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12 If anything in this document becomes out of da 12 If anything in this document becomes out of date, please send in patches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 mtk.manpages@gmail.com, and CC the list
>> 86 linux-api@vger.kernel.org.
86 87
87 Here is a list of files that are in the kernel 88 Here is a list of files that are in the kernel source tree that are
88 required reading: 89 required reading:
89 90
90 :ref:`Documentation/admin-guide/README.rst < 91 :ref:`Documentation/admin-guide/README.rst <readme>`
91 This file gives a short background on the 92 This file gives a short background on the Linux kernel and describes
92 what is necessary to do to configure and b 93 what is necessary to do to configure and build the kernel. People
93 who are new to the kernel should start her 94 who are new to the kernel should start here.
94 95
95 :ref:`Documentation/process/changes.rst <cha 96 :ref:`Documentation/process/changes.rst <changes>`
96 This file gives a list of the minimum leve 97 This file gives a list of the minimum levels of various software
97 packages that are necessary to build and r 98 packages that are necessary to build and run the kernel
98 successfully. 99 successfully.
99 100
100 :ref:`Documentation/process/coding-style.rst 101 :ref:`Documentation/process/coding-style.rst <codingstyle>`
101 This describes the Linux kernel coding sty 102 This describes the Linux kernel coding style, and some of the
102 rationale behind it. All new code is expec 103 rationale behind it. All new code is expected to follow the
103 guidelines in this document. Most maintain 104 guidelines in this document. Most maintainers will only accept
104 patches if these rules are followed, and m 105 patches if these rules are followed, and many people will only
105 review code if it is in the proper style. 106 review code if it is in the proper style.
106 107
107 :ref:`Documentation/process/submitting-patch !! 108 :ref:`Documentation/process/submitting-patches.rst <submittingpatches>` and :ref:`Documentation/process/submitting-drivers.rst <submittingdrivers>`
108 This file describes in explicit detail how !! 109 These files describe in explicit detail how to successfully create
109 and send a patch, including (but not limit 110 and send a patch, including (but not limited to):
110 111
111 - Email contents 112 - Email contents
112 - Email format 113 - Email format
113 - Who to send it to 114 - Who to send it to
114 115
115 Following these rules will not guarantee s 116 Following these rules will not guarantee success (as all patches are
116 subject to scrutiny for content and style) 117 subject to scrutiny for content and style), but not following them
117 will almost always prevent it. 118 will almost always prevent it.
118 119
119 Other excellent descriptions of how to cre 120 Other excellent descriptions of how to create patches properly are:
120 121
121 "The Perfect Patch" 122 "The Perfect Patch"
122 https://www.ozlabs.org/~akpm/s 123 https://www.ozlabs.org/~akpm/stuff/tpp.txt
123 124
124 "Linux kernel patch submission format" 125 "Linux kernel patch submission format"
125 https://web.archive.org/web/20 126 https://web.archive.org/web/20180829112450/http://linux.yyz.us/patch-format.html
126 127
127 :ref:`Documentation/process/stable-api-nonse 128 :ref:`Documentation/process/stable-api-nonsense.rst <stable_api_nonsense>`
128 This file describes the rationale behind t 129 This file describes the rationale behind the conscious decision to
129 not have a stable API within the kernel, i 130 not have a stable API within the kernel, including things like:
130 131
131 - Subsystem shim-layers (for compatibili 132 - Subsystem shim-layers (for compatibility?)
132 - Driver portability between Operating S 133 - Driver portability between Operating Systems.
133 - Mitigating rapid change within the ker 134 - Mitigating rapid change within the kernel source tree (or
134 preventing rapid change) 135 preventing rapid change)
135 136
136 This document is crucial for understanding 137 This document is crucial for understanding the Linux development
137 philosophy and is very important for peopl 138 philosophy and is very important for people moving to Linux from
138 development on other Operating Systems. 139 development on other Operating Systems.
139 140
140 :ref:`Documentation/process/security-bugs.rs !! 141 :ref:`Documentation/admin-guide/security-bugs.rst <securitybugs>`
141 If you feel you have found a security prob 142 If you feel you have found a security problem in the Linux kernel,
142 please follow the steps in this document t 143 please follow the steps in this document to help notify the kernel
143 developers, and help solve the issue. 144 developers, and help solve the issue.
