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Possible License(s): CC-BY-SA-3.0, GPL-2.0, LGPL-2.0, AGPL-1.0
  3The purpose of this document is to help developers (and their managers)
  4work with the development community with a minimum of frustration.  It is
  5an attempt to document how this community works in a way which is
  6accessible to those who are not intimately familiar with Linux kernel
  7development (or, indeed, free software development in general).  While
  8there is some technical material here, this is very much a process-oriented
  9discussion which does not require a deep knowledge of kernel programming to
 15The rest of this section covers the scope of the kernel development process
 16and the kinds of frustrations that developers and their employers can
 17encounter there.  There are a great many reasons why kernel code should be
 18merged into the official ("mainline") kernel, including automatic
 19availability to users, community support in many forms, and the ability to
 20influence the direction of kernel development.  Code contributed to the
 21Linux kernel must be made available under a GPL-compatible license.
 23Section 2 introduces the development process, the kernel release cycle, and
 24the mechanics of the merge window.  The various phases in the patch
 25development, review, and merging cycle are covered.  There is some
 26discussion of tools and mailing lists.  Developers wanting to get started
 27with kernel development are encouraged to track down and fix bugs as an
 28initial exercise.
 30Section 3 covers early-stage project planning, with an emphasis on
 31involving the development community as soon as possible.
 33Section 4 is about the coding process; several pitfalls which have been
 34encountered by other developers are discussed.  Some requirements for
 35patches are covered, and there is an introduction to some of the tools
 36which can help to ensure that kernel patches are correct.
 38Section 5 talks about the process of posting patches for review.  To be
 39taken seriously by the development community, patches must be properly
 40formatted and described, and they must be sent to the right place.
 41Following the advice in this section should help to ensure the best
 42possible reception for your work.
 44Section 6 covers what happens after posting patches; the job is far from
 45done at that point.  Working with reviewers is a crucial part of the
 46development process; this section offers a number of tips on how to avoid
 47problems at this important stage.  Developers are cautioned against
 48assuming that the job is done when a patch is merged into the mainline.
 50Section 7 introduces a couple of "advanced" topics: managing patches with
 51git and reviewing patches posted by others.
 53Section 8 concludes the document with pointers to sources for more
 54information on kernel development.
 59The Linux kernel, at over 6 million lines of code and well over 1000 active
 60contributors, is one of the largest and most active free software projects
 61in existence.  Since its humble beginning in 1991, this kernel has evolved
 62into a best-of-breed operating system component which runs on pocket-sized
 63digital music players, desktop PCs, the largest supercomputers in
 64existence, and all types of systems in between.  It is a robust, efficient,
 65and scalable solution for almost any situation.
 67With the growth of Linux has come an increase in the number of developers
 68(and companies) wishing to participate in its development.  Hardware
 69vendors want to ensure that Linux supports their products well, making
 70those products attractive to Linux users.  Embedded systems vendors, who
 71use Linux as a component in an integrated product, want Linux to be as
 72capable and well-suited to the task at hand as possible.  Distributors and
 73other software vendors who base their products on Linux have a clear
 74interest in the capabilities, performance, and reliability of the Linux
 75kernel.  And end users, too, will often wish to change Linux to make it
 76better suit their needs.
 78One of the most compelling features of Linux is that it is accessible to
 79these developers; anybody with the requisite skills can improve Linux and
 80influence the direction of its development.  Proprietary products cannot
 81offer this kind of openness, which is a characteristic of the free software
 82process.  But, if anything, the kernel is even more open than most other
 83free software projects.  A typical three-month kernel development cycle can
 84involve over 1000 developers working for more than 100 different companies
 85(or for no company at all).
 87Working with the kernel development community is not especially hard.  But,
 88that notwithstanding, many potential contributors have experienced
 89difficulties when trying to do kernel work.  The kernel community has
 90evolved its own distinct ways of operating which allow it to function
 91smoothly (and produce a high-quality product) in an environment where
 92thousands of lines of code are being changed every day.  So it is not
 93surprising that Linux kernel development process differs greatly from
 94proprietary development methods.
