1 .. _development_coding: 1 .. _development_coding:
2 2
3 Getting the code right 3 Getting the code right
4 ====================== 4 ======================
5 5
6 While there is much to be said for a solid and 6 While there is much to be said for a solid and community-oriented design
7 process, the proof of any kernel development p 7 process, the proof of any kernel development project is in the resulting
8 code. It is the code which will be examined b 8 code. It is the code which will be examined by other developers and merged
9 (or not) into the mainline tree. So it is the 9 (or not) into the mainline tree. So it is the quality of this code which
10 will determine the ultimate success of the pro 10 will determine the ultimate success of the project.
11 11
12 This section will examine the coding process. 12 This section will examine the coding process. We'll start with a look at a
13 number of ways in which kernel developers can 13 number of ways in which kernel developers can go wrong. Then the focus
14 will shift toward doing things right and the t 14 will shift toward doing things right and the tools which can help in that
15 quest. 15 quest.
16 16
17 17
18 Pitfalls 18 Pitfalls
19 --------- 19 ---------
20 20
21 Coding style 21 Coding style
22 ************ 22 ************
23 23
24 The kernel has long had a standard coding styl 24 The kernel has long had a standard coding style, described in
25 :ref:`Documentation/process/coding-style.rst < 25 :ref:`Documentation/process/coding-style.rst <codingstyle>`. For much of
26 that time, the policies described in that file 26 that time, the policies described in that file were taken as being, at most,
27 advisory. As a result, there is a substantial 27 advisory. As a result, there is a substantial amount of code in the kernel
28 which does not meet the coding style guideline 28 which does not meet the coding style guidelines. The presence of that code
29 leads to two independent hazards for kernel de 29 leads to two independent hazards for kernel developers.
30 30
31 The first of these is to believe that the kern 31 The first of these is to believe that the kernel coding standards do not
32 matter and are not enforced. The truth of the 32 matter and are not enforced. The truth of the matter is that adding new
33 code to the kernel is very difficult if that c 33 code to the kernel is very difficult if that code is not coded according to
34 the standard; many developers will request tha 34 the standard; many developers will request that the code be reformatted
35 before they will even review it. A code base 35 before they will even review it. A code base as large as the kernel
36 requires some uniformity of code to make it po 36 requires some uniformity of code to make it possible for developers to
37 quickly understand any part of it. So there i 37 quickly understand any part of it. So there is no longer room for
38 strangely-formatted code. 38 strangely-formatted code.
39 39
40 Occasionally, the kernel's coding style will r 40 Occasionally, the kernel's coding style will run into conflict with an
41 employer's mandated style. In such cases, the 41 employer's mandated style. In such cases, the kernel's style will have to
42 win before the code can be merged. Putting co 42 win before the code can be merged. Putting code into the kernel means
43 giving up a degree of control in a number of w 43 giving up a degree of control in a number of ways - including control over
44 how the code is formatted. 44 how the code is formatted.
45 45
46 The other trap is to assume that code which is 46 The other trap is to assume that code which is already in the kernel is
47 urgently in need of coding style fixes. Devel 47 urgently in need of coding style fixes. Developers may start to generate
48 reformatting patches as a way of gaining famil 48 reformatting patches as a way of gaining familiarity with the process, or
49 as a way of getting their name into the kernel 49 as a way of getting their name into the kernel changelogs - or both. But
50 pure coding style fixes are seen as noise by t 50 pure coding style fixes are seen as noise by the development community;
51 they tend to get a chilly reception. So this 51 they tend to get a chilly reception. So this type of patch is best
52 avoided. It is natural to fix the style of a 52 avoided. It is natural to fix the style of a piece of code while working
53 on it for other reasons, but coding style chan 53 on it for other reasons, but coding style changes should not be made for
54 their own sake. 54 their own sake.
55 55
56 The coding style document also should not be r 56 The coding style document also should not be read as an absolute law which
57 can never be transgressed. If there is a good 57 can never be transgressed. If there is a good reason to go against the
58 style (a line which becomes far less readable 58 style (a line which becomes far less readable if split to fit within the
59 80-column limit, for example), just do it. 59 80-column limit, for example), just do it.
