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 .. _development_process_intro:
 
 Introduction
 ============
 
 Executive summary
 -----------------
 
 The rest of this section covers the scope of the kernel development process
 and the kinds of frustrations that developers and their employers can
 encounter there.  There are a great many reasons why kernel code should be
 merged into the official ("mainline") kernel, including automatic
 availability to users, community support in many forms, and the ability to
 influence the direction of kernel development.  Code contributed to the
 Linux kernel must be made available under a GPL-compatible license.
 
 :ref:`development_process` introduces the development process, the kernel
 release cycle, and the mechanics of the merge window.  The various phases in
 the patch development, review, and merging cycle are covered.  There is some
 discussion of tools and mailing lists.  Developers wanting to get started
 with kernel development are encouraged to track down and fix bugs as an
 initial exercise.
 
 :ref:`development_early_stage` covers early-stage project planning, with an
 emphasis on involving the development community as soon as possible.
 
 :ref:`development_coding` is about the coding process; several pitfalls which
 have been encountered by other developers are discussed.  Some requirements for
 patches are covered, and there is an introduction to some of the tools
 which can help to ensure that kernel patches are correct.
 
 :ref:`development_posting` talks about the process of posting patches for
 review. To be taken seriously by the development community, patches must be
 properly formatted and described, and they must be sent to the right place.
 Following the advice in this section should help to ensure the best
 possible reception for your work.
 
 :ref:`development_followthrough` covers what happens after posting patches; the
 job is far from done at that point.  Working with reviewers is a crucial part
 of the development process; this section offers a number of tips on how to
 avoid problems at this important stage.  Developers are cautioned against
 assuming that the job is done when a patch is merged into the mainline.
 
 :ref:`development_advancedtopics` introduces a couple of "advanced" topics:
 managing patches with git and reviewing patches posted by others.
 
 :ref:`development_conclusion` concludes the document with pointers to sources
 for more information on kernel development.
 
 What this document is about
 ---------------------------
 
 The Linux kernel, at over 8 million lines of code and well over 1000
 contributors to each release, is one of the largest and most active free
 software projects in existence.  Since its humble beginning in 1991, this
 kernel has evolved into a best-of-breed operating system component which
 runs on pocket-sized digital music players, desktop PCs, the largest
 supercomputers in existence, and all types of systems in between.  It is a
 robust, efficient, and scalable solution for almost any situation.
 
 With the growth of Linux has come an increase in the number of developers
 (and companies) wishing to participate in its development.  Hardware
 vendors want to ensure that Linux supports their products well, making
 those products attractive to Linux users.  Embedded systems vendors, who
 use Linux as a component in an integrated product, want Linux to be as
 capable and well-suited to the task at hand as possible.  Distributors and
 other software vendors who base their products on Linux have a clear
 interest in the capabilities, performance, and reliability of the Linux
 kernel.  And end users, too, will often wish to change Linux to make it
 better suit their needs.
 
 One of the most compelling features of Linux is that it is accessible to
 these developers; anybody with the requisite skills can improve Linux and
 influence the direction of its development.  Proprietary products cannot
 offer this kind of openness, which is a characteristic of the free software
 process.  But, if anything, the kernel is even more open than most other
 free software projects.  A typical three-month kernel development cycle can
 involve over 1000 developers working for more than 100 different companies
 (or for no company at all).
 
 Working with the kernel development community is not especially hard.  But,
 that notwithstanding, many potential contributors have experienced
 difficulties when trying to do kernel work.  The kernel community has
 evolved its own distinct ways of operating which allow it to function
 smoothly (and produce a high-quality product) in an environment where
 thousands of lines of code are being changed every day.  So it is not
 surprising that Linux kernel development process differs greatly from
 proprietary development methods.
 
