1: A GUIDE TO THE KERNEL DEVELOPMENT PROCESS
  
  The purpose of this document is to help developers (and their managers)
  work with the development community with a minimum of frustration.  It is
  an attempt to document how this community works in a way which is
  accessible to those who are not intimately familiar with Linux kernel
  development (or, indeed, free software development in general).  While
  there is some technical material here, this is very much a process-oriented
  discussion which does not require a deep knowledge of kernel programming to
  understand.
  
  
  1.1: 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.
  
  Section 2 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.
  
  Section 3 covers early-stage project planning, with an emphasis on
  involving the development community as soon as possible.
  
  Section 4 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.
  
  Section 5 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.
  
  Section 6 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.
  
  Section 7 introduces a couple of "advanced" topics: managing patches with
  git and reviewing patches posted by others.
  
  Section 8 concludes the document with pointers to sources for more
  information on kernel development.
  
  
  1.2: 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.
  
  
  1.3: 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.
  
  
  1.4: 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.
  
  
  1.5: 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.