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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If anything in this document becomes out of date, please send in patches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 C11 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. The Linux kernel licensing
rules and how to use `SPDX <https://spdx.org/>`_ identifiers in source code are
described in :ref:`Documentation/process/license-rules.rst <kernel_licensing>`.
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:

	https://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 alx@kernel.org, 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:

  :ref:`Documentation/admin-guide/README.rst <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.

  :ref:`Documentation/process/changes.rst <changes>`
    This file gives a list of the minimum levels of various software
    packages that are necessary to build and run the kernel
    successfully.

  :ref:`Documentation/process/coding-style.rst <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.

  :ref:`Documentation/process/submitting-patches.rst <submittingpatches>`
    This file describes 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"
		https://www.ozlabs.org/~akpm/stuff/tpp.txt

	"Linux kernel patch submission format"
		https://web.archive.org/web/20180829112450/http://linux.yyz.us/patch-format.html

  :ref:`Documentation/process/stable-api-nonsense.rst <stable_api_nonsense>`
    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.

  :ref:`Documentation/process/security-bugs.rst <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.

  :ref:`Documentation/process/management-style.rst <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.

  :ref:`Documentation/process/stable-kernel-rules.rst <stable_kernel_rules>`
    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.

  :ref:`Documentation/process/kernel-docs.rst <kernel_docs>`
    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.

  :ref:`Documentation/process/applying-patches.rst <applying_patches>`
    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 or from
ReStructuredText markups (ReST), like this one. This includes a
full description of the in-kernel API, and rules on how to handle
locking properly.

All such documents can be generated as PDF or HTML by running::

	make pdfdocs
	make htmldocs

respectively from the main kernel source directory.

The documents that uses ReST markup will be generated at Documentation/output.
They can also be generated on LaTeX and ePub formats with::

	make latexdocs
	make epubdocs

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

If you do not know anything about Linux kernel development, you should
look at the Linux KernelNewbies project:

	https://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:

	https://kernelnewbies.org/KernelJanitors

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.

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:

	https://elixir.bootlin.com/


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:

  - Linus's mainline tree
  - Various stable trees with multiple major numbers
  - Subsystem-specific trees
  - linux-next integration testing tree

Mainline tree
~~~~~~~~~~~~~

The mainline tree is maintained by Linus Torvalds, and can be found at
https://kernel.org or in the repo.  Its development process is as follows:

  - As soon as a new kernel is released a two week 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
    linux-next 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 https://git-scm.com/) but plain patches are also just
    fine.
  - After two weeks a -rc1 kernel is released and the focus is on making the
    new kernel as rock solid as possible.  Most of the patches at this point
    should fix a regression.  Bugs that have always existed are not
    regressions, so only push these kinds of fixes if they are important.
    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.

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."*

Various stable trees with multiple major numbers
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Kernels with 3-part versions are -stable kernels. They contain
relatively small and critical fixes for security problems or significant
regressions discovered in a given major mainline release. Each release
in a major stable series increments the third part of the version
number, keeping the first two parts the same.

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

Stable trees are maintained by the "stable" team <stable@vger.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 :ref:`Documentation/process/stable-kernel-rules.rst <stable_kernel_rules>`
in the kernel tree documents what kinds of changes are acceptable for
the -stable tree, and how the release process works.

Subsystem-specific trees
~~~~~~~~~~~~~~~~~~~~~~~~

The maintainers of the various kernel subsystems --- and also many
kernel subsystem developers --- expose their current state of
development in source repositories.  That way, others can see what is
happening in the different areas of the kernel.  In areas where
development is rapid, a developer may be asked to base his submissions
onto such a subsystem kernel tree so that conflicts between the
submission and other already ongoing work are avoided.

Most of these repositories are git trees, but there are also other SCMs
in use, or patch queues being published as quilt series.  Addresses of
these subsystem repositories are listed in the MAINTAINERS file.  Many
of them can be browsed at https://git.kernel.org/.

Before a proposed patch is committed to such a subsystem tree, it is
subject to review which primarily happens on mailing lists (see the
respective section below).  For several kernel subsystems, this review
process is tracked with the tool patchwork.  Patchwork offers a web
interface which shows patch postings, any comments on a patch or
revisions to it, and maintainers can mark patches as under review,
accepted, or rejected.  Most of these patchwork sites are listed at
https://patchwork.kernel.org/.

linux-next integration testing tree
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Before updates from subsystem trees are merged into the mainline tree,
they need to be integration-tested.  For this purpose, a special
testing repository exists into which virtually all subsystem trees are
pulled on an almost daily basis:

	https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git

This way, the linux-next gives a summary outlook onto what will be
expected to go into the mainline kernel at the next merge period.
Adventurous testers are very welcome to runtime-test the linux-next.


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

The file 'Documentation/admin-guide/reporting-issues.rst' in the main kernel
source directory describes 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 will you help to make the kernel
more stable, but you'll also 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 on already reported bug reports, find a subsystem you are interested in.
Check the MAINTAINERS file where bugs for that subsystem get reported to; often
it will be a mailing list, rarely a bugtracker. Search the archives of said
place for recent reports and help where you see fit. You may also want to check
https://bugzilla.kernel.org for bug reports; only a handful of kernel subsystems
use it actively for reporting or tracking, nevertheless bugs for the whole
kernel get filed there.


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:

	https://subspace.kernel.org/subscribing.html

There are archives of the mailing list on the web in many different
places.  Use a search engine to find these archives.  For example:

	https://lore.kernel.org/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:

	https://subspace.kernel.org

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):

	https://subspace.kernel.org/etiquette.html

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 :ref:`Documentation/process/submitting-patches.rst <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 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"
      https://www.ozlabs.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"
(https://lwn.net/Articles/94386/) 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>