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