144 145
145 :ref:`Documentation/process/management-style 146 :ref:`Documentation/process/management-style.rst <managementstyle>`
146 This document describes how Linux kernel m 147 This document describes how Linux kernel maintainers operate and the
147 shared ethos behind their methodologies. 148 shared ethos behind their methodologies. This is important reading
148 for anyone new to kernel development (or a 149 for anyone new to kernel development (or anyone simply curious about
149 it), as it resolves a lot of common miscon 150 it), as it resolves a lot of common misconceptions and confusion
150 about the unique behavior of kernel mainta 151 about the unique behavior of kernel maintainers.
151 152
152 :ref:`Documentation/process/stable-kernel-ru 153 :ref:`Documentation/process/stable-kernel-rules.rst <stable_kernel_rules>`
153 This file describes the rules on how the s 154 This file describes the rules on how the stable kernel releases
154 happen, and what to do if you want to get 155 happen, and what to do if you want to get a change into one of these
155 releases. 156 releases.
156 157
157 :ref:`Documentation/process/kernel-docs.rst 158 :ref:`Documentation/process/kernel-docs.rst <kernel_docs>`
158 A list of external documentation that pert 159 A list of external documentation that pertains to kernel
159 development. Please consult this list if 160 development. Please consult this list if you do not find what you
160 are looking for within the in-kernel docum 161 are looking for within the in-kernel documentation.
161 162
162 :ref:`Documentation/process/applying-patches 163 :ref:`Documentation/process/applying-patches.rst <applying_patches>`
163 A good introduction describing exactly wha 164 A good introduction describing exactly what a patch is and how to
164 apply it to the different development bran 165 apply it to the different development branches of the kernel.
165 166
166 The kernel also has a large number of document 167 The kernel also has a large number of documents that can be
167 automatically generated from the source code i 168 automatically generated from the source code itself or from
168 ReStructuredText markups (ReST), like this one 169 ReStructuredText markups (ReST), like this one. This includes a
169 full description of the in-kernel API, and rul 170 full description of the in-kernel API, and rules on how to handle
170 locking properly. 171 locking properly.
171 172
172 All such documents can be generated as PDF or 173 All such documents can be generated as PDF or HTML by running::
173 174
174 make pdfdocs 175 make pdfdocs
175 make htmldocs 176 make htmldocs
176 177
177 respectively from the main kernel source direc 178 respectively from the main kernel source directory.
178 179
179 The documents that uses ReST markup will be ge 180 The documents that uses ReST markup will be generated at Documentation/output.
180 They can also be generated on LaTeX and ePub f 181 They can also be generated on LaTeX and ePub formats with::
181 182
182 make latexdocs 183 make latexdocs
183 make epubdocs 184 make epubdocs
184 185
185 Becoming A Kernel Developer 186 Becoming A Kernel Developer
186 --------------------------- 187 ---------------------------
187 188
188 If you do not know anything about Linux kernel 189 If you do not know anything about Linux kernel development, you should
189 look at the Linux KernelNewbies project: 190 look at the Linux KernelNewbies project:
190 191
191 https://kernelnewbies.org 192 https://kernelnewbies.org
192 193
193 It consists of a helpful mailing list where yo 194 It consists of a helpful mailing list where you can ask almost any type
194 of basic kernel development question (make sur 195 of basic kernel development question (make sure to search the archives
195 first, before asking something that has alread 196 first, before asking something that has already been answered in the
196 past.) It also has an IRC channel that you ca 197 past.) It also has an IRC channel that you can use to ask questions in
197 real-time, and a lot of helpful documentation 198 real-time, and a lot of helpful documentation that is useful for
198 learning about Linux kernel development. 199 learning about Linux kernel development.
199 200
200 The website has basic information about code o 201 The website has basic information about code organization, subsystems,
201 and current projects (both in-tree and out-of- 202 and current projects (both in-tree and out-of-tree). It also describes
202 some basic logistical information, like how to 203 some basic logistical information, like how to compile a kernel and
203 apply a patch. 204 apply a patch.
204 205
205 If you do not know where you want to start, bu 206 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 207 some task to start doing to join into the kernel development community,
207 go to the Linux Kernel Janitor's project: 208 go to the Linux Kernel Janitor's project:
208 209
209 https://kernelnewbies.org/KernelJanito 210 https://kernelnewbies.org/KernelJanitors
210 211
211 It is a great place to start. It describes a 212 It is a great place to start. It describes a list of relatively simple
212 problems that need to be cleaned up and fixed 213 problems that need to be cleaned up and fixed within the Linux kernel
213 source tree. Working with the developers in c 214 source tree. Working with the developers in charge of this project, you
214 will learn the basics of getting your patch in 215 will learn the basics of getting your patch into the Linux kernel tree,
215 and possibly be pointed in the direction of wh 216 and possibly be pointed in the direction of what to go work on next, if
216 you do not already have an idea. 217 you do not already have an idea.
