1.. _development_process_intro:
2
3Introduction
4============
5
6Executive summary
7-----------------
8
9The rest of this section covers the scope of the kernel development process
10and the kinds of frustrations that developers and their employers can
11encounter there.  There are a great many reasons why kernel code should be
12merged into the official ("mainline") kernel, including automatic
13availability to users, community support in many forms, and the ability to
14influence the direction of kernel development.  Code contributed to the
15Linux kernel must be made available under a GPL-compatible license.
16
17:ref:`development_process` introduces the development process, the kernel
18release cycle, and the mechanics of the merge window.  The various phases in
19the patch development, review, and merging cycle are covered.  There is some
20discussion of tools and mailing lists.  Developers wanting to get started
21with kernel development are encouraged to track down and fix bugs as an
22initial exercise.
23
24:ref:`development_early_stage` covers early-stage project planning, with an
25emphasis on involving the development community as soon as possible.
26
27:ref:`development_coding` is about the coding process; several pitfalls which
28have been encountered by other developers are discussed.  Some requirements for
29patches are covered, and there is an introduction to some of the tools
30which can help to ensure that kernel patches are correct.
31
32:ref:`development_posting` talks about the process of posting patches for
33review. To be taken seriously by the development community, patches must be
34properly formatted and described, and they must be sent to the right place.
35Following the advice in this section should help to ensure the best
36possible reception for your work.
37
38:ref:`development_followthrough` covers what happens after posting patches; the
39job is far from done at that point.  Working with reviewers is a crucial part
40of the development process; this section offers a number of tips on how to
41avoid problems at this important stage.  Developers are cautioned against
42assuming that the job is done when a patch is merged into the mainline.
43
44:ref:`development_advancedtopics` introduces a couple of "advanced" topics:
45managing patches with git and reviewing patches posted by others.
46
47:ref:`development_conclusion` concludes the document with pointers to sources
48for more information on kernel development.
49
50What this document is about
51---------------------------
52
53The Linux kernel, at over 8 million lines of code and well over 1000
54contributors to each release, is one of the largest and most active free
55software projects in existence.  Since its humble beginning in 1991, this
56kernel has evolved into a best-of-breed operating system component which
57runs on pocket-sized digital music players, desktop PCs, the largest
58supercomputers in existence, and all types of systems in between.  It is a
59robust, efficient, and scalable solution for almost any situation.
60
61With the growth of Linux has come an increase in the number of developers
62(and companies) wishing to participate in its development.  Hardware
63vendors want to ensure that Linux supports their products well, making
64those products attractive to Linux users.  Embedded systems vendors, who
65use Linux as a component in an integrated product, want Linux to be as
66capable and well-suited to the task at hand as possible.  Distributors and
67other software vendors who base their products on Linux have a clear
68interest in the capabilities, performance, and reliability of the Linux
69kernel.  And end users, too, will often wish to change Linux to make it
70better suit their needs.
71
72One of the most compelling features of Linux is that it is accessible to
73these developers; anybody with the requisite skills can improve Linux and
74influence the direction of its development.  Proprietary products cannot
75offer this kind of openness, which is a characteristic of the free software
76process.  But, if anything, the kernel is even more open than most other
77free software projects.  A typical three-month kernel development cycle can
78involve over 1000 developers working for more than 100 different companies
79(or for no company at all).
80
81Working with the kernel development community is not especially hard.  But,
82that notwithstanding, many potential contributors have experienced
83difficulties when trying to do kernel work.  The kernel community has
84evolved its own distinct ways of operating which allow it to function
85smoothly (and produce a high-quality product) in an environment where
86thousands of lines of code are being changed every day.  So it is not
87surprising that Linux kernel development process differs greatly from
88proprietary development methods.
89
90The kernel's development process may come across as strange and
91intimidating to new developers, but there are good reasons and solid
92experience behind it.  A developer who does not understand the kernel
93community's ways (or, worse, who tries to flout or circumvent them) will
94have a frustrating experience in store.  The development community, while
95being helpful to those who are trying to learn, has little time for those
96who will not listen or who do not care about the development process.
