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+.. _process_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+If anything in this document becomes out of date, please send in patches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 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 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:
+
+  :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/admin-guide/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/?p=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 you will 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:
+
+	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:
+
+	https://lore.kernel.org/lkml/
+
+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 :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>