60 60
61 Note that you can also use the ``clang-format` 61 Note that you can also use the ``clang-format`` tool to help you with
62 these rules, to quickly re-format parts of you 62 these rules, to quickly re-format parts of your code automatically,
63 and to review full files in order to spot codi 63 and to review full files in order to spot coding style mistakes,
64 typos and possible improvements. It is also ha 64 typos and possible improvements. It is also handy for sorting ``#includes``,
65 for aligning variables/macros, for reflowing t 65 for aligning variables/macros, for reflowing text and other similar tasks.
66 See the file :ref:`Documentation/dev-tools/cla 66 See the file :ref:`Documentation/dev-tools/clang-format.rst <clangformat>`
67 for more details. 67 for more details.
68 68
69 Some basic editor settings, such as indentatio 69 Some basic editor settings, such as indentation and line endings, will be
70 set automatically if you are using an editor t 70 set automatically if you are using an editor that is compatible with
71 EditorConfig. See the official EditorConfig we 71 EditorConfig. See the official EditorConfig website for more information:
72 https://editorconfig.org/ 72 https://editorconfig.org/
73 73
74 Abstraction layers 74 Abstraction layers
75 ****************** 75 ******************
76 76
77 Computer Science professors teach students to 77 Computer Science professors teach students to make extensive use of
78 abstraction layers in the name of flexibility 78 abstraction layers in the name of flexibility and information hiding.
79 Certainly the kernel makes extensive use of ab 79 Certainly the kernel makes extensive use of abstraction; no project
80 involving several million lines of code could 80 involving several million lines of code could do otherwise and survive.
81 But experience has shown that excessive or pre 81 But experience has shown that excessive or premature abstraction can be
82 just as harmful as premature optimization. Ab 82 just as harmful as premature optimization. Abstraction should be used to
83 the level required and no further. 83 the level required and no further.
84 84
85 At a simple level, consider a function which h 85 At a simple level, consider a function which has an argument which is
86 always passed as zero by all callers. One cou 86 always passed as zero by all callers. One could retain that argument just
87 in case somebody eventually needs to use the e 87 in case somebody eventually needs to use the extra flexibility that it
88 provides. By that time, though, chances are g 88 provides. By that time, though, chances are good that the code which
89 implements this extra argument has been broken 89 implements this extra argument has been broken in some subtle way which was
90 never noticed - because it has never been used 90 never noticed - because it has never been used. Or, when the need for
91 extra flexibility arises, it does not do so in 91 extra flexibility arises, it does not do so in a way which matches the
92 programmer's early expectation. Kernel develo 92 programmer's early expectation. Kernel developers will routinely submit
93 patches to remove unused arguments; they shoul 93 patches to remove unused arguments; they should, in general, not be added
94 in the first place. 94 in the first place.
95 95
96 Abstraction layers which hide access to hardwa 96 Abstraction layers which hide access to hardware - often to allow the bulk
97 of a driver to be used with multiple operating 97 of a driver to be used with multiple operating systems - are especially
98 frowned upon. Such layers obscure the code an 98 frowned upon. Such layers obscure the code and may impose a performance
99 penalty; they do not belong in the Linux kerne 99 penalty; they do not belong in the Linux kernel.
100 100
101 On the other hand, if you find yourself copyin 101 On the other hand, if you find yourself copying significant amounts of code
102 from another kernel subsystem, it is time to a 102 from another kernel subsystem, it is time to ask whether it would, in fact,
103 make sense to pull out some of that code into 103 make sense to pull out some of that code into a separate library or to
104 implement that functionality at a higher level 104 implement that functionality at a higher level. There is no value in
105 replicating the same code throughout the kerne 105 replicating the same code throughout the kernel.