97
98It is hoped that those who read this document will be able to avoid that
99frustrating experience.  There is a lot of material here, but the effort
100involved in reading it will be repaid in short order.  The development
101community is always in need of developers who will help to make the kernel
102better; the following text should help you - or those who work for you -
103join our community.
104
105Credits
106-------
107
108This document was written by Jonathan Corbet, corbet@lwn.net.  It has been
109improved by comments from Johannes Berg, James Berry, Alex Chiang, Roland
110Dreier, Randy Dunlap, Jake Edge, Jiri Kosina, Matt Mackall, Arthur Marsh,
111Amanda McPherson, Andrew Morton, Andrew Price, Tsugikazu Shibata, and
112Jochen Voß.
113
114This work was supported by the Linux Foundation; thanks especially to
115Amanda McPherson, who saw the value of this effort and made it all happen.
116
117The importance of getting code into the mainline
118------------------------------------------------
119
120Some companies and developers occasionally wonder why they should bother
121learning how to work with the kernel community and get their code into the
122mainline kernel (the "mainline" being the kernel maintained by Linus
123Torvalds and used as a base by Linux distributors).  In the short term,
124contributing code can look like an avoidable expense; it seems easier to
125just keep the code separate and support users directly.  The truth of the
126matter is that keeping code separate ("out of tree") is a false economy.
127
128As a way of illustrating the costs of out-of-tree code, here are a few
129relevant aspects of the kernel development process; most of these will be
130discussed in greater detail later in this document.  Consider:
131
132- Code which has been merged into the mainline kernel is available to all
133  Linux users.  It will automatically be present on all distributions which
134  enable it.  There is no need for driver disks, downloads, or the hassles
135  of supporting multiple versions of multiple distributions; it all just
136  works, for the developer and for the user.  Incorporation into the
137  mainline solves a large number of distribution and support problems.
138
139- While kernel developers strive to maintain a stable interface to user
140  space, the internal kernel API is in constant flux.  The lack of a stable
141  internal interface is a deliberate design decision; it allows fundamental
142  improvements to be made at any time and results in higher-quality code.
143  But one result of that policy is that any out-of-tree code requires
144  constant upkeep if it is to work with new kernels.  Maintaining
145  out-of-tree code requires significant amounts of work just to keep that
146  code working.
147
148  Code which is in the mainline, instead, does not require this work as the
149  result of a simple rule requiring any developer who makes an API change
150  to also fix any code that breaks as the result of that change.  So code
151  which has been merged into the mainline has significantly lower
152  maintenance costs.
153
154- Beyond that, code which is in the kernel will often be improved by other
155  developers.  Surprising results can come from empowering your user
156  community and customers to improve your product.
157
158- Kernel code is subjected to review, both before and after merging into
159  the mainline.  No matter how strong the original developer's skills are,
160  this review process invariably finds ways in which the code can be
161  improved.  Often review finds severe bugs and security problems.  This is
162  especially true for code which has been developed in a closed
163  environment; such code benefits strongly from review by outside
164  developers.  Out-of-tree code is lower-quality code.
165
166- Participation in the development process is your way to influence the
167  direction of kernel development.  Users who complain from the sidelines
168  are heard, but active developers have a stronger voice - and the ability
169  to implement changes which make the kernel work better for their needs.
170
171- When code is maintained separately, the possibility that a third party
172  will contribute a different implementation of a similar feature always
173  exists.  Should that happen, getting your code merged will become much
174  harder - to the point of impossibility.  Then you will be faced with the
175  unpleasant alternatives of either (1) maintaining a nonstandard feature
176  out of tree indefinitely, or (2) abandoning your code and migrating your
177  users over to the in-tree version.
