1 .. _process_howto: 1 .. _process_howto:
2 2
3 HOWTO do Linux kernel development 3 HOWTO do Linux kernel development
4 ================================= 4 =================================
5 5
6 This is the be-all, end-all document on this t 6 This is the be-all, end-all document on this topic. It contains
7 instructions on how to become a Linux kernel d 7 instructions on how to become a Linux kernel developer and how to learn
8 to work with the Linux kernel development comm 8 to work with the Linux kernel development community. It tries to not
9 contain anything related to the technical aspe 9 contain anything related to the technical aspects of kernel programming,
10 but will help point you in the right direction 10 but will help point you in the right direction for that.
11 11
12 If anything in this document becomes out of da 12 If anything in this document becomes out of date, please send in patches
13 to the maintainer of this file, who is listed 13 to the maintainer of this file, who is listed at the bottom of the
14 document. 14 document.
15 15
16 16
17 Introduction 17 Introduction
18 ------------ 18 ------------
19 19
20 So, you want to learn how to become a Linux ke 20 So, you want to learn how to become a Linux kernel developer? Or you
21 have been told by your manager, "Go write a Li 21 have been told by your manager, "Go write a Linux driver for this
22 device." This document's goal is to teach you 22 device." This document's goal is to teach you everything you need to
23 know to achieve this by describing the process 23 know to achieve this by describing the process you need to go through,
24 and hints on how to work with the community. 24 and hints on how to work with the community. It will also try to
25 explain some of the reasons why the community 25 explain some of the reasons why the community works like it does.
26 26
27 The kernel is written mostly in C, with some a 27 The kernel is written mostly in C, with some architecture-dependent
28 parts written in assembly. A good understandin 28 parts written in assembly. A good understanding of C is required for
29 kernel development. Assembly (any architectur 29 kernel development. Assembly (any architecture) is not required unless
30 you plan to do low-level development for that 30 you plan to do low-level development for that architecture. Though they
31 are not a good substitute for a solid C educat 31 are not a good substitute for a solid C education and/or years of
32 experience, the following books are good for, 32 experience, the following books are good for, if anything, reference:
33 33
34 - "The C Programming Language" by Kernighan a 34 - "The C Programming Language" by Kernighan and Ritchie [Prentice Hall]
35 - "Practical C Programming" by Steve Oualline 35 - "Practical C Programming" by Steve Oualline [O'Reilly]
36 - "C: A Reference Manual" by Harbison and St 36 - "C: A Reference Manual" by Harbison and Steele [Prentice Hall]
37 37
38 The kernel is written using GNU C and the GNU 38 The kernel is written using GNU C and the GNU toolchain. While it
39 adheres to the ISO C11 standard, it uses a num 39 adheres to the ISO C11 standard, it uses a number of extensions that are
40 not featured in the standard. The kernel is a 40 not featured in the standard. The kernel is a freestanding C
41 environment, with no reliance on the standard 41 environment, with no reliance on the standard C library, so some
42 portions of the C standard are not supported. 42 portions of the C standard are not supported. Arbitrary long long
43 divisions and floating point are not allowed. 43 divisions and floating point are not allowed. It can sometimes be
44 difficult to understand the assumptions the ke 44 difficult to understand the assumptions the kernel has on the toolchain
45 and the extensions that it uses, and unfortuna 45 and the extensions that it uses, and unfortunately there is no
46 definitive reference for them. Please check t 46 definitive reference for them. Please check the gcc info pages (`info
47 gcc`) for some information on them. 47 gcc`) for some information on them.
48 48
49 Please remember that you are trying to learn h 49 Please remember that you are trying to learn how to work with the
50 existing development community. It is a diver 50 existing development community. It is a diverse group of people, with
51 high standards for coding, style and procedure 51 high standards for coding, style and procedure. These standards have
52 been created over time based on what they have 52 been created over time based on what they have found to work best for
53 such a large and geographically dispersed team 53 such a large and geographically dispersed team. Try to learn as much as
54 possible about these standards ahead of time, 54 possible about these standards ahead of time, as they are well
55 documented; do not expect people to adapt to y 55 documented; do not expect people to adapt to you or your company's way
56 of doing things. 56 of doing things.
57 57
58 58
59 Legal Issues 59 Legal Issues
60 ------------ 60 ------------
61 61
62 The Linux kernel source code is released under 62 The Linux kernel source code is released under the GPL. Please see the file
63 COPYING in the main directory of the source tr 63 COPYING in the main directory of the source tree. The Linux kernel licensing
64 rules and how to use `SPDX <https://spdx.org/> 64 rules and how to use `SPDX <https://spdx.org/>`_ identifiers in source code are
65 described in :ref:`Documentation/process/licen 65 described in :ref:`Documentation/process/license-rules.rst <kernel_licensing>`.
66 If you have further questions about the licens 66 If you have further questions about the license, please contact a lawyer, and do
67 not ask on the Linux kernel mailing list. The 67 not ask on the Linux kernel mailing list. The people on the mailing lists are
68 not lawyers, and you should not rely on their 68 not lawyers, and you should not rely on their statements on legal matters.
69 69
70 For common questions and answers about the GPL 70 For common questions and answers about the GPL, please see:
71 71
72 https://www.gnu.org/licenses/gpl-faq.h 72 https://www.gnu.org/licenses/gpl-faq.html
73 73
74 74
75 Documentation 75 Documentation
76 ------------- 76 -------------
77 77
78 The Linux kernel source tree has a large range 78 The Linux kernel source tree has a large range of documents that are
79 invaluable for learning how to interact with t 79 invaluable for learning how to interact with the kernel community. When
80 new features are added to the kernel, it is re 80 new features are added to the kernel, it is recommended that new
81 documentation files are also added which expla 81 documentation files are also added which explain how to use the feature.