106 106
107 107
108 #ifdef and preprocessor use in general 108 #ifdef and preprocessor use in general
109 ************************************** 109 **************************************
110 110
111 The C preprocessor seems to present a powerful 111 The C preprocessor seems to present a powerful temptation to some C
112 programmers, who see it as a way to efficientl 112 programmers, who see it as a way to efficiently encode a great deal of
113 flexibility into a source file. But the prepr 113 flexibility into a source file. But the preprocessor is not C, and heavy
114 use of it results in code which is much harder 114 use of it results in code which is much harder for others to read and
115 harder for the compiler to check for correctne 115 harder for the compiler to check for correctness. Heavy preprocessor use
116 is almost always a sign of code which needs so 116 is almost always a sign of code which needs some cleanup work.
117 117
118 Conditional compilation with #ifdef is, indeed 118 Conditional compilation with #ifdef is, indeed, a powerful feature, and it
119 is used within the kernel. But there is littl 119 is used within the kernel. But there is little desire to see code which is
120 sprinkled liberally with #ifdef blocks. As a 120 sprinkled liberally with #ifdef blocks. As a general rule, #ifdef use
121 should be confined to header files whenever po 121 should be confined to header files whenever possible.
122 Conditionally-compiled code can be confined to 122 Conditionally-compiled code can be confined to functions which, if the code
123 is not to be present, simply become empty. Th 123 is not to be present, simply become empty. The compiler will then quietly
124 optimize out the call to the empty function. 124 optimize out the call to the empty function. The result is far cleaner
125 code which is easier to follow. 125 code which is easier to follow.
126 126
127 C preprocessor macros present a number of haza 127 C preprocessor macros present a number of hazards, including possible
128 multiple evaluation of expressions with side e 128 multiple evaluation of expressions with side effects and no type safety.
129 If you are tempted to define a macro, consider 129 If you are tempted to define a macro, consider creating an inline function
130 instead. The code which results will be the s 130 instead. The code which results will be the same, but inline functions are
131 easier to read, do not evaluate their argument 131 easier to read, do not evaluate their arguments multiple times, and allow
132 the compiler to perform type checking on the a 132 the compiler to perform type checking on the arguments and return value.
133 133
134 134
135 Inline functions 135 Inline functions
136 **************** 136 ****************
137 137
138 Inline functions present a hazard of their own 138 Inline functions present a hazard of their own, though. Programmers can
139 become enamored of the perceived efficiency in 139 become enamored of the perceived efficiency inherent in avoiding a function
140 call and fill a source file with inline functi 140 call and fill a source file with inline functions. Those functions,
141 however, can actually reduce performance. Sin 141 however, can actually reduce performance. Since their code is replicated
142 at each call site, they end up bloating the si 142 at each call site, they end up bloating the size of the compiled kernel.
143 That, in turn, creates pressure on the process 143 That, in turn, creates pressure on the processor's memory caches, which can
144 slow execution dramatically. Inline functions 144 slow execution dramatically. Inline functions, as a rule, should be quite
145 small and relatively rare. The cost of a func 145 small and relatively rare. The cost of a function call, after all, is not
146 that high; the creation of large numbers of in 146 that high; the creation of large numbers of inline functions is a classic
147 example of premature optimization. 147 example of premature optimization.
148 148
149 In general, kernel programmers ignore cache ef 149 In general, kernel programmers ignore cache effects at their peril. The
150 classic time/space tradeoff taught in beginnin 150 classic time/space tradeoff taught in beginning data structures classes
151 often does not apply to contemporary hardware. 151 often does not apply to contemporary hardware. Space *is* time, in that a
152 larger program will run slower than one which 152 larger program will run slower than one which is more compact.
153 153
154 More recent compilers take an increasingly act 154 More recent compilers take an increasingly active role in deciding whether
155 a given function should actually be inlined or 155 a given function should actually be inlined or not. So the liberal
156 placement of "inline" keywords may not just be 156 placement of "inline" keywords may not just be excessive; it could also be
157 irrelevant. 157 irrelevant.
158 158
159 159
160 Locking 160 Locking
161 ******* 161 *******
162 162
163 In May, 2006, the "Devicescape" networking sta 163 In May, 2006, the "Devicescape" networking stack was, with great
164 fanfare, released under the GPL and made avail 164 fanfare, released under the GPL and made available for inclusion in the
165 mainline kernel. This donation was welcome ne 165 mainline kernel. This donation was welcome news; support for wireless
166 networking in Linux was considered substandard 166 networking in Linux was considered substandard at best, and the Devicescape
167 stack offered the promise of fixing that situa 167 stack offered the promise of fixing that situation. Yet, this code did not
168 actually make it into the mainline until June, 168 actually make it into the mainline until June, 2007 (2.6.22). What
169 happened? 169 happened?