 96The kernel's development process may come across as strange and
 97intimidating to new developers, but there are good reasons and solid
 98experience behind it.  A developer who does not understand the kernel
 99community's ways (or, worse, who tries to flout or circumvent them) will
100have a frustrating experience in store.  The development community, while
101being helpful to those who are trying to learn, has little time for those
102who will not listen or who do not care about the development process.
104It is hoped that those who read this document will be able to avoid that
105frustrating experience.  There is a lot of material here, but the effort
106involved in reading it will be repaid in short order.  The development
107community is always in need of developers who will help to make the kernel
108better; the following text should help you - or those who work for you -
109join our community.
1121.3: CREDITS
114This document was written by Jonathan Corbet,  It has been
115improved by comments from Johannes Berg, James Berry, Alex Chiang, Roland
116Dreier, Randy Dunlap, Jake Edge, Jiri Kosina, Matt Mackall, Arthur Marsh,
117Amanda McPherson, Andrew Morton, Andrew Price, Tsugikazu Shibata, and
118Jochen Voß. 
120This work was supported by the Linux Foundation; thanks especially to
121Amanda McPherson, who saw the value of this effort and made it all happen.
126Some companies and developers occasionally wonder why they should bother
127learning how to work with the kernel community and get their code into the
128mainline kernel (the "mainline" being the kernel maintained by Linus
129Torvalds and used as a base by Linux distributors).  In the short term,
130contributing code can look like an avoidable expense; it seems easier to
131just keep the code separate and support users directly.  The truth of the
132matter is that keeping code separate ("out of tree") is a false economy.
134As a way of illustrating the costs of out-of-tree code, here are a few
135relevant aspects of the kernel development process; most of these will be
136discussed in greater detail later in this document.  Consider:
138- Code which has been merged into the mainline kernel is available to all
139  Linux users.  It will automatically be present on all distributions which
140  enable it.  There is no need for driver disks, downloads, or the hassles
141  of supporting multiple versions of multiple distributions; it all just
142  works, for the developer and for the user.  Incorporation into the
143  mainline solves a large number of distribution and support problems.
145- While kernel developers strive to maintain a stable interface to user
146  space, the internal kernel API is in constant flux.  The lack of a stable
147  internal interface is a deliberate design decision; it allows fundamental
148  improvements to be made at any time and results in higher-quality code.
149  But one result of that policy is that any out-of-tree code requires
150  constant upkeep if it is to work with new kernels.  Maintaining
151  out-of-tree code requires significant amounts of work just to keep that
152  code working.
154  Code which is in the mainline, instead, does not require this work as the
155  result of a simple rule requiring any developer who makes an API change
156  to also fix any code that breaks as the result of that change.  So code
157  which has been merged into the mainline has significantly lower
158  maintenance costs.
160- Beyond that, code which is in the kernel will often be improved by other
161  developers.  Surprising results can come from empowering your user
162  community and customers to improve your product.
164- Kernel code is subjected to review, both before and after merging into
165  the mainline.  No matter how strong the original developer's skills are,
166  this review process invariably finds ways in which the code can be
167  improved.  Often review finds severe bugs and security problems.  This is
168  especially true for code which has been developed in a closed
169  environment; such code benefits strongly from review by outside
170  developers.  Out-of-tree code is lower-quality code.
172- Participation in the development process is your way to influence the
173  direction of kernel development.  Users who complain from the sidelines
174  are heard, but active developers have a stronger voice - and the ability
175  to implement changes which make the kernel work better for their needs.
177- When code is maintained separately, the possibility that a third party
178  will contribute a different implementation of a similar feature always
179  exists.  Should that happen, getting your code merged will become much
180  harder - to the point of impossibility.  Then you will be faced with the
181  unpleasant alternatives of either (1) maintaining a nonstandard feature
182  out of tree indefinitely, or (2) abandoning your code and migrating your
183  users over to the in-tree version.