217 218
218 Before making any actual modifications to the 219 Before making any actual modifications to the Linux kernel code, it is
219 imperative to understand how the code in quest 220 imperative to understand how the code in question works. For this
220 purpose, nothing is better than reading throug 221 purpose, nothing is better than reading through it directly (most tricky
221 bits are commented well), perhaps even with th 222 bits are commented well), perhaps even with the help of specialized
222 tools. One such tool that is particularly rec 223 tools. One such tool that is particularly recommended is the Linux
223 Cross-Reference project, which is able to pres 224 Cross-Reference project, which is able to present source code in a
224 self-referential, indexed webpage format. An e 225 self-referential, indexed webpage format. An excellent up-to-date
225 repository of the kernel code may be found at: 226 repository of the kernel code may be found at:
226 227
227 https://elixir.bootlin.com/ 228 https://elixir.bootlin.com/
228 229
229 230
230 The development process 231 The development process
231 ----------------------- 232 -----------------------
232 233
233 Linux kernel development process currently con 234 Linux kernel development process currently consists of a few different
234 main kernel "branches" and lots of different s 235 main kernel "branches" and lots of different subsystem-specific kernel
235 branches. These different branches are: 236 branches. These different branches are:
236 237
237 - Linus's mainline tree 238 - Linus's mainline tree
238 - Various stable trees with multiple major n 239 - Various stable trees with multiple major numbers
239 - Subsystem-specific trees 240 - Subsystem-specific trees
240 - linux-next integration testing tree 241 - linux-next integration testing tree
241 242
242 Mainline tree 243 Mainline tree
243 ~~~~~~~~~~~~~ 244 ~~~~~~~~~~~~~
244 245
245 The mainline tree is maintained by Linus Torva 246 The mainline tree is maintained by Linus Torvalds, and can be found at
246 https://kernel.org or in the repo. Its develo 247 https://kernel.org or in the repo. Its development process is as follows:
247 248
248 - As soon as a new kernel is released a two 249 - As soon as a new kernel is released a two week window is open,
249 during this period of time maintainers can 250 during this period of time maintainers can submit big diffs to
250 Linus, usually the patches that have alrea 251 Linus, usually the patches that have already been included in the
251 linux-next for a few weeks. The preferred 252 linux-next for a few weeks. The preferred way to submit big changes
252 is using git (the kernel's source manageme 253 is using git (the kernel's source management tool, more information
253 can be found at https://git-scm.com/) but 254 can be found at https://git-scm.com/) but plain patches are also just
254 fine. 255 fine.
255 - After two weeks a -rc1 kernel is released 256 - After two weeks a -rc1 kernel is released and the focus is on making the
256 new kernel as rock solid as possible. Mos 257 new kernel as rock solid as possible. Most of the patches at this point
257 should fix a regression. Bugs that have a 258 should fix a regression. Bugs that have always existed are not
258 regressions, so only push these kinds of f 259 regressions, so only push these kinds of fixes if they are important.
259 Please note that a whole new driver (or fi 260 Please note that a whole new driver (or filesystem) might be accepted
260 after -rc1 because there is no risk of cau 261 after -rc1 because there is no risk of causing regressions with such a
261 change as long as the change is self-conta 262 change as long as the change is self-contained and does not affect areas
262 outside of the code that is being added. 263 outside of the code that is being added. git can be used to send
263 patches to Linus after -rc1 is released, b 264 patches to Linus after -rc1 is released, but the patches need to also be
264 sent to a public mailing list for review. 265 sent to a public mailing list for review.
265 - A new -rc is released whenever Linus deems 266 - A new -rc is released whenever Linus deems the current git tree to
266 be in a reasonably sane state adequate for 267 be in a reasonably sane state adequate for testing. The goal is to
267 release a new -rc kernel every week. 268 release a new -rc kernel every week.
268 - Process continues until the kernel is cons 269 - Process continues until the kernel is considered "ready", the
269 process should last around 6 weeks. 270 process should last around 6 weeks.
270 271
271 It is worth mentioning what Andrew Morton wrot 272 It is worth mentioning what Andrew Morton wrote on the linux-kernel
272 mailing list about kernel releases: 273 mailing list about kernel releases:
273 274
274 *"Nobody knows when a kernel will be r 275 *"Nobody knows when a kernel will be released, because it's
275 released according to perceived bug st 276 released according to perceived bug status, not according to a
276 preconceived timeline."* 277 preconceived timeline."*
277 278
278 Various stable trees with multiple major numbe 279 Various stable trees with multiple major numbers
279 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 280 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
280 281
281 Kernels with 3-part versions are -stable kerne 282 Kernels with 3-part versions are -stable kernels. They contain
282 relatively small and critical fixes for securi 283 relatively small and critical fixes for security problems or significant
283 regressions discovered in a given major mainli 284 regressions discovered in a given major mainline release. Each release
284 in a major stable series increments the third 285 in a major stable series increments the third part of the version
285 number, keeping the first two parts the same. 286 number, keeping the first two parts the same.