82 When a kernel change causes the interface that 82 When a kernel change causes the interface that the kernel exposes to
83 userspace to change, it is recommended that yo 83 userspace to change, it is recommended that you send the information or
84 a patch to the manual pages explaining the cha 84 a patch to the manual pages explaining the change to the manual pages
85 maintainer at alx@kernel.org, and CC the list 85 maintainer at alx@kernel.org, and CC the list linux-api@vger.kernel.org.
86 86
87 Here is a list of files that are in the kernel 87 Here is a list of files that are in the kernel source tree that are
88 required reading: 88 required reading:
89 89
90 :ref:`Documentation/admin-guide/README.rst < 90 :ref:`Documentation/admin-guide/README.rst <readme>`
91 This file gives a short background on the 91 This file gives a short background on the Linux kernel and describes
92 what is necessary to do to configure and b 92 what is necessary to do to configure and build the kernel. People
93 who are new to the kernel should start her 93 who are new to the kernel should start here.
94 94
95 :ref:`Documentation/process/changes.rst <cha 95 :ref:`Documentation/process/changes.rst <changes>`
96 This file gives a list of the minimum leve 96 This file gives a list of the minimum levels of various software
97 packages that are necessary to build and r 97 packages that are necessary to build and run the kernel
98 successfully. 98 successfully.
99 99
100 :ref:`Documentation/process/coding-style.rst 100 :ref:`Documentation/process/coding-style.rst <codingstyle>`
101 This describes the Linux kernel coding sty 101 This describes the Linux kernel coding style, and some of the
102 rationale behind it. All new code is expec 102 rationale behind it. All new code is expected to follow the
103 guidelines in this document. Most maintain 103 guidelines in this document. Most maintainers will only accept
104 patches if these rules are followed, and m 104 patches if these rules are followed, and many people will only
105 review code if it is in the proper style. 105 review code if it is in the proper style.
106 106
107 :ref:`Documentation/process/submitting-patch 107 :ref:`Documentation/process/submitting-patches.rst <submittingpatches>`
108 This file describes in explicit detail how 108 This file describes in explicit detail how to successfully create
109 and send a patch, including (but not limit 109 and send a patch, including (but not limited to):
110 110
111 - Email contents 111 - Email contents
112 - Email format 112 - Email format
113 - Who to send it to 113 - Who to send it to
114 114
115 Following these rules will not guarantee s 115 Following these rules will not guarantee success (as all patches are
116 subject to scrutiny for content and style) 116 subject to scrutiny for content and style), but not following them
117 will almost always prevent it. 117 will almost always prevent it.
118 118
119 Other excellent descriptions of how to cre 119 Other excellent descriptions of how to create patches properly are:
120 120
121 "The Perfect Patch" 121 "The Perfect Patch"
122 https://www.ozlabs.org/~akpm/s 122 https://www.ozlabs.org/~akpm/stuff/tpp.txt
123 123
124 "Linux kernel patch submission format" 124 "Linux kernel patch submission format"
125 https://web.archive.org/web/20 125 https://web.archive.org/web/20180829112450/http://linux.yyz.us/patch-format.html
126 126
127 :ref:`Documentation/process/stable-api-nonse 127 :ref:`Documentation/process/stable-api-nonsense.rst <stable_api_nonsense>`
128 This file describes the rationale behind t 128 This file describes the rationale behind the conscious decision to
129 not have a stable API within the kernel, i 129 not have a stable API within the kernel, including things like:
130 130
131 - Subsystem shim-layers (for compatibili 131 - Subsystem shim-layers (for compatibility?)
132 - Driver portability between Operating S 132 - Driver portability between Operating Systems.
133 - Mitigating rapid change within the ker 133 - Mitigating rapid change within the kernel source tree (or
134 preventing rapid change) 134 preventing rapid change)
135 135
136 This document is crucial for understanding 136 This document is crucial for understanding the Linux development
137 philosophy and is very important for peopl 137 philosophy and is very important for people moving to Linux from
138 development on other Operating Systems. 138 development on other Operating Systems.
139 139
140 :ref:`Documentation/process/security-bugs.rs 140 :ref:`Documentation/process/security-bugs.rst <securitybugs>`
141 If you feel you have found a security prob 141 If you feel you have found a security problem in the Linux kernel,
142 please follow the steps in this document t 142 please follow the steps in this document to help notify the kernel
143 developers, and help solve the issue. 143 developers, and help solve the issue.
144 144
145 :ref:`Documentation/process/management-style 145 :ref:`Documentation/process/management-style.rst <managementstyle>`
146 This document describes how Linux kernel m 146 This document describes how Linux kernel maintainers operate and the
147 shared ethos behind their methodologies. 147 shared ethos behind their methodologies. This is important reading
148 for anyone new to kernel development (or a 148 for anyone new to kernel development (or anyone simply curious about
149 it), as it resolves a lot of common miscon 149 it), as it resolves a lot of common misconceptions and confusion
150 about the unique behavior of kernel mainta 150 about the unique behavior of kernel maintainers.