170 170
171 This code showed a number of signs of having b 171 This code showed a number of signs of having been developed behind
172 corporate doors. But one large problem in par 172 corporate doors. But one large problem in particular was that it was not
173 designed to work on multiprocessor systems. B 173 designed to work on multiprocessor systems. Before this networking stack
174 (now called mac80211) could be merged, a locki 174 (now called mac80211) could be merged, a locking scheme needed to be
175 retrofitted onto it. 175 retrofitted onto it.
176 176
177 Once upon a time, Linux kernel code could be d 177 Once upon a time, Linux kernel code could be developed without thinking
178 about the concurrency issues presented by mult 178 about the concurrency issues presented by multiprocessor systems. Now,
179 however, this document is being written on a d 179 however, this document is being written on a dual-core laptop. Even on
180 single-processor systems, work being done to i 180 single-processor systems, work being done to improve responsiveness will
181 raise the level of concurrency within the kern 181 raise the level of concurrency within the kernel. The days when kernel
182 code could be written without thinking about l 182 code could be written without thinking about locking are long past.
183 183
184 Any resource (data structures, hardware regist 184 Any resource (data structures, hardware registers, etc.) which could be
185 accessed concurrently by more than one thread 185 accessed concurrently by more than one thread must be protected by a lock.
186 New code should be written with this requireme 186 New code should be written with this requirement in mind; retrofitting
187 locking after the fact is a rather more diffic 187 locking after the fact is a rather more difficult task. Kernel developers
188 should take the time to understand the availab 188 should take the time to understand the available locking primitives well
189 enough to pick the right tool for the job. Co 189 enough to pick the right tool for the job. Code which shows a lack of
190 attention to concurrency will have a difficult 190 attention to concurrency will have a difficult path into the mainline.
191 191
192 192
193 Regressions 193 Regressions
194 *********** 194 ***********
195 195
196 One final hazard worth mentioning is this: it 196 One final hazard worth mentioning is this: it can be tempting to make a
197 change (which may bring big improvements) whic 197 change (which may bring big improvements) which causes something to break
198 for existing users. This kind of change is ca 198 for existing users. This kind of change is called a "regression," and
199 regressions have become most unwelcome in the 199 regressions have become most unwelcome in the mainline kernel. With few
200 exceptions, changes which cause regressions wi 200 exceptions, changes which cause regressions will be backed out if the
201 regression cannot be fixed in a timely manner. 201 regression cannot be fixed in a timely manner. Far better to avoid the
202 regression in the first place. 202 regression in the first place.
203 203
204 It is often argued that a regression can be ju 204 It is often argued that a regression can be justified if it causes things
205 to work for more people than it creates proble 205 to work for more people than it creates problems for. Why not make a
206 change if it brings new functionality to ten s 206 change if it brings new functionality to ten systems for each one it
207 breaks? The best answer to this question was 207 breaks? The best answer to this question was expressed by Linus in July,
208 2007: 208 2007:
209 209
210 :: 210 ::
211 211
212 So we don't fix bugs by introducing ne 212 So we don't fix bugs by introducing new problems. That way lies
213 madness, and nobody ever knows if you 213 madness, and nobody ever knows if you actually make any real
214 progress at all. Is it two steps forwa 214 progress at all. Is it two steps forwards, one step back, or one
215 step forward and two steps back? 215 step forward and two steps back?
216 216
217 (https://lwn.net/Articles/243460/). 217 (https://lwn.net/Articles/243460/).
