This tree serves as a proving ground for
new features and patches. Once a patch has proved its worth in -mm for
a while Andrew or the subsystem maintainer pushes it on to Linus for
inclusion in mainline.

It is heavily encouraged that all new patches get tested in the -mm tree
before they are sent to Linus for inclusion in the main kernel tree.

These kernels are not appropriate for use on systems that are supposed
to be stable and they are more risky to run than any of the other
branches.

If you wish to help out with the kernel development process, please test
and use these kernel releases and provide feedback to the linux-kernel
mailing list if you have any problems, and if everything works properly.

These kernels in addition to all the other experimental patches they
contain usually also contain any changes in the mainline -git kernels
available at the time of release.

The -mm kernels are not released on a fixed schedule, but usually a few
-mm kernels are released in between each -rc kernel (1 to 3 is common).

Subsystem Specific kernel trees and patches
-------------------------------------------
A number of the different kernel subsystem developers expose their
development trees so that others can see what is happening in the
different areas of the kernel. These trees are pulled into the -mm
kernel releases as described above.

Here is a list of some of the different kernel trees available:
git trees:
- Kbuild development tree, Sam Ravnborg <sam@ravnborg.org>
kernel.org:/pub/scm/linux/kernel/git/sam/kbuild.git

- ACPI development tree, Len Brown <len.brown@intel.com>
kernel.org:/pub/scm/linux/kernel/git/lenb/linux-acpi-2.6.git

- Block development tree, Jens Axboe <axboe@suse.de>
kernel.org:/pub/scm/linux/kernel/git/axboe/linux-2.6-block.git

- DRM development tree, Dave Airlie <airlied@linux.ie>
kernel.org:/pub/scm/linux/kernel/git/airlied/drm-2.6.git

- ia64 development tree, Tony Luck <tony.luck@intel.com>
kernel.org:/pub/scm/linux/kernel/git/aegl/linux-2.6.git

- ieee1394 development tree, Jody McIntyre <scjody@modernduck.com>
kernel.org:/pub/scm/linux/kernel/git/scjody/ieee1394.git

- infiniband, Roland Dreier <rolandd@cisco.com>
kernel.org:/pub/scm/linux/kernel/git/roland/infiniband.git

- libata, Jeff Garzik <jgarzik@pobox.com>
kernel.org:/pub/scm/linux/kernel/git/jgarzik/libata-dev.git

- network drivers, Jeff Garzik <jgarzik@pobox.com>
kernel.org:/pub/scm/linux/kernel/git/jgarzik/netdev-2.6.git

- pcmcia, Dominik Brodowski <linux@dominikbrodowski.net>
kernel.org:/pub/scm/linux/kernel/git/brodo/pcmcia-2.6.git

- SCSI, James Bottomley <James.Bottomley@SteelEye.com>
kernel.org:/pub/scm/linux/kernel/git/jejb/scsi-misc-2.6.git

quilt trees:
- USB, PCI, Driver Core, and I2C, Greg Kroah-Hartman <gregkh@suse.de>
kernel.org/pub/linux/kernel/people/gregkh/gregkh-2.6/

Bug Reporting
-------------

bugzilla.kernel.org is where the Linux kernel developers track kernel
bugs. Users are encouraged to report all bugs that they find in this
tool.

The file REPORTING-BUGS in the main kernel source directory has a good
template for how to report a possible kernel bug, and details what kind
of information is needed by the kernel developers to help track down the
problem.

Mailing lists
-------------

As some of the above documents describe, the majority of the core kernel
developers participate on the Linux Kernel Mailing list. Details on how
to subscribe and unsubscribe from the list can be found at:
http://vger.kernel.org/vger-lists.html#linux-kernel
There are archives of the mailing list on the web in many different
places. Use a search engine to find these archives. For example:
http://dir.gmane.org/gmane.linux.kernel
It is highly recommended that you search the archives about the topic
you want to bring up, before you post it to the list. A lot of things
are already discussed in detail and are only recorded at the mailing
list archives.

Most of the individual kernel subsystems also have their own separate
mailing list where they do their development efforts. See the
MAINTAINERS file for a list of what these lists are for the different
groups.

Many of the lists are hosted on kernel.org. Information on them can be
found at:
http://vger.kernel.org/vger-lists.html

Please remember to follow good behavioral habits when using the lists.
Though a bit cheesy, the following URL has some simple guidelines for
interacting with the list (or any list):
http://www.albion.com/netiquette/

If multiple people respond to your mail, the CC: list of recipients may
get pretty large. Don't remove anybody from the CC: list without a good
reason, or don't reply only to the list address. Get used to receiving the
mail twice, one from the sender and the one from the list, and don't try
to tune that by adding fancy mail-headers, people will not like it.

Remember to keep the context and the attribution of your replies intact,
keep the "John Kernelhacker wrote ...:" lines at the top of your reply, and
add your statements between the individual quoted sections instead of
writing at the top of the mail.

If you add patches to your mail, make sure they are plain readable text
as stated in Documentation/SubmittingPatches. Kernel developers don't
want to deal with attachments or compressed patches; they may want
to comment on individual lines of your patch, which works only that way.
Make sure you use a mail program that does not mangle spaces and tab
characters. A good first test is to send the mail to yourself and try
to apply your own patch by yourself. If that doesn't work, get your
mail program fixed or change it until it works.

Above all, please remember to show respect to other subscribers.

Working with the community
--------------------------

The goal of the kernel community is to provide the best possible kernel
there is. When you submit a patch for acceptance, it will be reviewed
on its technical merits and those alone. So, what should you be
expecting?
- criticism
- comments
- requests for change
- requests justification
- silence

Remember, this is part of getting your patch into the kernel. You have
to be able to take criticism and comments about your patches, evaluate
them at a technical level and either rework your patches or provide
clear and concise reasoning as to why those changes should not be made.
If there are no responses to your posting, wait a few days and try
again, sometimes things get lost in the huge volume.