151 151
152 :ref:`Documentation/process/stable-kernel-ru 152 :ref:`Documentation/process/stable-kernel-rules.rst <stable_kernel_rules>`
153 This file describes the rules on how the s 153 This file describes the rules on how the stable kernel releases
154 happen, and what to do if you want to get 154 happen, and what to do if you want to get a change into one of these
155 releases. 155 releases.
156 156
157 :ref:`Documentation/process/kernel-docs.rst 157 :ref:`Documentation/process/kernel-docs.rst <kernel_docs>`
158 A list of external documentation that pert 158 A list of external documentation that pertains to kernel
159 development. Please consult this list if 159 development. Please consult this list if you do not find what you
160 are looking for within the in-kernel docum 160 are looking for within the in-kernel documentation.
161 161
162 :ref:`Documentation/process/applying-patches 162 :ref:`Documentation/process/applying-patches.rst <applying_patches>`
163 A good introduction describing exactly wha 163 A good introduction describing exactly what a patch is and how to
164 apply it to the different development bran 164 apply it to the different development branches of the kernel.
165 165
166 The kernel also has a large number of document 166 The kernel also has a large number of documents that can be
167 automatically generated from the source code i 167 automatically generated from the source code itself or from
168 ReStructuredText markups (ReST), like this one 168 ReStructuredText markups (ReST), like this one. This includes a
169 full description of the in-kernel API, and rul 169 full description of the in-kernel API, and rules on how to handle
170 locking properly. 170 locking properly.
171 171
172 All such documents can be generated as PDF or 172 All such documents can be generated as PDF or HTML by running::
173 173
174 make pdfdocs 174 make pdfdocs
175 make htmldocs 175 make htmldocs
176 176
177 respectively from the main kernel source direc 177 respectively from the main kernel source directory.
178 178
179 The documents that uses ReST markup will be ge 179 The documents that uses ReST markup will be generated at Documentation/output.
180 They can also be generated on LaTeX and ePub f 180 They can also be generated on LaTeX and ePub formats with::
181 181
182 make latexdocs 182 make latexdocs
183 make epubdocs 183 make epubdocs
184 184
185 Becoming A Kernel Developer 185 Becoming A Kernel Developer
186 --------------------------- 186 ---------------------------
187 187
188 If you do not know anything about Linux kernel 188 If you do not know anything about Linux kernel development, you should
189 look at the Linux KernelNewbies project: 189 look at the Linux KernelNewbies project:
190 190
191 https://kernelnewbies.org 191 https://kernelnewbies.org
192 192
193 It consists of a helpful mailing list where yo 193 It consists of a helpful mailing list where you can ask almost any type
194 of basic kernel development question (make sur 194 of basic kernel development question (make sure to search the archives
195 first, before asking something that has alread 195 first, before asking something that has already been answered in the
196 past.) It also has an IRC channel that you ca 196 past.) It also has an IRC channel that you can use to ask questions in
197 real-time, and a lot of helpful documentation 197 real-time, and a lot of helpful documentation that is useful for
198 learning about Linux kernel development. 198 learning about Linux kernel development.
199 199
200 The website has basic information about code o 200 The website has basic information about code organization, subsystems,
201 and current projects (both in-tree and out-of- 201 and current projects (both in-tree and out-of-tree). It also describes
202 some basic logistical information, like how to 202 some basic logistical information, like how to compile a kernel and
203 apply a patch. 203 apply a patch.
204 204
205 If you do not know where you want to start, bu 205 If you do not know where you want to start, but you want to look for
206 some task to start doing to join into the kern 206 some task to start doing to join into the kernel development community,
207 go to the Linux Kernel Janitor's project: 207 go to the Linux Kernel Janitor's project:
208 208
209 https://kernelnewbies.org/KernelJanito 209 https://kernelnewbies.org/KernelJanitors
210 210
211 It is a great place to start. It describes a 211 It is a great place to start. It describes a list of relatively simple
212 problems that need to be cleaned up and fixed 212 problems that need to be cleaned up and fixed within the Linux kernel
213 source tree. Working with the developers in c 213 source tree. Working with the developers in charge of this project, you
214 will learn the basics of getting your patch in 214 will learn the basics of getting your patch into the Linux kernel tree,
215 and possibly be pointed in the direction of wh 215 and possibly be pointed in the direction of what to go work on next, if
216 you do not already have an idea. 216 you do not already have an idea.