286 287
287 This is the recommended branch for users who w 288 This is the recommended branch for users who want the most recent stable
288 kernel and are not interested in helping test 289 kernel and are not interested in helping test development/experimental
289 versions. 290 versions.
290 291
291 Stable trees are maintained by the "stable" te< 292 Stable trees are maintained by the "stable" team <stable@vger.kernel.org>, and
292 are released as needs dictate. The normal rel 293 are released as needs dictate. The normal release period is approximately
293 two weeks, but it can be longer if there are n 294 two weeks, but it can be longer if there are no pressing problems. A
294 security-related problem, instead, can cause a 295 security-related problem, instead, can cause a release to happen almost
295 instantly. 296 instantly.
296 297
297 The file :ref:`Documentation/process/stable-ke 298 The file :ref:`Documentation/process/stable-kernel-rules.rst <stable_kernel_rules>`
298 in the kernel tree documents what kinds of cha 299 in the kernel tree documents what kinds of changes are acceptable for
299 the -stable tree, and how the release process 300 the -stable tree, and how the release process works.
300 301
301 Subsystem-specific trees 302 Subsystem-specific trees
302 ~~~~~~~~~~~~~~~~~~~~~~~~ 303 ~~~~~~~~~~~~~~~~~~~~~~~~
303 304
304 The maintainers of the various kernel subsyste 305 The maintainers of the various kernel subsystems --- and also many
305 kernel subsystem developers --- expose their c 306 kernel subsystem developers --- expose their current state of
306 development in source repositories. That way, 307 development in source repositories. That way, others can see what is
307 happening in the different areas of the kernel 308 happening in the different areas of the kernel. In areas where
308 development is rapid, a developer may be asked 309 development is rapid, a developer may be asked to base his submissions
309 onto such a subsystem kernel tree so that conf 310 onto such a subsystem kernel tree so that conflicts between the
310 submission and other already ongoing work are 311 submission and other already ongoing work are avoided.
311 312
312 Most of these repositories are git trees, but 313 Most of these repositories are git trees, but there are also other SCMs
313 in use, or patch queues being published as qui 314 in use, or patch queues being published as quilt series. Addresses of
314 these subsystem repositories are listed in the 315 these subsystem repositories are listed in the MAINTAINERS file. Many
315 of them can be browsed at https://git.kernel.o 316 of them can be browsed at https://git.kernel.org/.
316 317
317 Before a proposed patch is committed to such a 318 Before a proposed patch is committed to such a subsystem tree, it is
318 subject to review which primarily happens on m 319 subject to review which primarily happens on mailing lists (see the
319 respective section below). For several kernel 320 respective section below). For several kernel subsystems, this review
320 process is tracked with the tool patchwork. P 321 process is tracked with the tool patchwork. Patchwork offers a web
321 interface which shows patch postings, any comm 322 interface which shows patch postings, any comments on a patch or
322 revisions to it, and maintainers can mark patc 323 revisions to it, and maintainers can mark patches as under review,
323 accepted, or rejected. Most of these patchwor 324 accepted, or rejected. Most of these patchwork sites are listed at
324 https://patchwork.kernel.org/. 325 https://patchwork.kernel.org/.
325 326
326 linux-next integration testing tree 327 linux-next integration testing tree
327 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 328 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
328 329
329 Before updates from subsystem trees are merged 330 Before updates from subsystem trees are merged into the mainline tree,
330 they need to be integration-tested. For this 331 they need to be integration-tested. For this purpose, a special
331 testing repository exists into which virtually 332 testing repository exists into which virtually all subsystem trees are
332 pulled on an almost daily basis: 333 pulled on an almost daily basis:
333 334
334 https://git.kernel.org/pub/scm/linux/k !! 335 https://git.kernel.org/?p=linux/kernel/git/next/linux-next.git
335 336
336 This way, the linux-next gives a summary outlo 337 This way, the linux-next gives a summary outlook onto what will be
337 expected to go into the mainline kernel at the 338 expected to go into the mainline kernel at the next merge period.
