FreeBSD/Linux Kernel Cross Reference
sys/Documentation/HOWTO

     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    If anything in this document becomes out of date, please send in patches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://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:
           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://lxr.linux.no/+trees
   
   
   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 3.x kernel tree
     - 3.x.y -stable kernel tree
     - 3.x -git kernel patches
     - subsystem specific kernel trees and patches
     - the 3.x -next kernel tree for integration tests
   
   3.x kernel tree
   -----------------
   3.x kernels are maintained by Linus Torvalds, and can be found on
   kernel.org in the pub/linux/kernel/v3.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 http://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.
     - 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."
   
   3.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 3.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 3.x.y kernel is available, then the highest numbered 3.x
   kernel is the current stable kernel.
   
   3.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.
   
   3.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 http://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
   http://patchwork.kernel.org/.
   
   3.x -next kernel tree for integration tests
   ---------------------------------------------
   Before updates from subsystem trees are merged into the mainline 3.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:
           http://git.kernel.org/?p=linux/kernel/git/next/linux-next.git
           http://linux.f-seidel.de/linux-next/pmwiki/
   
   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
   -------------
   
   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 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://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>

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