178
179- Contribution of code is the fundamental action which makes the whole
180  process work.  By contributing your code you can add new functionality to
181  the kernel and provide capabilities and examples which are of use to
182  other kernel developers.  If you have developed code for Linux (or are
183  thinking about doing so), you clearly have an interest in the continued
184  success of this platform; contributing code is one of the best ways to
185  help ensure that success.
186
187All of the reasoning above applies to any out-of-tree kernel code,
188including code which is distributed in proprietary, binary-only form.
189There are, however, additional factors which should be taken into account
190before considering any sort of binary-only kernel code distribution.  These
191include:
192
193- The legal issues around the distribution of proprietary kernel modules
194  are cloudy at best; quite a few kernel copyright holders believe that
195  most binary-only modules are derived products of the kernel and that, as
196  a result, their distribution is a violation of the GNU General Public
197  license (about which more will be said below).  Your author is not a
198  lawyer, and nothing in this document can possibly be considered to be
199  legal advice.  The true legal status of closed-source modules can only be
200  determined by the courts.  But the uncertainty which haunts those modules
201  is there regardless.
202
203- Binary modules greatly increase the difficulty of debugging kernel
204  problems, to the point that most kernel developers will not even try.  So
205  the distribution of binary-only modules will make it harder for your
206  users to get support from the community.
207
208- Support is also harder for distributors of binary-only modules, who must
209  provide a version of the module for every distribution and every kernel
210  version they wish to support.  Dozens of builds of a single module can
211  be required to provide reasonably comprehensive coverage, and your users
212  will have to upgrade your module separately every time they upgrade their
213  kernel.
214
215- Everything that was said above about code review applies doubly to
216  closed-source code.  Since this code is not available at all, it cannot
217  have been reviewed by the community and will, beyond doubt, have serious
218  problems.
219
220Makers of embedded systems, in particular, may be tempted to disregard much
221of what has been said in this section in the belief that they are shipping
222a self-contained product which uses a frozen kernel version and requires no
223more development after its release.  This argument misses the value of
224widespread code review and the value of allowing your users to add
225capabilities to your product.  But these products, too, have a limited
226commercial life, after which a new version must be released.  At that
227point, vendors whose code is in the mainline and well maintained will be
228much better positioned to get the new product ready for market quickly.
229
230Licensing
231---------
232
233Code is contributed to the Linux kernel under a number of licenses, but all
234code must be compatible with version 2 of the GNU General Public License
235(GPLv2), which is the license covering the kernel distribution as a whole.
236In practice, that means that all code contributions are covered either by
237GPLv2 (with, optionally, language allowing distribution under later
238versions of the GPL) or the three-clause BSD license.  Any contributions
239which are not covered by a compatible license will not be accepted into the
240kernel.
241
242Copyright assignments are not required (or requested) for code contributed
243to the kernel.  All code merged into the mainline kernel retains its
244original ownership; as a result, the kernel now has thousands of owners.
245
246One implication of this ownership structure is that any attempt to change
247the licensing of the kernel is doomed to almost certain failure.  There are
248few practical scenarios where the agreement of all copyright holders could
249be obtained (or their code removed from the kernel).  So, in particular,
250there is no prospect of a migration to version 3 of the GPL in the
251foreseeable future.
252
253It is imperative that all code contributed to the kernel be legitimately
254free software.  For that reason, code from anonymous (or pseudonymous)
255contributors will not be accepted.  All contributors are required to "sign
256off" on their code, stating that the code can be distributed with the
257kernel under the GPL.  Code which has not been licensed as free software by
258its owner, or which risks creating copyright-related problems for the
259kernel (such as code which derives from reverse-engineering efforts lacking
260proper safeguards) cannot be contributed.
261
262Questions about copyright-related issues are common on Linux development
263mailing lists.  Such questions will normally receive no shortage of
264answers, but one should bear in mind that the people answering those
265questions are not lawyers and cannot provide legal advice.  If you have
266legal questions relating to Linux source code, there is no substitute for
267talking with a lawyer who understands this field.  Relying on answers
268obtained on technical mailing lists is a risky affair.
269