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  If anything in this document becomes out of date, please send in patches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

  	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 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.

    :ref:`Documentation/Changes <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/CodingStyle <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/SubmittingPatches <submittingpatches>` and :ref:`Documentation/SubmittingDrivers <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"

  		https://www.ozlabs.org/~akpm/stuff/tpp.txt

  	"Linux kernel patch submission format"
  		http://linux.yyz.us/patch-format.html

    :ref:`Documentation/stable_api_nonsense.txt <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/SecurityBugs <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/ManagementStyle <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/stable_kernel_rules.txt <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/kernel-docs.txt <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/applying-patches.txt <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
  
  Currently, there are some documents written on DocBook that are in
  the process of conversion to ReST. Such documents will be created in the
  Documentation/DocBook/ directory and can be generated also as

  Postscript or man pages by running::

  
  	make psdocs
  	make mandocs

  
  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.
  
  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:

  	https://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://lxr.free-electrons.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:

    - main 4.x kernel tree
    - 4.x.y -stable kernel tree
    - 4.x -git kernel patches

    - subsystem specific kernel trees and patches

    - the 4.x -next kernel tree for integration tests

  4.x kernel tree

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

  4.x kernels are maintained by Linus Torvalds, and can be found on

  https://kernel.org in the pub/linux/kernel/v4.x/ 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

      -next 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 https://git-scm.com/) 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.
  
  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."*

  4.x.y -stable kernel tree

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

  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 4.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 4.x.y kernel is available, then the highest numbered 4.x

  kernel is the current stable kernel.

  4.x.y 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 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.

  4.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.

  Subsystem Specific kernel trees and patches
  -------------------------------------------

  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/.

  4.x -next kernel tree for integration tests

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

  Before updates from subsystem trees are merged into the mainline 4.x

  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/?p=linux/kernel/git/next/linux-next.git

  
  This way, the -next kernel 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 -next kernel.

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

  https://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:

  	https://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 https://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)

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