~ubuntu-branches/ubuntu/maverick/linux-mvl-dove/maverick

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.  It tries to not

contain anything related to the technical aspects of kernel programming,

but will help point you in the right direction for that.

If anything in this document becomes out of date, please send in patches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]

 - "C:  A Reference Manual" by Harbison and Steele [Prentice Hall]

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 diverse group of people, with

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.

For common questions and answers about the GPL, please see:

	http://www.gnu.org/licenses/gpl-faq.html

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.

When a kernel change causes the interface that the kernel exposes to

userspace to change, it is recommended that you send the information or

a patch to the manual pages explaining the change to the manual pages

maintainer at mtk.manpages@gmail.com, and CC the list

linux-api@vger.kernel.org.

Here is a list of files that are in the kernel source tree that are

required reading:

  README

    This file gives a short background on the Linux kernel and describes

    what is necessary to do to configure and build the kernel.  People

    who are new to the kernel should start here.

  Documentation/Changes

    This file gives a list of the minimum levels of various software

    packages that are necessary to build and run the kernel

    successfully.

  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://userweb.kernel.org/~akpm/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 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 understanding 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 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 an 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://users.sosdg.org/~qiyong/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.  Its development

process is as follows:

  - As soon as a new kernel is released a two weeks window is open,

    during this period of time maintainers can submit big diffs to

    Linus, usually the patches that have already been included in the

    -mm kernel for a few weeks.  The preferred way to submit big changes

    is using git (the kernel's source management tool, more information

    can be found at http://git.or.cz/) but plain patches are also just

    fine.

  - After two weeks a -rc1 kernel is released it is now possible to push

    only patches that do not include new features 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 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.

  - Known regressions in each release are periodically posted to the 

    linux-kernel mailing list.  The goal is to reduce the length of 

    that list to zero before declaring the kernel to be "ready," but, in

    the real world, a small number of regressions often remain at 

    release time.

It is worth mentioning 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-part versions are -stable kernels. They contain

relatively 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>, and are

released as needs dictate.  The normal release period is approximately 

two weeks, but it can be longer if there are no pressing problems.  A

security-related problem, instead, can cause a release to happen almost

instantly.

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 are 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.  Code

which does not make an appearance in -mm before the opening of the merge

window will prove hard to merge into the mainline.

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.

In addition to all the other experimental patches, these kernels 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>

	git.kernel.org:/pub/scm/linux/kernel/git/sam/kbuild.git

    - ACPI development tree, Len Brown <len.brown@intel.com>

	git.kernel.org:/pub/scm/linux/kernel/git/lenb/linux-acpi-2.6.git

    - Block development tree, Jens Axboe <jens.axboe@oracle.com>

	git.kernel.org:/pub/scm/linux/kernel/git/axboe/linux-2.6-block.git

    - DRM development tree, Dave Airlie <airlied@linux.ie>

	git.kernel.org:/pub/scm/linux/kernel/git/airlied/drm-2.6.git

    - ia64 development tree, Tony Luck <tony.luck@intel.com>

	git.kernel.org:/pub/scm/linux/kernel/git/aegl/linux-2.6.git

    - infiniband, Roland Dreier <rolandd@cisco.com>

	git.kernel.org:/pub/scm/linux/kernel/git/roland/infiniband.git

    - libata, Jeff Garzik <jgarzik@pobox.com>

	git.kernel.org:/pub/scm/linux/kernel/git/jgarzik/libata-dev.git

    - network drivers, Jeff Garzik <jgarzik@pobox.com>

	git.kernel.org:/pub/scm/linux/kernel/git/jgarzik/netdev-2.6.git

    - pcmcia, Dominik Brodowski <linux@dominikbrodowski.net>

	git.kernel.org:/pub/scm/linux/kernel/git/brodo/pcmcia-2.6.git

    - SCSI, James Bottomley <James.Bottomley@hansenpartnership.com>

	git.kernel.org:/pub/scm/linux/kernel/git/jejb/scsi-misc-2.6.git

    - x86, Ingo Molnar <mingo@elte.hu>

	git://git.kernel.org/pub/scm/linux/kernel/git/x86/linux-2.6-x86.git

  quilt trees:

    - USB, Driver Core, and I2C, Greg Kroah-Hartman <gregkh@suse.de>

	kernel.org/pub/linux/kernel/people/gregkh/gregkh-2.6/

  Other kernel trees can be found listed at http://git.kernel.org/ and in

  the MAINTAINERS file.

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.  For details on how to use the kernel bugzilla, please see:

	http://bugzilla.kernel.org/page.cgi?id=faq.html

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.

Managing bug reports

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

One of the best ways to put into practice your hacking skills is by fixing

bugs reported by other people. Not only you will help to make the kernel

more stable, you'll learn to fix real world problems and you will improve

your skills, and other developers will be aware of your presence. Fixing

bugs is one of the best ways to get merits among other developers, because

not many people like wasting time fixing other people's bugs.

To work in the already reported bug reports, go to http://bugzilla.kernel.org.

If you want to be advised of the future bug reports, you can subscribe to the

bugme-new mailing list (only new bug reports are mailed here) or to the

bugme-janitor mailing list (every change in the bugzilla is mailed here)

	http://lists.linux-foundation.org/mailman/listinfo/bugme-new

	http://lists.linux-foundation.org/mailman/listinfo/bugme-janitors

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

already discussed in detail 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 for 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.

It is normal that the answers to your first patch might simply be a list

of a dozen things you should correct.  This does _not_ imply that your

patch will not be accepted, and it is _not_ meant against you

personally.  Simply correct all issues raised against your patch and

resend it.

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..."

    - "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.

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 up your changes

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

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 discussing 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 this should all look like, please see the

ChangeLog section of the document:

  "The Perfect Patch"

      http://userweb.kernel.org/~akpm/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"

(http://linux.tar.bz/articles/2.6-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, Jesper Juhl, Adrian Bunk, Keri Harris, Frans Pop,

David A. Wheeler, Junio Hamano, Michael Kerrisk, and Alex Shepard for

their review, comments, and contributions.  Without their help, this

document would not have been possible.

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