338 Adventurous testers are very welcome to runtim 339 Adventurous testers are very welcome to runtime-test the linux-next.
339 340
340 341
341 Bug Reporting 342 Bug Reporting
342 ------------- 343 -------------
343 344
344 The file 'Documentation/admin-guide/reporting- 345 The file 'Documentation/admin-guide/reporting-issues.rst' in the main kernel
345 source directory describes how to report a pos 346 source directory describes how to report a possible kernel bug, and details
346 what kind of information is needed by the kern 347 what kind of information is needed by the kernel developers to help track
347 down the problem. 348 down the problem.
348 349
349 350
350 Managing bug reports 351 Managing bug reports
351 -------------------- 352 --------------------
352 353
353 One of the best ways to put into practice your 354 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 !! 355 bugs reported by other people. Not only you will help to make the kernel
355 more stable, but you'll also learn to fix real !! 356 more stable, but you'll also learn to fix real world problems and you will
356 improve your skills, and other developers will 357 improve your skills, and other developers will be aware of your presence.
357 Fixing bugs is one of the best ways to get mer 358 Fixing bugs is one of the best ways to get merits among other developers,
358 because not many people like wasting time fixi 359 because not many people like wasting time fixing other people's bugs.
359 360
360 To work on already reported bug reports, find 361 To work on already reported bug reports, find a subsystem you are interested in.
361 Check the MAINTAINERS file where bugs for that 362 Check the MAINTAINERS file where bugs for that subsystem get reported to; often
362 it will be a mailing list, rarely a bugtracker 363 it will be a mailing list, rarely a bugtracker. Search the archives of said
363 place for recent reports and help where you se 364 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 365 https://bugzilla.kernel.org for bug reports; only a handful of kernel subsystems
365 use it actively for reporting or tracking, nev 366 use it actively for reporting or tracking, nevertheless bugs for the whole
366 kernel get filed there. 367 kernel get filed there.
367 368
368 369
369 Mailing lists 370 Mailing lists
370 ------------- 371 -------------
371 372
372 As some of the above documents describe, the m 373 As some of the above documents describe, the majority of the core kernel
373 developers participate on the Linux Kernel Mai 374 developers participate on the Linux Kernel Mailing list. Details on how
374 to subscribe and unsubscribe from the list can 375 to subscribe and unsubscribe from the list can be found at:
375 376
376 https://subspace.kernel.org/subscribin !! 377 http://vger.kernel.org/vger-lists.html#linux-kernel
377 378
378 There are archives of the mailing list on the 379 There are archives of the mailing list on the web in many different
379 places. Use a search engine to find these arc 380 places. Use a search engine to find these archives. For example:
380 381
381 https://lore.kernel.org/linux-kernel/ !! 382 http://dir.gmane.org/gmane.linux.kernel
382 383
383 It is highly recommended that you search the a 384 It is highly recommended that you search the archives about the topic
384 you want to bring up, before you post it to th 385 you want to bring up, before you post it to the list. A lot of things
385 already discussed in detail are only recorded 386 already discussed in detail are only recorded at the mailing list
386 archives. 387 archives.
387 388
388 Most of the individual kernel subsystems also 389 Most of the individual kernel subsystems also have their own separate
389 mailing list where they do their development e 390 mailing list where they do their development efforts. See the
390 MAINTAINERS file for a list of what these list 391 MAINTAINERS file for a list of what these lists are for the different
391 groups. 392 groups.
392 393
393 Many of the lists are hosted on kernel.org. In 394 Many of the lists are hosted on kernel.org. Information on them can be
394 found at: 395 found at:
395 396
396 https://subspace.kernel.org !! 397 http://vger.kernel.org/vger-lists.html
397 398
398 Please remember to follow good behavioral habi 399 Please remember to follow good behavioral habits when using the lists.
399 Though a bit cheesy, the following URL has som 400 Though a bit cheesy, the following URL has some simple guidelines for
400 interacting with the list (or any list): 401 interacting with the list (or any list):
401 402
402 https://subspace.kernel.org/etiquette. !! 403 http://www.albion.com/netiquette/
403 404
404 If multiple people respond to your mail, the C 405 If multiple people respond to your mail, the CC: list of recipients may
405 get pretty large. Don't remove anybody from th 406 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 407 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 408 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 409 to tune that by adding fancy mail-headers, people will not like it.
409 410
410 Remember to keep the context and the attributi 411 Remember to keep the context and the attribution of your replies intact,
411 keep the "John Kernelhacker wrote ...:" lines 412 keep the "John Kernelhacker wrote ...:" lines at the top of your reply, and
412 add your statements between the individual quo 413 add your statements between the individual quoted sections instead of
413 writing at the top of the mail. 414 writing at the top of the mail.