218 218
219 An especially unwelcome type of regression is 219 An especially unwelcome type of regression is any sort of change to the
220 user-space ABI. Once an interface has been ex 220 user-space ABI. Once an interface has been exported to user space, it must
221 be supported indefinitely. This fact makes th 221 be supported indefinitely. This fact makes the creation of user-space
222 interfaces particularly challenging: since the 222 interfaces particularly challenging: since they cannot be changed in
223 incompatible ways, they must be done right the 223 incompatible ways, they must be done right the first time. For this
224 reason, a great deal of thought, clear documen 224 reason, a great deal of thought, clear documentation, and wide review for
225 user-space interfaces is always required. 225 user-space interfaces is always required.
226 226
227 227
228 Code checking tools 228 Code checking tools
229 ------------------- 229 -------------------
230 230
231 For now, at least, the writing of error-free c 231 For now, at least, the writing of error-free code remains an ideal that few
232 of us can reach. What we can hope to do, thou 232 of us can reach. What we can hope to do, though, is to catch and fix as
233 many of those errors as possible before our co 233 many of those errors as possible before our code goes into the mainline
234 kernel. To that end, the kernel developers ha 234 kernel. To that end, the kernel developers have put together an impressive
235 array of tools which can catch a wide variety 235 array of tools which can catch a wide variety of obscure problems in an
236 automated way. Any problem caught by the comp 236 automated way. Any problem caught by the computer is a problem which will
237 not afflict a user later on, so it stands to r 237 not afflict a user later on, so it stands to reason that the automated
238 tools should be used whenever possible. 238 tools should be used whenever possible.
239 239
240 The first step is simply to heed the warnings 240 The first step is simply to heed the warnings produced by the compiler.
241 Contemporary versions of gcc can detect (and w 241 Contemporary versions of gcc can detect (and warn about) a large number of
242 potential errors. Quite often, these warnings 242 potential errors. Quite often, these warnings point to real problems.
243 Code submitted for review should, as a rule, n 243 Code submitted for review should, as a rule, not produce any compiler
244 warnings. When silencing warnings, take care 244 warnings. When silencing warnings, take care to understand the real cause
245 and try to avoid "fixes" which make the warnin 245 and try to avoid "fixes" which make the warning go away without addressing
246 its cause. 246 its cause.
247 247
248 Note that not all compiler warnings are enable 248 Note that not all compiler warnings are enabled by default. Build the
249 kernel with "make KCFLAGS=-W" to get the full 249 kernel with "make KCFLAGS=-W" to get the full set.
250 250
251 The kernel provides several configuration opti 251 The kernel provides several configuration options which turn on debugging
252 features; most of these are found in the "kern 252 features; most of these are found in the "kernel hacking" submenu. Several
253 of these options should be turned on for any k 253 of these options should be turned on for any kernel used for development or
254 testing purposes. In particular, you should t 254 testing purposes. In particular, you should turn on:
255 255
256 - FRAME_WARN to get warnings for stack frames 256 - FRAME_WARN to get warnings for stack frames larger than a given amount.
257 The output generated can be verbose, but on 257 The output generated can be verbose, but one need not worry about
258 warnings from other parts of the kernel. 258 warnings from other parts of the kernel.
259 259
260 - DEBUG_OBJECTS will add code to track the li 260 - DEBUG_OBJECTS will add code to track the lifetime of various objects
261 created by the kernel and warn when things 261 created by the kernel and warn when things are done out of order. If
262 you are adding a subsystem which creates (a 262 you are adding a subsystem which creates (and exports) complex objects
263 of its own, consider adding support for the 263 of its own, consider adding support for the object debugging
264 infrastructure. 264 infrastructure.
265 265
266 - DEBUG_SLAB can find a variety of memory all 266 - DEBUG_SLAB can find a variety of memory allocation and use errors; it
267 should be used on most development kernels. 267 should be used on most development kernels.
268 268
269 - DEBUG_SPINLOCK, DEBUG_ATOMIC_SLEEP, and DEB 269 - DEBUG_SPINLOCK, DEBUG_ATOMIC_SLEEP, and DEBUG_MUTEXES will find a
270 number of common locking errors. 270 number of common locking errors.
271 271
272 There are quite a few other debugging options, 272 There are quite a few other debugging options, some of which will be
273 discussed below. Some of them have a signific 273 discussed below. Some of them have a significant performance impact and
274 should not be used all of the time. But some 274 should not be used all of the time. But some time spent learning the
275 available options will likely be paid back man 275 available options will likely be paid back many times over in short order.
