1 1: A GUIDE TO THE KERNEL DEVELOPMENT PROCESS
3 The purpose of this document is to help developers (and their managers)
4 work with the development community with a minimum of frustration. It is
5 an attempt to document how this community works in a way which is
6 accessible to those who are not intimately familiar with Linux kernel
7 development (or, indeed, free software development in general). While
8 there is some technical material here, this is very much a process-oriented
9 discussion which does not require a deep knowledge of kernel programming to
13 1.1: EXECUTIVE SUMMARY
15 The rest of this section covers the scope of the kernel development process
16 and the kinds of frustrations that developers and their employers can
17 encounter there. There are a great many reasons why kernel code should be
18 merged into the official ("mainline") kernel, including automatic
19 availability to users, community support in many forms, and the ability to
20 influence the direction of kernel development. Code contributed to the
21 Linux kernel must be made available under a GPL-compatible license.
23 Section 2 introduces the development process, the kernel release cycle, and
24 the mechanics of the merge window. The various phases in the patch
25 development, review, and merging cycle are covered. There is some
26 discussion of tools and mailing lists. Developers wanting to get started
27 with kernel development are encouraged to track down and fix bugs as an
30 Section 3 covers early-stage project planning, with an emphasis on
31 involving the development community as soon as possible.
33 Section 4 is about the coding process; several pitfalls which have been
34 encountered by other developers are discussed. Some requirements for
35 patches are covered, and there is an introduction to some of the tools
36 which can help to ensure that kernel patches are correct.
38 Section 5 talks about the process of posting patches for review. To be
39 taken seriously by the development community, patches must be properly
40 formatted and described, and they must be sent to the right place.
41 Following the advice in this section should help to ensure the best
42 possible reception for your work.
44 Section 6 covers what happens after posting patches; the job is far from
45 done at that point. Working with reviewers is a crucial part of the
46 development process; this section offers a number of tips on how to avoid
47 problems at this important stage. Developers are cautioned against
48 assuming that the job is done when a patch is merged into the mainline.
50 Section 7 introduces a couple of "advanced" topics: managing patches with
51 git and reviewing patches posted by others.
53 Section 8 concludes the document with pointers to sources for more
54 information on kernel development.
57 1.2: WHAT THIS DOCUMENT IS ABOUT
59 The Linux kernel, at over 6 million lines of code and well over 1000 active
60 contributors, is one of the largest and most active free software projects
61 in existence. Since its humble beginning in 1991, this kernel has evolved
62 into a best-of-breed operating system component which runs on pocket-sized
63 digital music players, desktop PCs, the largest supercomputers in
64 existence, and all types of systems in between. It is a robust, efficient,
65 and scalable solution for almost any situation.
67 With the growth of Linux has come an increase in the number of developers
68 (and companies) wishing to participate in its development. Hardware
69 vendors want to ensure that Linux supports their products well, making
70 those products attractive to Linux users. Embedded systems vendors, who
71 use Linux as a component in an integrated product, want Linux to be as
72 capable and well-suited to the task at hand as possible. Distributors and
73 other software vendors who base their products on Linux have a clear
74 interest in the capabilities, performance, and reliability of the Linux
75 kernel. And end users, too, will often wish to change Linux to make it
76 better suit their needs.
78 One of the most compelling features of Linux is that it is accessible to
79 these developers; anybody with the requisite skills can improve Linux and
80 influence the direction of its development. Proprietary products cannot
81 offer this kind of openness, which is a characteristic of the free software
82 process. But, if anything, the kernel is even more open than most other
83 free software projects. A typical three-month kernel development cycle can
84 involve over 1000 developers working for more than 100 different companies
85 (or for no company at all).
87 Working with the kernel development community is not especially hard. But,
88 that notwithstanding, many potential contributors have experienced
89 difficulties when trying to do kernel work. The kernel community has
90 evolved its own distinct ways of operating which allow it to function
91 smoothly (and produce a high-quality product) in an environment where
92 thousands of lines of code are being changed every day. So it is not
93 surprising that Linux kernel development process differs greatly from
94 proprietary development methods.