414 415
415 If you add patches to your mail, make sure the 416 If you add patches to your mail, make sure they are plain readable text
416 as stated in :ref:`Documentation/process/submi 417 as stated in :ref:`Documentation/process/submitting-patches.rst <submittingpatches>`.
417 Kernel developers don't want to deal with 418 Kernel developers don't want to deal with
418 attachments or compressed patches; they may wa 419 attachments or compressed patches; they may want to comment on
419 individual lines of your patch, which works on 420 individual lines of your patch, which works only that way. Make sure you
420 use a mail program that does not mangle spaces 421 use a mail program that does not mangle spaces and tab characters. A
421 good first test is to send the mail to yoursel 422 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 423 own patch by yourself. If that doesn't work, get your mail program fixed
423 or change it until it works. 424 or change it until it works.
424 425
425 Above all, please remember to show respect to 426 Above all, please remember to show respect to other subscribers.
426 427
427 428
428 Working with the community 429 Working with the community
429 -------------------------- 430 --------------------------
430 431
431 The goal of the kernel community is to provide 432 The goal of the kernel community is to provide the best possible kernel
432 there is. When you submit a patch for accepta 433 there is. When you submit a patch for acceptance, it will be reviewed
433 on its technical merits and those alone. So, 434 on its technical merits and those alone. So, what should you be
434 expecting? 435 expecting?
435 436
436 - criticism 437 - criticism
437 - comments 438 - comments
438 - requests for change 439 - requests for change
439 - requests for justification 440 - requests for justification
440 - silence 441 - silence
441 442
442 Remember, this is part of getting your patch i 443 Remember, this is part of getting your patch into the kernel. You have
443 to be able to take criticism and comments abou 444 to be able to take criticism and comments about your patches, evaluate
444 them at a technical level and either rework yo 445 them at a technical level and either rework your patches or provide
445 clear and concise reasoning as to why those ch 446 clear and concise reasoning as to why those changes should not be made.
446 If there are no responses to your posting, wai 447 If there are no responses to your posting, wait a few days and try
447 again, sometimes things get lost in the huge v 448 again, sometimes things get lost in the huge volume.
448 449
449 What should you not do? 450 What should you not do?
450 451
451 - expect your patch to be accepted without q 452 - expect your patch to be accepted without question
452 - become defensive 453 - become defensive
453 - ignore comments 454 - ignore comments
454 - resubmit the patch without making any of t 455 - resubmit the patch without making any of the requested changes
455 456
456 In a community that is looking for the best te 457 In a community that is looking for the best technical solution possible,
457 there will always be differing opinions on how 458 there will always be differing opinions on how beneficial a patch is.
458 You have to be cooperative, and willing to ada 459 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 460 the kernel. Or at least be willing to prove your idea is worth it.
460 Remember, being wrong is acceptable as long as 461 Remember, being wrong is acceptable as long as you are willing to work
461 toward a solution that is right. 462 toward a solution that is right.
462 463
463 It is normal that the answers to your first pa 464 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 465 of a dozen things you should correct. This does **not** imply that your
465 patch will not be accepted, and it is **not** 466 patch will not be accepted, and it is **not** meant against you
466 personally. Simply correct all issues raised 467 personally. Simply correct all issues raised against your patch and
467 resend it. 468 resend it.
468 469
469 470
470 Differences between the kernel community and c 471 Differences between the kernel community and corporate structures
471 ---------------------------------------------- 472 -----------------------------------------------------------------
472 473
473 The kernel community works differently than mo 474 The kernel community works differently than most traditional corporate
474 development environments. Here are a list of 475 development environments. Here are a list of things that you can try to
475 do to avoid problems: 476 do to avoid problems:
476 477
477 Good things to say regarding your proposed c 478 Good things to say regarding your proposed changes:
478 479
479 - "This solves multiple problems." 480 - "This solves multiple problems."
480 - "This deletes 2000 lines of code." 481 - "This deletes 2000 lines of code."
481 - "Here is a patch that explains what I am 482 - "Here is a patch that explains what I am trying to describe."
482 - "I tested it on 5 different architecture 483 - "I tested it on 5 different architectures..."
483 - "Here is a series of small patches that. 484 - "Here is a series of small patches that..."
484 - "This increases performance on typical m 485 - "This increases performance on typical machines..."
485 486
486 Bad things you should avoid saying: 487 Bad things you should avoid saying:
487 488
488 - "We did it this way in AIX/ptx/Solaris, 489 - "We did it this way in AIX/ptx/Solaris, so therefore it must be
489 good..." 490 good..."