276 276
277 One of the heavier debugging tools is the lock 277 One of the heavier debugging tools is the locking checker, or "lockdep."
278 This tool will track the acquisition and relea 278 This tool will track the acquisition and release of every lock (spinlock or
279 mutex) in the system, the order in which locks 279 mutex) in the system, the order in which locks are acquired relative to
280 each other, the current interrupt environment, 280 each other, the current interrupt environment, and more. It can then
281 ensure that locks are always acquired in the s 281 ensure that locks are always acquired in the same order, that the same
282 interrupt assumptions apply in all situations, 282 interrupt assumptions apply in all situations, and so on. In other words,
283 lockdep can find a number of scenarios in whic 283 lockdep can find a number of scenarios in which the system could, on rare
284 occasion, deadlock. This kind of problem can 284 occasion, deadlock. This kind of problem can be painful (for both
285 developers and users) in a deployed system; lo 285 developers and users) in a deployed system; lockdep allows them to be found
286 in an automated manner ahead of time. Code wi 286 in an automated manner ahead of time. Code with any sort of non-trivial
287 locking should be run with lockdep enabled bef 287 locking should be run with lockdep enabled before being submitted for
288 inclusion. 288 inclusion.
289 289
290 As a diligent kernel programmer, you will, bey 290 As a diligent kernel programmer, you will, beyond doubt, check the return
291 status of any operation (such as a memory allo 291 status of any operation (such as a memory allocation) which can fail. The
292 fact of the matter, though, is that the result 292 fact of the matter, though, is that the resulting failure recovery paths
293 are, probably, completely untested. Untested 293 are, probably, completely untested. Untested code tends to be broken code;
294 you could be much more confident of your code 294 you could be much more confident of your code if all those error-handling
295 paths had been exercised a few times. 295 paths had been exercised a few times.
296 296
297 The kernel provides a fault injection framewor 297 The kernel provides a fault injection framework which can do exactly that,
298 especially where memory allocations are involv 298 especially where memory allocations are involved. With fault injection
299 enabled, a configurable percentage of memory a 299 enabled, a configurable percentage of memory allocations will be made to
300 fail; these failures can be restricted to a sp 300 fail; these failures can be restricted to a specific range of code.
301 Running with fault injection enabled allows th 301 Running with fault injection enabled allows the programmer to see how the
302 code responds when things go badly. See 302 code responds when things go badly. See
303 Documentation/fault-injection/fault-injection. 303 Documentation/fault-injection/fault-injection.rst for more information on
304 how to use this facility. 304 how to use this facility.
305 305
306 Other kinds of errors can be found with the "s 306 Other kinds of errors can be found with the "sparse" static analysis tool.
307 With sparse, the programmer can be warned abou 307 With sparse, the programmer can be warned about confusion between
308 user-space and kernel-space addresses, mixture 308 user-space and kernel-space addresses, mixture of big-endian and
309 small-endian quantities, the passing of intege 309 small-endian quantities, the passing of integer values where a set of bit
310 flags is expected, and so on. Sparse must be 310 flags is expected, and so on. Sparse must be installed separately (it can
311 be found at https://sparse.wiki.kernel.org/ind 311 be found at https://sparse.wiki.kernel.org/index.php/Main_Page if your
312 distributor does not package it); it can then 312 distributor does not package it); it can then be run on the code by adding
313 "C=1" to your make command. 313 "C=1" to your make command.
314 314
315 The "Coccinelle" tool (http://coccinelle.lip6. 315 The "Coccinelle" tool (http://coccinelle.lip6.fr/) is able to find a wide
316 variety of potential coding problems; it can a 316 variety of potential coding problems; it can also propose fixes for those
317 problems. Quite a few "semantic patches" for 317 problems. Quite a few "semantic patches" for the kernel have been packaged
318 under the scripts/coccinelle directory; runnin 318 under the scripts/coccinelle directory; running "make coccicheck" will run
319 through those semantic patches and report on a 319 through those semantic patches and report on any problems found. See
320 :ref:`Documentation/dev-tools/coccinelle.rst < 320 :ref:`Documentation/dev-tools/coccinelle.rst <devtools_coccinelle>`
321 for more information. 321 for more information.