217 217
218 Before making any actual modifications to the 218 Before making any actual modifications to the Linux kernel code, it is
219 imperative to understand how the code in quest 219 imperative to understand how the code in question works. For this
220 purpose, nothing is better than reading throug 220 purpose, nothing is better than reading through it directly (most tricky
221 bits are commented well), perhaps even with th 221 bits are commented well), perhaps even with the help of specialized
222 tools. One such tool that is particularly rec 222 tools. One such tool that is particularly recommended is the Linux
223 Cross-Reference project, which is able to pres 223 Cross-Reference project, which is able to present source code in a
224 self-referential, indexed webpage format. An e 224 self-referential, indexed webpage format. An excellent up-to-date
225 repository of the kernel code may be found at: 225 repository of the kernel code may be found at:
226 226
227 https://elixir.bootlin.com/ 227 https://elixir.bootlin.com/
228 228
229 229
230 The development process 230 The development process
231 ----------------------- 231 -----------------------
232 232
233 Linux kernel development process currently con 233 Linux kernel development process currently consists of a few different
234 main kernel "branches" and lots of different s 234 main kernel "branches" and lots of different subsystem-specific kernel
235 branches. These different branches are: 235 branches. These different branches are:
236 236
237 - Linus's mainline tree 237 - Linus's mainline tree
238 - Various stable trees with multiple major n 238 - Various stable trees with multiple major numbers
239 - Subsystem-specific trees 239 - Subsystem-specific trees
240 - linux-next integration testing tree 240 - linux-next integration testing tree
241 241
242 Mainline tree 242 Mainline tree
243 ~~~~~~~~~~~~~ 243 ~~~~~~~~~~~~~
244 244
245 The mainline tree is maintained by Linus Torva 245 The mainline tree is maintained by Linus Torvalds, and can be found at
246 https://kernel.org or in the repo. Its develo 246 https://kernel.org or in the repo. Its development process is as follows:
247 247
248 - As soon as a new kernel is released a two 248 - As soon as a new kernel is released a two week window is open,
249 during this period of time maintainers can 249 during this period of time maintainers can submit big diffs to
250 Linus, usually the patches that have alrea 250 Linus, usually the patches that have already been included in the
251 linux-next for a few weeks. The preferred 251 linux-next for a few weeks. The preferred way to submit big changes
252 is using git (the kernel's source manageme 252 is using git (the kernel's source management tool, more information
253 can be found at https://git-scm.com/) but 253 can be found at https://git-scm.com/) but plain patches are also just
254 fine. 254 fine.
255 - After two weeks a -rc1 kernel is released 255 - After two weeks a -rc1 kernel is released and the focus is on making the
256 new kernel as rock solid as possible. Mos 256 new kernel as rock solid as possible. Most of the patches at this point
257 should fix a regression. Bugs that have a 257 should fix a regression. Bugs that have always existed are not
258 regressions, so only push these kinds of f 258 regressions, so only push these kinds of fixes if they are important.
259 Please note that a whole new driver (or fi 259 Please note that a whole new driver (or filesystem) might be accepted
260 after -rc1 because there is no risk of cau 260 after -rc1 because there is no risk of causing regressions with such a
261 change as long as the change is self-conta 261 change as long as the change is self-contained and does not affect areas
262 outside of the code that is being added. 262 outside of the code that is being added. git can be used to send
263 patches to Linus after -rc1 is released, b 263 patches to Linus after -rc1 is released, but the patches need to also be
264 sent to a public mailing list for review. 264 sent to a public mailing list for review.
265 - A new -rc is released whenever Linus deems 265 - A new -rc is released whenever Linus deems the current git tree to
266 be in a reasonably sane state adequate for 266 be in a reasonably sane state adequate for testing. The goal is to
267 release a new -rc kernel every week. 267 release a new -rc kernel every week.
268 - Process continues until the kernel is cons 268 - Process continues until the kernel is considered "ready", the
269 process should last around 6 weeks. 269 process should last around 6 weeks.
270 270
271 It is worth mentioning what Andrew Morton wrot 271 It is worth mentioning what Andrew Morton wrote on the linux-kernel
272 mailing list about kernel releases: 272 mailing list about kernel releases:
273 273
274 *"Nobody knows when a kernel will be r 274 *"Nobody knows when a kernel will be released, because it's
275 released according to perceived bug st 275 released according to perceived bug status, not according to a
276 preconceived timeline."* 276 preconceived timeline."*
277 277
278 Various stable trees with multiple major numbe 278 Various stable trees with multiple major numbers
279 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 279 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
280 280
281 Kernels with 3-part versions are -stable kerne 281 Kernels with 3-part versions are -stable kernels. They contain
282 relatively small and critical fixes for securi 282 relatively small and critical fixes for security problems or significant
283 regressions discovered in a given major mainli 283 regressions discovered in a given major mainline release. Each release
284 in a major stable series increments the third 284 in a major stable series increments the third part of the version
285 number, keeping the first two parts the same. 285 number, keeping the first two parts the same.
286 286
287 This is the recommended branch for users who w 287 This is the recommended branch for users who want the most recent stable
288 kernel and are not interested in helping test 288 kernel and are not interested in helping test development/experimental
289 versions. 289 versions.
290 290
291 Stable trees are maintained by the "stable" te< 291 Stable trees are maintained by the "stable" team <stable@vger.kernel.org>, and
292 are released as needs dictate. The normal rel 292 are released as needs dictate. The normal release period is approximately
293 two weeks, but it can be longer if there are n 293 two weeks, but it can be longer if there are no pressing problems. A
294 security-related problem, instead, can cause a 294 security-related problem, instead, can cause a release to happen almost
295 instantly. 295 instantly.
296 296
297 The file :ref:`Documentation/process/stable-ke 297 The file :ref:`Documentation/process/stable-kernel-rules.rst <stable_kernel_rules>`
298 in the kernel tree documents what kinds of cha 298 in the kernel tree documents what kinds of changes are acceptable for
299 the -stable tree, and how the release process 299 the -stable tree, and how the release process works.
300 300
301 Subsystem-specific trees 301 Subsystem-specific trees
302 ~~~~~~~~~~~~~~~~~~~~~~~~ 302 ~~~~~~~~~~~~~~~~~~~~~~~~
303 303
304 The maintainers of the various kernel subsyste 304 The maintainers of the various kernel subsystems --- and also many
305 kernel subsystem developers --- expose their c 305 kernel subsystem developers --- expose their current state of
306 development in source repositories. That way, 306 development in source repositories. That way, others can see what is
307 happening in the different areas of the kernel 307 happening in the different areas of the kernel. In areas where
308 development is rapid, a developer may be asked 308 development is rapid, a developer may be asked to base his submissions
309 onto such a subsystem kernel tree so that conf 309 onto such a subsystem kernel tree so that conflicts between the
310 submission and other already ongoing work are 310 submission and other already ongoing work are avoided.