490 - "I've being doing this for 20 years, so. 491 - "I've being doing this for 20 years, so..."
491 - "This is required for my company to make 492 - "This is required for my company to make money"
492 - "This is for our Enterprise product line 493 - "This is for our Enterprise product line."
493 - "Here is my 1000 page design document th 494 - "Here is my 1000 page design document that describes my idea"
494 - "I've been working on this for 6 months. 495 - "I've been working on this for 6 months..."
495 - "Here's a 5000 line patch that..." 496 - "Here's a 5000 line patch that..."
496 - "I rewrote all of the current mess, and 497 - "I rewrote all of the current mess, and here it is..."
497 - "I have a deadline, and this patch needs 498 - "I have a deadline, and this patch needs to be applied now."
498 499
499 Another way the kernel community is different 500 Another way the kernel community is different than most traditional
500 software engineering work environments is the 501 software engineering work environments is the faceless nature of
501 interaction. One benefit of using email and i 502 interaction. One benefit of using email and irc as the primary forms of
502 communication is the lack of discrimination ba 503 communication is the lack of discrimination based on gender or race.
503 The Linux kernel work environment is accepting 504 The Linux kernel work environment is accepting of women and minorities
504 because all you are is an email address. The 505 because all you are is an email address. The international aspect also
505 helps to level the playing field because you c 506 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 507 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 508 Most women who have worked in the Linux kernel and have expressed an
508 opinion have had positive experiences. 509 opinion have had positive experiences.
509 510
510 The language barrier can cause problems for so 511 The language barrier can cause problems for some people who are not
511 comfortable with English. A good grasp of the 512 comfortable with English. A good grasp of the language can be needed in
512 order to get ideas across properly on mailing 513 order to get ideas across properly on mailing lists, so it is
513 recommended that you check your emails to make 514 recommended that you check your emails to make sure they make sense in
514 English before sending them. 515 English before sending them.
515 516
516 517
517 Break up your changes 518 Break up your changes
518 --------------------- 519 ---------------------
519 520
520 The Linux kernel community does not gladly acc 521 The Linux kernel community does not gladly accept large chunks of code
521 dropped on it all at once. The changes need t 522 dropped on it all at once. The changes need to be properly introduced,
522 discussed, and broken up into tiny, individual 523 discussed, and broken up into tiny, individual portions. This is almost
523 the exact opposite of what companies are used 524 the exact opposite of what companies are used to doing. Your proposal
524 should also be introduced very early in the de 525 should also be introduced very early in the development process, so that
525 you can receive feedback on what you are doing 526 you can receive feedback on what you are doing. It also lets the
526 community feel that you are working with them, 527 community feel that you are working with them, and not simply using them
527 as a dumping ground for your feature. However 528 as a dumping ground for your feature. However, don't send 50 emails at
528 one time to a mailing list, your patch series 529 one time to a mailing list, your patch series should be smaller than
529 that almost all of the time. 530 that almost all of the time.
530 531
531 The reasons for breaking things up are the fol 532 The reasons for breaking things up are the following:
532 533
533 1) Small patches increase the likelihood that 534 1) Small patches increase the likelihood that your patches will be
534 applied, since they don't take much time or 535 applied, since they don't take much time or effort to verify for
535 correctness. A 5 line patch can be applied 536 correctness. A 5 line patch can be applied by a maintainer with
536 barely a second glance. However, a 500 line 537 barely a second glance. However, a 500 line patch may take hours to
537 review for correctness (the time it takes i 538 review for correctness (the time it takes is exponentially
538 proportional to the size of the patch, or s 539 proportional to the size of the patch, or something).
539 540
540 Small patches also make it very easy to deb 541 Small patches also make it very easy to debug when something goes
541 wrong. It's much easier to back out patche 542 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 543 to dissect a very large patch after it's been applied (and broken
543 something). 544 something).
544 545
545 2) It's important not only to send small patch 546 2) It's important not only to send small patches, but also to rewrite
546 and simplify (or simply re-order) patches b 547 and simplify (or simply re-order) patches before submitting them.