322 322
323 Other kinds of portability errors are best fou 323 Other kinds of portability errors are best found by compiling your code for
324 other architectures. If you do not happen to 324 other architectures. If you do not happen to have an S/390 system or a
325 Blackfin development board handy, you can stil 325 Blackfin development board handy, you can still perform the compilation
326 step. A large set of cross compilers for x86 326 step. A large set of cross compilers for x86 systems can be found at
327 327
328 https://www.kernel.org/pub/tools/cross 328 https://www.kernel.org/pub/tools/crosstool/
329 329
330 Some time spent installing and using these com 330 Some time spent installing and using these compilers will help avoid
331 embarrassment later. 331 embarrassment later.
332 332
333 333
334 Documentation 334 Documentation
335 ------------- 335 -------------
336 336
337 Documentation has often been more the exceptio 337 Documentation has often been more the exception than the rule with kernel
338 development. Even so, adequate documentation 338 development. Even so, adequate documentation will help to ease the merging
339 of new code into the kernel, make life easier 339 of new code into the kernel, make life easier for other developers, and
340 will be helpful for your users. In many cases 340 will be helpful for your users. In many cases, the addition of
341 documentation has become essentially mandatory 341 documentation has become essentially mandatory.
342 342
343 The first piece of documentation for any patch 343 The first piece of documentation for any patch is its associated
344 changelog. Log entries should describe the pr 344 changelog. Log entries should describe the problem being solved, the form
345 of the solution, the people who worked on the 345 of the solution, the people who worked on the patch, any relevant
346 effects on performance, and anything else that 346 effects on performance, and anything else that might be needed to
347 understand the patch. Be sure that the change 347 understand the patch. Be sure that the changelog says *why* the patch is
348 worth applying; a surprising number of develop 348 worth applying; a surprising number of developers fail to provide that
349 information. 349 information.
350 350
351 Any code which adds a new user-space interface 351 Any code which adds a new user-space interface - including new sysfs or
352 /proc files - should include documentation of 352 /proc files - should include documentation of that interface which enables
353 user-space developers to know what they are wo 353 user-space developers to know what they are working with. See
354 Documentation/ABI/README for a description of 354 Documentation/ABI/README for a description of how this documentation should
355 be formatted and what information needs to be 355 be formatted and what information needs to be provided.
356 356
357 The file :ref:`Documentation/admin-guide/kerne 357 The file :ref:`Documentation/admin-guide/kernel-parameters.rst
358 <kernelparameters>` describes all of the kerne 358 <kernelparameters>` describes all of the kernel's boot-time parameters.
359 Any patch which adds new parameters should add 359 Any patch which adds new parameters should add the appropriate entries to
360 this file. 360 this file.
361 361
362 Any new configuration options must be accompan 362 Any new configuration options must be accompanied by help text which
363 clearly explains the options and when the user 363 clearly explains the options and when the user might want to select them.
364 364
365 Internal API information for many subsystems i 365 Internal API information for many subsystems is documented by way of
366 specially-formatted comments; these comments c 366 specially-formatted comments; these comments can be extracted and formatted
367 in a number of ways by the "kernel-doc" script 367 in a number of ways by the "kernel-doc" script. If you are working within
368 a subsystem which has kerneldoc comments, you 368 a subsystem which has kerneldoc comments, you should maintain them and add
369 them, as appropriate, for externally-available 369 them, as appropriate, for externally-available functions. Even in areas
370 which have not been so documented, there is no 370 which have not been so documented, there is no harm in adding kerneldoc
371 comments for the future; indeed, this can be a 371 comments for the future; indeed, this can be a useful activity for
372 beginning kernel developers. The format of th 372 beginning kernel developers. The format of these comments, along with some
373 information on how to create kerneldoc templat 373 information on how to create kerneldoc templates can be found at
374 :ref:`Documentation/doc-guide/ <doc_guide>`. 374 :ref:`Documentation/doc-guide/ <doc_guide>`.