185- Contribution of code is the fundamental action which makes the whole
186  process work.  By contributing your code you can add new functionality to
187  the kernel and provide capabilities and examples which are of use to
188  other kernel developers.  If you have developed code for Linux (or are
189  thinking about doing so), you clearly have an interest in the continued
190  success of this platform; contributing code is one of the best ways to
191  help ensure that success.
193All of the reasoning above applies to any out-of-tree kernel code,
194including code which is distributed in proprietary, binary-only form.
195There are, however, additional factors which should be taken into account
196before considering any sort of binary-only kernel code distribution.  These
199- The legal issues around the distribution of proprietary kernel modules
200  are cloudy at best; quite a few kernel copyright holders believe that
201  most binary-only modules are derived products of the kernel and that, as
202  a result, their distribution is a violation of the GNU General Public
203  license (about which more will be said below).  Your author is not a
204  lawyer, and nothing in this document can possibly be considered to be
205  legal advice.  The true legal status of closed-source modules can only be
206  determined by the courts.  But the uncertainty which haunts those modules
207  is there regardless.
209- Binary modules greatly increase the difficulty of debugging kernel
210  problems, to the point that most kernel developers will not even try.  So
211  the distribution of binary-only modules will make it harder for your
212  users to get support from the community.
214- Support is also harder for distributors of binary-only modules, who must
215  provide a version of the module for every distribution and every kernel
216  version they wish to support.  Dozens of builds of a single module can
217  be required to provide reasonably comprehensive coverage, and your users
218  will have to upgrade your module separately every time they upgrade their
219  kernel.
221- Everything that was said above about code review applies doubly to
222  closed-source code.  Since this code is not available at all, it cannot
223  have been reviewed by the community and will, beyond doubt, have serious
224  problems. 
226Makers of embedded systems, in particular, may be tempted to disregard much
227of what has been said in this section in the belief that they are shipping
228a self-contained product which uses a frozen kernel version and requires no
229more development after its release.  This argument misses the value of
230widespread code review and the value of allowing your users to add
231capabilities to your product.  But these products, too, have a limited
232commercial life, after which a new version must be released.  At that
233point, vendors whose code is in the mainline and well maintained will be
234much better positioned to get the new product ready for market quickly.
239Code is contributed to the Linux kernel under a number of licenses, but all
240code must be compatible with version 2 of the GNU General Public License
241(GPLv2), which is the license covering the kernel distribution as a whole.
242In practice, that means that all code contributions are covered either by
243GPLv2 (with, optionally, language allowing distribution under later
244versions of the GPL) or the three-clause BSD license.  Any contributions
245which are not covered by a compatible license will not be accepted into the
248Copyright assignments are not required (or requested) for code contributed
249to the kernel.  All code merged into the mainline kernel retains its
250original ownership; as a result, the kernel now has thousands of owners.
252One implication of this ownership structure is that any attempt to change
253the licensing of the kernel is doomed to almost certain failure.  There are
254few practical scenarios where the agreement of all copyright holders could
255be obtained (or their code removed from the kernel).  So, in particular,
256there is no prospect of a migration to version 3 of the GPL in the
257foreseeable future.
259It is imperative that all code contributed to the kernel be legitimately
260free software.  For that reason, code from anonymous (or pseudonymous)
261contributors will not be accepted.  All contributors are required to "sign
262off" on their code, stating that the code can be distributed with the
263kernel under the GPL.  Code which has not been licensed as free software by
264its owner, or which risks creating copyright-related problems for the
265kernel (such as code which derives from reverse-engineering efforts lacking
266proper safeguards) cannot be contributed.
268Questions about copyright-related issues are common on Linux development
269mailing lists.  Such questions will normally receive no shortage of
270answers, but one should bear in mind that the people answering those
271questions are not lawyers and cannot provide legal advice.  If you have
272legal questions relating to Linux source code, there is no substitute for
273talking with a lawyer who understands this field.  Relying on answers
274obtained on technical mailing lists is a risky affair.