547 548
548 Here is an analogy from kernel developer Al Vi 549 Here is an analogy from kernel developer Al Viro:
549 550
550 *"Think of a teacher grading homework 551 *"Think of a teacher grading homework from a math student. The
551 teacher does not want to see the stude 552 teacher does not want to see the student's trials and errors
552 before they came up with the solution. 553 before they came up with the solution. They want to see the
553 cleanest, most elegant answer. A good 554 cleanest, most elegant answer. A good student knows this, and
554 would never submit her intermediate wo 555 would never submit her intermediate work before the final
555 solution.* 556 solution.*
556 557
557 *The same is true of kernel developmen 558 *The same is true of kernel development. The maintainers and
558 reviewers do not want to see the thoug 559 reviewers do not want to see the thought process behind the
559 solution to the problem one is solving 560 solution to the problem one is solving. They want to see a
560 simple and elegant solution."* 561 simple and elegant solution."*
561 562
562 It may be challenging to keep the balance betw 563 It may be challenging to keep the balance between presenting an elegant
563 solution and working together with the communi 564 solution and working together with the community and discussing your
564 unfinished work. Therefore it is good to get e 565 unfinished work. Therefore it is good to get early in the process to
565 get feedback to improve your work, but also ke 566 get feedback to improve your work, but also keep your changes in small
566 chunks that they may get already accepted, eve 567 chunks that they may get already accepted, even when your whole task is
567 not ready for inclusion now. 568 not ready for inclusion now.
568 569
569 Also realize that it is not acceptable to send 570 Also realize that it is not acceptable to send patches for inclusion
570 that are unfinished and will be "fixed up late 571 that are unfinished and will be "fixed up later."
571 572
572 573
573 Justify your change 574 Justify your change
574 ------------------- 575 -------------------
575 576
576 Along with breaking up your patches, it is ver 577 Along with breaking up your patches, it is very important for you to let
577 the Linux community know why they should add t 578 the Linux community know why they should add this change. New features
578 must be justified as being needed and useful. 579 must be justified as being needed and useful.
579 580
580 581
581 Document your change 582 Document your change
582 -------------------- 583 --------------------
583 584
584 When sending in your patches, pay special atte 585 When sending in your patches, pay special attention to what you say in
585 the text in your email. This information will 586 the text in your email. This information will become the ChangeLog
586 information for the patch, and will be preserv 587 information for the patch, and will be preserved for everyone to see for
587 all time. It should describe the patch comple 588 all time. It should describe the patch completely, containing:
588 589
589 - why the change is necessary 590 - why the change is necessary
590 - the overall design approach in the patch 591 - the overall design approach in the patch
591 - implementation details 592 - implementation details
592 - testing results 593 - testing results
593 594
594 For more details on what this should all look 595 For more details on what this should all look like, please see the
595 ChangeLog section of the document: 596 ChangeLog section of the document:
596 597
597 "The Perfect Patch" 598 "The Perfect Patch"
598 https://www.ozlabs.org/~akpm/stuff/tpp.t 599 https://www.ozlabs.org/~akpm/stuff/tpp.txt
599 600
600 601
601 All of these things are sometimes very hard to 602 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 603 perfect these practices (if at all). It's a continuous process of
603 improvement that requires a lot of patience an 604 improvement that requires a lot of patience and determination. But
604 don't give up, it's possible. Many have done i 605 don't give up, it's possible. Many have done it before, and each had to
605 start exactly where you are now. 606 start exactly where you are now.
606 607
607 608
608 609
609 610
610 ---------- 611 ----------
611 612
612 Thanks to Paolo Ciarrocchi who allowed the "De 613 Thanks to Paolo Ciarrocchi who allowed the "Development Process"
613 (https://lwn.net/Articles/94386/) section 614 (https://lwn.net/Articles/94386/) section
614 to be based on text he had written, and to Ran 615 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 616 Huizenga for some of the list of things you should and should not say.
616 Also thanks to Pat Mochel, Hanna Linder, Randy 617 Also thanks to Pat Mochel, Hanna Linder, Randy Dunlap, Kay Sievers,
617 Vojtech Pavlik, Jan Kara, Josh Boyer, Kees Coo 618 Vojtech Pavlik, Jan Kara, Josh Boyer, Kees Cook, Andrew Morton, Andi
618 Kleen, Vadim Lobanov, Jesper Juhl, Adrian Bunk 619 Kleen, Vadim Lobanov, Jesper Juhl, Adrian Bunk, Keri Harris, Frans Pop,
619 David A. Wheeler, Junio Hamano, Michael Kerris 620 David A. Wheeler, Junio Hamano, Michael Kerrisk, and Alex Shepard for
620 their review, comments, and contributions. Wi 621 their review, comments, and contributions. Without their help, this
621 document would not have been possible. 622 document would not have been possible.
622 623
623 624
624 625
625 Maintainer: Greg Kroah-Hartman <greg@kroah.com> 626 Maintainer: Greg Kroah-Hartman <greg@kroah.com>
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