375 375
376 Anybody who reads through a significant amount 376 Anybody who reads through a significant amount of existing kernel code will
377 note that, often, comments are most notable by 377 note that, often, comments are most notable by their absence. Once again,
378 the expectations for new code are higher than 378 the expectations for new code are higher than they were in the past;
379 merging uncommented code will be harder. That 379 merging uncommented code will be harder. That said, there is little desire
380 for verbosely-commented code. The code should 380 for verbosely-commented code. The code should, itself, be readable, with
381 comments explaining the more subtle aspects. 381 comments explaining the more subtle aspects.
382 382
383 Certain things should always be commented. Us 383 Certain things should always be commented. Uses of memory barriers should
384 be accompanied by a line explaining why the ba 384 be accompanied by a line explaining why the barrier is necessary. The
385 locking rules for data structures generally ne 385 locking rules for data structures generally need to be explained somewhere.
386 Major data structures need comprehensive docum 386 Major data structures need comprehensive documentation in general.
387 Non-obvious dependencies between separate bits 387 Non-obvious dependencies between separate bits of code should be pointed
388 out. Anything which might tempt a code janito 388 out. Anything which might tempt a code janitor to make an incorrect
389 "cleanup" needs a comment saying why it is don 389 "cleanup" needs a comment saying why it is done the way it is. And so on.
390 390
391 391
392 Internal API changes 392 Internal API changes
393 -------------------- 393 --------------------
394 394
395 The binary interface provided by the kernel to 395 The binary interface provided by the kernel to user space cannot be broken
396 except under the most severe circumstances. T 396 except under the most severe circumstances. The kernel's internal
397 programming interfaces, instead, are highly fl 397 programming interfaces, instead, are highly fluid and can be changed when
398 the need arises. If you find yourself having 398 the need arises. If you find yourself having to work around a kernel API,
399 or simply not using a specific functionality b 399 or simply not using a specific functionality because it does not meet your
400 needs, that may be a sign that the API needs t 400 needs, that may be a sign that the API needs to change. As a kernel
401 developer, you are empowered to make such chan 401 developer, you are empowered to make such changes.
402 402
403 There are, of course, some catches. API chang 403 There are, of course, some catches. API changes can be made, but they need
404 to be well justified. So any patch making an 404 to be well justified. So any patch making an internal API change should be
405 accompanied by a description of what the chang 405 accompanied by a description of what the change is and why it is
406 necessary. This kind of change should also be 406 necessary. This kind of change should also be broken out into a separate
407 patch, rather than buried within a larger patc 407 patch, rather than buried within a larger patch.
408 408
409 The other catch is that a developer who change 409 The other catch is that a developer who changes an internal API is
410 generally charged with the task of fixing any 410 generally charged with the task of fixing any code within the kernel tree
411 which is broken by the change. For a widely-u 411 which is broken by the change. For a widely-used function, this duty can
412 lead to literally hundreds or thousands of cha 412 lead to literally hundreds or thousands of changes - many of which are
413 likely to conflict with work being done by oth 413 likely to conflict with work being done by other developers. Needless to
414 say, this can be a large job, so it is best to 414 say, this can be a large job, so it is best to be sure that the
415 justification is solid. Note that the Coccine 415 justification is solid. Note that the Coccinelle tool can help with
416 wide-ranging API changes. 416 wide-ranging API changes.
417 417
418 When making an incompatible API change, one sh 418 When making an incompatible API change, one should, whenever possible,
419 ensure that code which has not been updated is 419 ensure that code which has not been updated is caught by the compiler.
420 This will help you to be sure that you have fo 420 This will help you to be sure that you have found all in-tree uses of that
421 interface. It will also alert developers of o 421 interface. It will also alert developers of out-of-tree code that there is
422 a change that they need to respond to. Suppor 422 a change that they need to respond to. Supporting out-of-tree code is not
423 something that kernel developers need to be wo 423 something that kernel developers need to be worried about, but we also do
424 not have to make life harder for out-of-tree d 424 not have to make life harder for out-of-tree developers than it needs to
425 be. 425 be.
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