311 311
312 Most of these repositories are git trees, but 312 Most of these repositories are git trees, but there are also other SCMs
313 in use, or patch queues being published as qui 313 in use, or patch queues being published as quilt series. Addresses of
314 these subsystem repositories are listed in the 314 these subsystem repositories are listed in the MAINTAINERS file. Many
315 of them can be browsed at https://git.kernel.o 315 of them can be browsed at https://git.kernel.org/.
316 316
317 Before a proposed patch is committed to such a 317 Before a proposed patch is committed to such a subsystem tree, it is
318 subject to review which primarily happens on m 318 subject to review which primarily happens on mailing lists (see the
319 respective section below). For several kernel 319 respective section below). For several kernel subsystems, this review
320 process is tracked with the tool patchwork. P 320 process is tracked with the tool patchwork. Patchwork offers a web
321 interface which shows patch postings, any comm 321 interface which shows patch postings, any comments on a patch or
322 revisions to it, and maintainers can mark patc 322 revisions to it, and maintainers can mark patches as under review,
323 accepted, or rejected. Most of these patchwor 323 accepted, or rejected. Most of these patchwork sites are listed at
324 https://patchwork.kernel.org/. 324 https://patchwork.kernel.org/.
325 325
326 linux-next integration testing tree 326 linux-next integration testing tree
327 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 327 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
328 328
329 Before updates from subsystem trees are merged 329 Before updates from subsystem trees are merged into the mainline tree,
330 they need to be integration-tested. For this 330 they need to be integration-tested. For this purpose, a special
331 testing repository exists into which virtually 331 testing repository exists into which virtually all subsystem trees are
332 pulled on an almost daily basis: 332 pulled on an almost daily basis:
333 333
334 https://git.kernel.org/pub/scm/linux/k 334 https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git
335 335
336 This way, the linux-next gives a summary outlo 336 This way, the linux-next gives a summary outlook onto what will be
337 expected to go into the mainline kernel at the 337 expected to go into the mainline kernel at the next merge period.
338 Adventurous testers are very welcome to runtim 338 Adventurous testers are very welcome to runtime-test the linux-next.
339 339
340 340
341 Bug Reporting 341 Bug Reporting
342 ------------- 342 -------------
343 343
344 The file 'Documentation/admin-guide/reporting- 344 The file 'Documentation/admin-guide/reporting-issues.rst' in the main kernel
345 source directory describes how to report a pos 345 source directory describes how to report a possible kernel bug, and details
346 what kind of information is needed by the kern 346 what kind of information is needed by the kernel developers to help track
347 down the problem. 347 down the problem.
348 348
349 349
350 Managing bug reports 350 Managing bug reports
351 -------------------- 351 --------------------
352 352
353 One of the best ways to put into practice your 353 One of the best ways to put into practice your hacking skills is by fixing
354 bugs reported by other people. Not only will y 354 bugs reported by other people. Not only will you help to make the kernel
355 more stable, but you'll also learn to fix real 355 more stable, but you'll also learn to fix real-world problems and you will
356 improve your skills, and other developers will 356 improve your skills, and other developers will be aware of your presence.
357 Fixing bugs is one of the best ways to get mer 357 Fixing bugs is one of the best ways to get merits among other developers,
358 because not many people like wasting time fixi 358 because not many people like wasting time fixing other people's bugs.
359 359
360 To work on already reported bug reports, find 360 To work on already reported bug reports, find a subsystem you are interested in.
361 Check the MAINTAINERS file where bugs for that 361 Check the MAINTAINERS file where bugs for that subsystem get reported to; often
362 it will be a mailing list, rarely a bugtracker 362 it will be a mailing list, rarely a bugtracker. Search the archives of said
363 place for recent reports and help where you se 363 place for recent reports and help where you see fit. You may also want to check
364 https://bugzilla.kernel.org for bug reports; o 364 https://bugzilla.kernel.org for bug reports; only a handful of kernel subsystems
365 use it actively for reporting or tracking, nev 365 use it actively for reporting or tracking, nevertheless bugs for the whole
366 kernel get filed there. 366 kernel get filed there.
367 367
368 368
369 Mailing lists 369 Mailing lists
370 ------------- 370 -------------
371 371
372 As some of the above documents describe, the m 372 As some of the above documents describe, the majority of the core kernel
373 developers participate on the Linux Kernel Mai 373 developers participate on the Linux Kernel Mailing list. Details on how
374 to subscribe and unsubscribe from the list can 374 to subscribe and unsubscribe from the list can be found at:
375 375
376 https://subspace.kernel.org/subscribin 376 https://subspace.kernel.org/subscribing.html
377 377
378 There are archives of the mailing list on the 378 There are archives of the mailing list on the web in many different
379 places. Use a search engine to find these arc 379 places. Use a search engine to find these archives. For example:
380 380
381 https://lore.kernel.org/linux-kernel/ 381 https://lore.kernel.org/linux-kernel/
382 382
383 It is highly recommended that you search the a 383 It is highly recommended that you search the archives about the topic
384 you want to bring up, before you post it to th 384 you want to bring up, before you post it to the list. A lot of things
385 already discussed in detail are only recorded 385 already discussed in detail are only recorded at the mailing list
386 archives. 386 archives.