What should you not do?
- expect your patch to be accepted without question
- become defensive
- ignore comments
- resubmit the patch without making any of the requested changes

In a community that is looking for the best technical solution possible,
there will always be differing opinions on how beneficial a patch is.
You have to be cooperative, and willing to adapt your idea to fit within
the kernel. Or at least be willing to prove your idea is worth it.
Remember, being wrong is acceptable as long as you are willing to work
toward a solution that is right.

Differences between the kernel community and corporate structures
-----------------------------------------------------------------

The kernel community works differently than most traditional corporate
development environments. Here are a list of things that you can try to
do to try to avoid problems:
Good things to say regarding your proposed changes:
- "This solves multiple problems."
- "This deletes 2000 lines of code."
- "Here is a patch that explains what I am trying to describe."
- "I tested it on 5 different architectures..."
- "Here is a series of small patches that..."
- "This increases performance on typical machines..."

Bad things you should avoid saying:
- "We did it this way in AIX/ptx/Solaris, so therefore it must be
good..."
- "I've being doing this for 20 years, so..."
- "It makes this proprietary benchmark go faster"
- "This is required for my company to make money"
- "This is for our Enterprise product line."
- "Here is my 1000 page design document that describes my idea"
- "I've been working on this for 6 months..."
- "Here's a 5000 line patch that..."
- "I rewrote all of the current mess, and here it is..."
- "I have a deadline, and this patch needs to be applied now."

Another way the kernel community is different than most traditional
software engineering work environments is the faceless nature of
interaction. One benefit of using email and irc as the primary forms of
communication is the lack of discrimination based on gender or race.
The Linux kernel work environment is accepting of women and minorities
because all you are is an email address. The international aspect also
helps to level the playing field because you can't guess gender based on
a person's name. A man may be named Andrea and a woman may be named Pat.
Most women who have worked in the Linux kernel and have expressed an
opinion have had positive experiences. Here is a group that is a good
starting point for women interested in contributing to Linux:
http://www.linuxchix.org/

The language barrier can cause problems for some people who are not
comfortable with English. A good grasp of the language can be needed in
order to get ideas across properly on mailing lists, so it is
recommended that you check your emails to make sure they make sense in
English before sending them.

Break your changes up
---------------------

The Linux kernel community does not gladly accept large chunks of code
dropped on it all at once. The changes need to be properly introduced,
discussed, and broken up into tiny, individual portions. This is almost
the exact opposite of what companies are used to doing. Your proposal
should also be introduced very early in the development process, so that
you can receive feedback on what you are doing. It also lets the
community feel that you are working with them, and not simply using them
as a dumping ground for your feature. However, don't send 50 emails at
one time to a mailing list, your patch series should be smaller than
that almost all of the time.

The reasons for breaking things up are the following:

1) Small patches increase the likelihood that your patches will be
applied, since they don't take much time or effort to verify for
correctness. A 5 line patch can be applied by a maintainer with
barely a second glance. However, a 500 line patch may take hours to
review for correctness (the time it takes is exponentially
proportional to the size of the patch, or something).

Small patches also make it very easy to debug when something goes
wrong. It's much easier to back out patches one by one than it is
to dissect a very large patch after it's been applied (and broken
something).

2) It's important not only to send small patches, but also to rewrite
and simplify (or simply re-order) patches before submitting them.

Here is an analogy from kernel developer Al Viro:
"Think of a teacher grading homework from a math student. The
teacher does not want to see the student's trials and errors
before they came up with the solution. They want to see the
cleanest, most elegant answer. A good student knows this, and
would never submit her intermediate work before the final
solution."

The same is true of kernel development. The maintainers and
reviewers do not want to see the thought process behind the
solution to the problem one is solving. They want to see a
simple and elegant solution."

It may be challenging to keep the balance between presenting an elegant
solution and working together with the community and discuss your
unfinished work. Therefore it is good to get early in the process to
get feedback to improve your work, but also keep your changes in small
chunks that they may get already accepted, even when your whole task is
not ready for inclusion now.

Also realize that it is not acceptable to send patches for inclusion
that are unfinished and will be "fixed up later."

Justify your change
-------------------

Along with breaking up your patches, it is very important for you to let
the Linux community know why they should add this change. New features
must be justified as being needed and useful.

Document your change
--------------------

When sending in your patches, pay special attention to what you say in
the text in your email. This information will become the ChangeLog
information for the patch, and will be preserved for everyone to see for
all time. It should describe the patch completely, containing:
- why the change is necessary
- the overall design approach in the patch
- implementation details
- testing results

For more details on what the this should all look like, please see the
ChangeLog section of the document:
"The Perfect Patch"
http://www.zip.com.au/~akpm/linux/patches/stuff/tpp.txt

All of these things are sometimes very hard to do. It can take years to
perfect these practices (if at all). It's a continuous process of
improvement that requires a lot of patience and determination. But
don't give up, it's possible. Many have done it before, and each had to
start exactly where you are now.

----------
Thanks to Paolo Ciarrocchi who allowed the "Development Process" section
to be based on text he had written, and to Randy Dunlap and Gerrit
Huizenga for some of the list of things you should and should not say.
Also thanks to Pat Mochel, Hanna Linder, Randy Dunlap, Kay Sievers,
Vojtech Pavlik, Jan Kara, Josh Boyer, Kees Cook, Andrew Morton, Andi
Kleen, Vadim Lobanov, and Alex Shepard for their review and comments on
early drafts of this document.

Maintainer: Greg Kroah-Hartman <greg@kroah.com>