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