387 387
388 Most of the individual kernel subsystems also 388 Most of the individual kernel subsystems also have their own separate
389 mailing list where they do their development e 389 mailing list where they do their development efforts. See the
390 MAINTAINERS file for a list of what these list 390 MAINTAINERS file for a list of what these lists are for the different
391 groups. 391 groups.
392 392
393 Many of the lists are hosted on kernel.org. In 393 Many of the lists are hosted on kernel.org. Information on them can be
394 found at: 394 found at:
395 395
396 https://subspace.kernel.org 396 https://subspace.kernel.org
397 397
398 Please remember to follow good behavioral habi 398 Please remember to follow good behavioral habits when using the lists.
399 Though a bit cheesy, the following URL has som 399 Though a bit cheesy, the following URL has some simple guidelines for
400 interacting with the list (or any list): 400 interacting with the list (or any list):
401 401
402 https://subspace.kernel.org/etiquette. 402 https://subspace.kernel.org/etiquette.html
403 403
404 If multiple people respond to your mail, the C 404 If multiple people respond to your mail, the CC: list of recipients may
405 get pretty large. Don't remove anybody from th 405 get pretty large. Don't remove anybody from the CC: list without a good
406 reason, or don't reply only to the list addres 406 reason, or don't reply only to the list address. Get used to receiving the
407 mail twice, one from the sender and the one fr 407 mail twice, one from the sender and the one from the list, and don't try
408 to tune that by adding fancy mail-headers, peo 408 to tune that by adding fancy mail-headers, people will not like it.
409 409
410 Remember to keep the context and the attributi 410 Remember to keep the context and the attribution of your replies intact,
411 keep the "John Kernelhacker wrote ...:" lines 411 keep the "John Kernelhacker wrote ...:" lines at the top of your reply, and
412 add your statements between the individual quo 412 add your statements between the individual quoted sections instead of
413 writing at the top of the mail. 413 writing at the top of the mail.
414 414
415 If you add patches to your mail, make sure the 415 If you add patches to your mail, make sure they are plain readable text
416 as stated in :ref:`Documentation/process/submi 416 as stated in :ref:`Documentation/process/submitting-patches.rst <submittingpatches>`.
417 Kernel developers don't want to deal with 417 Kernel developers don't want to deal with
418 attachments or compressed patches; they may wa 418 attachments or compressed patches; they may want to comment on
419 individual lines of your patch, which works on 419 individual lines of your patch, which works only that way. Make sure you
420 use a mail program that does not mangle spaces 420 use a mail program that does not mangle spaces and tab characters. A
421 good first test is to send the mail to yoursel 421 good first test is to send the mail to yourself and try to apply your
422 own patch by yourself. If that doesn't work, g 422 own patch by yourself. If that doesn't work, get your mail program fixed
423 or change it until it works. 423 or change it until it works.
424 424
425 Above all, please remember to show respect to 425 Above all, please remember to show respect to other subscribers.
426 426
427 427
428 Working with the community 428 Working with the community
429 -------------------------- 429 --------------------------
430 430
431 The goal of the kernel community is to provide 431 The goal of the kernel community is to provide the best possible kernel
432 there is. When you submit a patch for accepta 432 there is. When you submit a patch for acceptance, it will be reviewed
433 on its technical merits and those alone. So, 433 on its technical merits and those alone. So, what should you be
434 expecting? 434 expecting?
435 435
436 - criticism 436 - criticism
437 - comments 437 - comments
438 - requests for change 438 - requests for change
439 - requests for justification 439 - requests for justification
440 - silence 440 - silence
441 441
442 Remember, this is part of getting your patch i 442 Remember, this is part of getting your patch into the kernel. You have
443 to be able to take criticism and comments abou 443 to be able to take criticism and comments about your patches, evaluate
444 them at a technical level and either rework yo 444 them at a technical level and either rework your patches or provide
445 clear and concise reasoning as to why those ch 445 clear and concise reasoning as to why those changes should not be made.
446 If there are no responses to your posting, wai 446 If there are no responses to your posting, wait a few days and try
447 again, sometimes things get lost in the huge v 447 again, sometimes things get lost in the huge volume.
448 448
449 What should you not do? 449 What should you not do?
450 450
451 - expect your patch to be accepted without q 451 - expect your patch to be accepted without question
452 - become defensive 452 - become defensive
453 - ignore comments 453 - ignore comments
454 - resubmit the patch without making any of t 454 - resubmit the patch without making any of the requested changes
455 455
456 In a community that is looking for the best te 456 In a community that is looking for the best technical solution possible,
457 there will always be differing opinions on how 457 there will always be differing opinions on how beneficial a patch is.
458 You have to be cooperative, and willing to ada 458 You have to be cooperative, and willing to adapt your idea to fit within
459 the kernel. Or at least be willing to prove y 459 the kernel. Or at least be willing to prove your idea is worth it.
460 Remember, being wrong is acceptable as long as 460 Remember, being wrong is acceptable as long as you are willing to work
461 toward a solution that is right. 461 toward a solution that is right.
462 462
463 It is normal that the answers to your first pa 463 It is normal that the answers to your first patch might simply be a list
464 of a dozen things you should correct. This do 464 of a dozen things you should correct. This does **not** imply that your
465 patch will not be accepted, and it is **not** 465 patch will not be accepted, and it is **not** meant against you
466 personally. Simply correct all issues raised 466 personally. Simply correct all issues raised against your patch and
467 resend it. 467 resend it.