 The kernel's development process may come across as strange and
 intimidating to new developers, but there are good reasons and solid
 experience behind it.  A developer who does not understand the kernel
 community's ways (or, worse, who tries to flout or circumvent them) will
 have a frustrating experience in store.  The development community, while
 being helpful to those who are trying to learn, has little time for those
 who will not listen or who do not care about the development process.
 
 It is hoped that those who read this document will be able to avoid that
 frustrating experience.  There is a lot of material here, but the effort
 involved in reading it will be repaid in short order.  The development
 community is always in need of developers who will help to make the kernel
 better; the following text should help you - or those who work for you -
 join our community.
 
 Credits
 -------
 
 This document was written by Jonathan Corbet, corbet@lwn.net.  It has been
 improved by comments from Johannes Berg, James Berry, Alex Chiang, Roland
 Dreier, Randy Dunlap, Jake Edge, Jiri Kosina, Matt Mackall, Arthur Marsh,
 Amanda McPherson, Andrew Morton, Andrew Price, Tsugikazu Shibata, and
 Jochen Voß.
 
 This work was supported by the Linux Foundation; thanks especially to
 Amanda McPherson, who saw the value of this effort and made it all happen.
 
 The importance of getting code into the mainline
 ------------------------------------------------
 
 Some companies and developers occasionally wonder why they should bother
 learning how to work with the kernel community and get their code into the
 mainline kernel (the "mainline" being the kernel maintained by Linus
 Torvalds and used as a base by Linux distributors).  In the short term,
 contributing code can look like an avoidable expense; it seems easier to
 just keep the code separate and support users directly.  The truth of the
 matter is that keeping code separate ("out of tree") is a false economy.
 
 As a way of illustrating the costs of out-of-tree code, here are a few
 relevant aspects of the kernel development process; most of these will be
 discussed in greater detail later in this document.  Consider:
 
 - Code which has been merged into the mainline kernel is available to all
   Linux users.  It will automatically be present on all distributions which
   enable it.  There is no need for driver disks, downloads, or the hassles
   of supporting multiple versions of multiple distributions; it all just
   works, for the developer and for the user.  Incorporation into the
   mainline solves a large number of distribution and support problems.
 
 - While kernel developers strive to maintain a stable interface to user
   space, the internal kernel API is in constant flux.  The lack of a stable
   internal interface is a deliberate design decision; it allows fundamental
   improvements to be made at any time and results in higher-quality code.
   But one result of that policy is that any out-of-tree code requires
   constant upkeep if it is to work with new kernels.  Maintaining
   out-of-tree code requires significant amounts of work just to keep that
   code working.
 
   Code which is in the mainline, instead, does not require this work as the
   result of a simple rule requiring any developer who makes an API change
   to also fix any code that breaks as the result of that change.  So code
   which has been merged into the mainline has significantly lower
   maintenance costs.
 
 - Beyond that, code which is in the kernel will often be improved by other
   developers.  Surprising results can come from empowering your user
   community and customers to improve your product.
 
 - Kernel code is subjected to review, both before and after merging into
   the mainline.  No matter how strong the original developer's skills are,
   this review process invariably finds ways in which the code can be
   improved.  Often review finds severe bugs and security problems.  This is
   especially true for code which has been developed in a closed
   environment; such code benefits strongly from review by outside
   developers.  Out-of-tree code is lower-quality code.
 
 - Participation in the development process is your way to influence the
   direction of kernel development.  Users who complain from the sidelines
   are heard, but active developers have a stronger voice - and the ability
   to implement changes which make the kernel work better for their needs.
 
 - When code is maintained separately, the possibility that a third party
   will contribute a different implementation of a similar feature always
   exists.  Should that happen, getting your code merged will become much
   harder - to the point of impossibility.  Then you will be faced with the
   unpleasant alternatives of either (1) maintaining a nonstandard feature
   out of tree indefinitely, or (2) abandoning your code and migrating your
   users over to the in-tree version.
 