96 The kernel's development process may come across as strange and
97 intimidating to new developers, but there are good reasons and solid
98 experience behind it. A developer who does not understand the kernel
99 community's ways (or, worse, who tries to flout or circumvent them) will
100 have a frustrating experience in store. The development community, while
101 being helpful to those who are trying to learn, has little time for those
102 who will not listen or who do not care about the development process.
104 It is hoped that those who read this document will be able to avoid that
105 frustrating experience. There is a lot of material here, but the effort
106 involved in reading it will be repaid in short order. The development
107 community is always in need of developers who will help to make the kernel
108 better; the following text should help you - or those who work for you -
114 This document was written by Jonathan Corbet, corbet@lwn.net. It has been
115 improved by comments from Johannes Berg, James Berry, Alex Chiang, Roland
116 Dreier, Randy Dunlap, Jake Edge, Jiri Kosina, Matt Mackall, Arthur Marsh,
117 Amanda McPherson, Andrew Morton, Andrew Price, Tsugikazu Shibata, and
120 This work was supported by the Linux Foundation; thanks especially to
121 Amanda McPherson, who saw the value of this effort and made it all happen.
124 1.4: THE IMPORTANCE OF GETTING CODE INTO THE MAINLINE
126 Some companies and developers occasionally wonder why they should bother
127 learning how to work with the kernel community and get their code into the
128 mainline kernel (the "mainline" being the kernel maintained by Linus
129 Torvalds and used as a base by Linux distributors). In the short term,
130 contributing code can look like an avoidable expense; it seems easier to
131 just keep the code separate and support users directly. The truth of the
132 matter is that keeping code separate ("out of tree") is a false economy.
134 As a way of illustrating the costs of out-of-tree code, here are a few
135 relevant aspects of the kernel development process; most of these will be
136 discussed 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
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
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.
193 All of the reasoning above applies to any out-of-tree kernel code,
194 including code which is distributed in proprietary, binary-only form.
195 There are, however, additional factors which should be taken into account
196 before 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
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
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
226 Makers of embedded systems, in particular, may be tempted to disregard much
227 of what has been said in this section in the belief that they are shipping
228 a self-contained product which uses a frozen kernel version and requires no
229 more development after its release. This argument misses the value of
230 widespread code review and the value of allowing your users to add
231 capabilities to your product. But these products, too, have a limited
232 commercial life, after which a new version must be released. At that
233 point, vendors whose code is in the mainline and well maintained will be
234 much better positioned to get the new product ready for market quickly.
239 Code is contributed to the Linux kernel under a number of licenses, but all
240 code 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.
242 In practice, that means that all code contributions are covered either by
243 GPLv2 (with, optionally, language allowing distribution under later
244 versions of the GPL) or the three-clause BSD license. Any contributions
245 which are not covered by a compatible license will not be accepted into the
248 Copyright assignments are not required (or requested) for code contributed
249 to the kernel. All code merged into the mainline kernel retains its
250 original ownership; as a result, the kernel now has thousands of owners.
252 One implication of this ownership structure is that any attempt to change
253 the licensing of the kernel is doomed to almost certain failure. There are
254 few practical scenarios where the agreement of all copyright holders could
255 be obtained (or their code removed from the kernel). So, in particular,
256 there is no prospect of a migration to version 3 of the GPL in the
259 It is imperative that all code contributed to the kernel be legitimately
260 free software. For that reason, code from anonymous (or pseudonymous)
261 contributors will not be accepted. All contributors are required to "sign
262 off" on their code, stating that the code can be distributed with the
263 kernel under the GPL. Code which has not been licensed as free software by
264 its owner, or which risks creating copyright-related problems for the
265 kernel (such as code which derives from reverse-engineering efforts lacking
266 proper safeguards) cannot be contributed.
268 Questions about copyright-related issues are common on Linux development
269 mailing lists. Such questions will normally receive no shortage of
270 answers, but one should bear in mind that the people answering those
271 questions are not lawyers and cannot provide legal advice. If you have
272 legal questions relating to Linux source code, there is no substitute for
273 talking with a lawyer who understands this field. Relying on answers
274 obtained on technical mailing lists is a risky affair.
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