468 468
469 469
470 Differences between the kernel community and c 470 Differences between the kernel community and corporate structures
471 ---------------------------------------------- 471 -----------------------------------------------------------------
472 472
473 The kernel community works differently than mo 473 The kernel community works differently than most traditional corporate
474 development environments. Here are a list of 474 development environments. Here are a list of things that you can try to
475 do to avoid problems: 475 do to avoid problems:
476 476
477 Good things to say regarding your proposed c 477 Good things to say regarding your proposed changes:
478 478
479 - "This solves multiple problems." 479 - "This solves multiple problems."
480 - "This deletes 2000 lines of code." 480 - "This deletes 2000 lines of code."
481 - "Here is a patch that explains what I am 481 - "Here is a patch that explains what I am trying to describe."
482 - "I tested it on 5 different architecture 482 - "I tested it on 5 different architectures..."
483 - "Here is a series of small patches that. 483 - "Here is a series of small patches that..."
484 - "This increases performance on typical m 484 - "This increases performance on typical machines..."
485 485
486 Bad things you should avoid saying: 486 Bad things you should avoid saying:
487 487
488 - "We did it this way in AIX/ptx/Solaris, 488 - "We did it this way in AIX/ptx/Solaris, so therefore it must be
489 good..." 489 good..."
490 - "I've being doing this for 20 years, so. 490 - "I've being doing this for 20 years, so..."
491 - "This is required for my company to make 491 - "This is required for my company to make money"
492 - "This is for our Enterprise product line 492 - "This is for our Enterprise product line."
493 - "Here is my 1000 page design document th 493 - "Here is my 1000 page design document that describes my idea"
494 - "I've been working on this for 6 months. 494 - "I've been working on this for 6 months..."
495 - "Here's a 5000 line patch that..." 495 - "Here's a 5000 line patch that..."
496 - "I rewrote all of the current mess, and 496 - "I rewrote all of the current mess, and here it is..."
497 - "I have a deadline, and this patch needs 497 - "I have a deadline, and this patch needs to be applied now."
498 498
499 Another way the kernel community is different 499 Another way the kernel community is different than most traditional
500 software engineering work environments is the 500 software engineering work environments is the faceless nature of
501 interaction. One benefit of using email and i 501 interaction. One benefit of using email and irc as the primary forms of
502 communication is the lack of discrimination ba 502 communication is the lack of discrimination based on gender or race.
503 The Linux kernel work environment is accepting 503 The Linux kernel work environment is accepting of women and minorities
504 because all you are is an email address. The 504 because all you are is an email address. The international aspect also
505 helps to level the playing field because you c 505 helps to level the playing field because you can't guess gender based on
506 a person's name. A man may be named Andrea and 506 a person's name. A man may be named Andrea and a woman may be named Pat.
507 Most women who have worked in the Linux kernel 507 Most women who have worked in the Linux kernel and have expressed an
508 opinion have had positive experiences. 508 opinion have had positive experiences.
509 509
510 The language barrier can cause problems for so 510 The language barrier can cause problems for some people who are not
511 comfortable with English. A good grasp of the 511 comfortable with English. A good grasp of the language can be needed in
512 order to get ideas across properly on mailing 512 order to get ideas across properly on mailing lists, so it is
513 recommended that you check your emails to make 513 recommended that you check your emails to make sure they make sense in
514 English before sending them. 514 English before sending them.
515 515
516 516
517 Break up your changes 517 Break up your changes
518 --------------------- 518 ---------------------
519 519
520 The Linux kernel community does not gladly acc 520 The Linux kernel community does not gladly accept large chunks of code
521 dropped on it all at once. The changes need t 521 dropped on it all at once. The changes need to be properly introduced,
522 discussed, and broken up into tiny, individual 522 discussed, and broken up into tiny, individual portions. This is almost
523 the exact opposite of what companies are used 523 the exact opposite of what companies are used to doing. Your proposal
524 should also be introduced very early in the de 524 should also be introduced very early in the development process, so that
525 you can receive feedback on what you are doing 525 you can receive feedback on what you are doing. It also lets the
526 community feel that you are working with them, 526 community feel that you are working with them, and not simply using them
527 as a dumping ground for your feature. However 527 as a dumping ground for your feature. However, don't send 50 emails at
528 one time to a mailing list, your patch series 528 one time to a mailing list, your patch series should be smaller than
529 that almost all of the time. 529 that almost all of the time.
530 530
531 The reasons for breaking things up are the fol 531 The reasons for breaking things up are the following:
532 532
533 1) Small patches increase the likelihood that 533 1) Small patches increase the likelihood that your patches will be
534 applied, since they don't take much time or 534 applied, since they don't take much time or effort to verify for
535 correctness. A 5 line patch can be applied 535 correctness. A 5 line patch can be applied by a maintainer with
536 barely a second glance. However, a 500 line 536 barely a second glance. However, a 500 line patch may take hours to
537 review for correctness (the time it takes i 537 review for correctness (the time it takes is exponentially
538 proportional to the size of the patch, or s 538 proportional to the size of the patch, or something).
539 539
540 Small patches also make it very easy to deb 540 Small patches also make it very easy to debug when something goes
541 wrong. It's much easier to back out patche 541 wrong. It's much easier to back out patches one by one than it is
542 to dissect a very large patch after it's be 542 to dissect a very large patch after it's been applied (and broken
543 something). 543 something).