 - Contribution of code is the fundamental action which makes the whole
   process work.  By contributing your code you can add new functionality to
   the kernel and provide capabilities and examples which are of use to
   other kernel developers.  If you have developed code for Linux (or are
   thinking about doing so), you clearly have an interest in the continued
   success of this platform; contributing code is one of the best ways to
   help ensure that success.
 
 All of the reasoning above applies to any out-of-tree kernel code,
 including code which is distributed in proprietary, binary-only form.
 There are, however, additional factors which should be taken into account
 before considering any sort of binary-only kernel code distribution.  These
 include:
 
 - The legal issues around the distribution of proprietary kernel modules
   are cloudy at best; quite a few kernel copyright holders believe that
   most binary-only modules are derived products of the kernel and that, as
   a result, their distribution is a violation of the GNU General Public
   license (about which more will be said below).  Your author is not a
   lawyer, and nothing in this document can possibly be considered to be
   legal advice.  The true legal status of closed-source modules can only be
   determined by the courts.  But the uncertainty which haunts those modules
   is there regardless.
 
 - Binary modules greatly increase the difficulty of debugging kernel
   problems, to the point that most kernel developers will not even try.  So
   the distribution of binary-only modules will make it harder for your
   users to get support from the community.
 
 - Support is also harder for distributors of binary-only modules, who must
   provide a version of the module for every distribution and every kernel
   version they wish to support.  Dozens of builds of a single module can
   be required to provide reasonably comprehensive coverage, and your users
   will have to upgrade your module separately every time they upgrade their
   kernel.
 
 - Everything that was said above about code review applies doubly to
   closed-source code.  Since this code is not available at all, it cannot
   have been reviewed by the community and will, beyond doubt, have serious
   problems.
 
 Makers of embedded systems, in particular, may be tempted to disregard much
 of what has been said in this section in the belief that they are shipping
 a self-contained product which uses a frozen kernel version and requires no
 more development after its release.  This argument misses the value of
 widespread code review and the value of allowing your users to add
 capabilities to your product.  But these products, too, have a limited
 commercial life, after which a new version must be released.  At that
 point, vendors whose code is in the mainline and well maintained will be
 much better positioned to get the new product ready for market quickly.
 
 Licensing
 ---------
 
 Code is contributed to the Linux kernel under a number of licenses, but all
 code must be compatible with version 2 of the GNU General Public License
 (GPLv2), which is the license covering the kernel distribution as a whole.
 In practice, that means that all code contributions are covered either by
 GPLv2 (with, optionally, language allowing distribution under later
 versions of the GPL) or the three-clause BSD license.  Any contributions
 which are not covered by a compatible license will not be accepted into the
 kernel.
 
 Copyright assignments are not required (or requested) for code contributed
 to the kernel.  All code merged into the mainline kernel retains its
 original ownership; as a result, the kernel now has thousands of owners.
 
 One implication of this ownership structure is that any attempt to change
 the licensing of the kernel is doomed to almost certain failure.  There are
 few practical scenarios where the agreement of all copyright holders could
 be obtained (or their code removed from the kernel).  So, in particular,
 there is no prospect of a migration to version 3 of the GPL in the
 foreseeable future.
 
 It is imperative that all code contributed to the kernel be legitimately
 free software.  For that reason, code from anonymous (or pseudonymous)
 contributors will not be accepted.  All contributors are required to "sign
 off" on their code, stating that the code can be distributed with the
 kernel under the GPL.  Code which has not been licensed as free software by
 its owner, or which risks creating copyright-related problems for the
 kernel (such as code which derives from reverse-engineering efforts lacking
 proper safeguards) cannot be contributed.
 
 Questions about copyright-related issues are common on Linux development
 mailing lists.  Such questions will normally receive no shortage of
 answers, but one should bear in mind that the people answering those
 questions are not lawyers and cannot provide legal advice.  If you have
 legal questions relating to Linux source code, there is no substitute for
 talking with a lawyer who understands this field.  Relying on answers
 obtained on technical mailing lists is a risky affair.

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