544 544
545 2) It's important not only to send small patch 545 2) It's important not only to send small patches, but also to rewrite
546 and simplify (or simply re-order) patches b 546 and simplify (or simply re-order) patches before submitting them.
547 547
548 Here is an analogy from kernel developer Al Vi 548 Here is an analogy from kernel developer Al Viro:
549 549
550 *"Think of a teacher grading homework 550 *"Think of a teacher grading homework from a math student. The
551 teacher does not want to see the stude 551 teacher does not want to see the student's trials and errors
552 before they came up with the solution. 552 before they came up with the solution. They want to see the
553 cleanest, most elegant answer. A good 553 cleanest, most elegant answer. A good student knows this, and
554 would never submit her intermediate wo 554 would never submit her intermediate work before the final
555 solution.* 555 solution.*
556 556
557 *The same is true of kernel developmen 557 *The same is true of kernel development. The maintainers and
558 reviewers do not want to see the thoug 558 reviewers do not want to see the thought process behind the
559 solution to the problem one is solving 559 solution to the problem one is solving. They want to see a
560 simple and elegant solution."* 560 simple and elegant solution."*
561 561
562 It may be challenging to keep the balance betw 562 It may be challenging to keep the balance between presenting an elegant
563 solution and working together with the communi 563 solution and working together with the community and discussing your
564 unfinished work. Therefore it is good to get e 564 unfinished work. Therefore it is good to get early in the process to
565 get feedback to improve your work, but also ke 565 get feedback to improve your work, but also keep your changes in small
566 chunks that they may get already accepted, eve 566 chunks that they may get already accepted, even when your whole task is
567 not ready for inclusion now. 567 not ready for inclusion now.
568 568
569 Also realize that it is not acceptable to send 569 Also realize that it is not acceptable to send patches for inclusion
570 that are unfinished and will be "fixed up late 570 that are unfinished and will be "fixed up later."
571 571
572 572
573 Justify your change 573 Justify your change
574 ------------------- 574 -------------------
575 575
576 Along with breaking up your patches, it is ver 576 Along with breaking up your patches, it is very important for you to let
577 the Linux community know why they should add t 577 the Linux community know why they should add this change. New features
578 must be justified as being needed and useful. 578 must be justified as being needed and useful.
579 579
580 580
581 Document your change 581 Document your change
582 -------------------- 582 --------------------
583 583
584 When sending in your patches, pay special atte 584 When sending in your patches, pay special attention to what you say in
585 the text in your email. This information will 585 the text in your email. This information will become the ChangeLog
586 information for the patch, and will be preserv 586 information for the patch, and will be preserved for everyone to see for
587 all time. It should describe the patch comple 587 all time. It should describe the patch completely, containing:
588 588
589 - why the change is necessary 589 - why the change is necessary
590 - the overall design approach in the patch 590 - the overall design approach in the patch
591 - implementation details 591 - implementation details
592 - testing results 592 - testing results
593 593
594 For more details on what this should all look 594 For more details on what this should all look like, please see the
595 ChangeLog section of the document: 595 ChangeLog section of the document:
596 596
597 "The Perfect Patch" 597 "The Perfect Patch"
598 https://www.ozlabs.org/~akpm/stuff/tpp.t 598 https://www.ozlabs.org/~akpm/stuff/tpp.txt
599 599
600 600
601 All of these things are sometimes very hard to 601 All of these things are sometimes very hard to do. It can take years to
602 perfect these practices (if at all). It's a co 602 perfect these practices (if at all). It's a continuous process of
603 improvement that requires a lot of patience an 603 improvement that requires a lot of patience and determination. But
604 don't give up, it's possible. Many have done i 604 don't give up, it's possible. Many have done it before, and each had to
605 start exactly where you are now. 605 start exactly where you are now.
606 606
607 607
608 608
609 609
610 ---------- 610 ----------
611 611
612 Thanks to Paolo Ciarrocchi who allowed the "De 612 Thanks to Paolo Ciarrocchi who allowed the "Development Process"
613 (https://lwn.net/Articles/94386/) section 613 (https://lwn.net/Articles/94386/) section
614 to be based on text he had written, and to Ran 614 to be based on text he had written, and to Randy Dunlap and Gerrit
615 Huizenga for some of the list of things you sh 615 Huizenga for some of the list of things you should and should not say.
616 Also thanks to Pat Mochel, Hanna Linder, Randy 616 Also thanks to Pat Mochel, Hanna Linder, Randy Dunlap, Kay Sievers,
617 Vojtech Pavlik, Jan Kara, Josh Boyer, Kees Coo 617 Vojtech Pavlik, Jan Kara, Josh Boyer, Kees Cook, Andrew Morton, Andi
618 Kleen, Vadim Lobanov, Jesper Juhl, Adrian Bunk 618 Kleen, Vadim Lobanov, Jesper Juhl, Adrian Bunk, Keri Harris, Frans Pop,
619 David A. Wheeler, Junio Hamano, Michael Kerris 619 David A. Wheeler, Junio Hamano, Michael Kerrisk, and Alex Shepard for
620 their review, comments, and contributions. Wi 620 their review, comments, and contributions. Without their help, this
621 document would not have been possible. 621 document would not have been possible.
622 622
623 623
624 624
625 Maintainer: Greg Kroah-Hartman <greg@kroah.com> 625 Maintainer: Greg Kroah-Hartman <greg@kroah.com>
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