1 .. SPDX-License-Identifier: GPL-2.0-only
2
3 ==========
4 Checkpatch
5 ==========
6
7 Checkpatch (scripts/checkpatch.pl) is a perl script which checks for trivial
8 style violations in patches and optionally corrects them. Checkpatch can
9 also be run on file contexts and without the kernel tree.
10
11 Checkpatch is not always right. Your judgement takes precedence over checkpatch
12 messages. If your code looks better with the violations, then its probably
13 best left alone.
14
15
16 Options
17 =======
18
19 This section will describe the options checkpatch can be run with.
20
21 Usage::
22
23 ./scripts/checkpatch.pl [OPTION]... [FILE]...
24
25 Available options:
26
27 - -q, --quiet
28
29 Enable quiet mode.
30
31 - -v, --verbose
32 Enable verbose mode. Additional verbose test descriptions are output
33 so as to provide information on why that particular message is shown.
34
35 - --no-tree
36
37 Run checkpatch without the kernel tree.
38
39 - --no-signoff
40
41 Disable the 'Signed-off-by' line check. The sign-off is a simple line at
42 the end of the explanation for the patch, which certifies that you wrote it
43 or otherwise have the right to pass it on as an open-source patch.
44
45 Example::
46
47 Signed-off-by: Random J Developer <random@developer.example.org>
48
49 Setting this flag effectively stops a message for a missing signed-off-by
50 line in a patch context.
51
52 - --patch
53
54 Treat FILE as a patch. This is the default option and need not be
55 explicitly specified.
56
57 - --emacs
58
59 Set output to emacs compile window format. This allows emacs users to jump
60 from the error in the compile window directly to the offending line in the
61 patch.
62
63 - --terse
64
65 Output only one line per report.
66
67 - --showfile
68
69 Show the diffed file position instead of the input file position.
70
71 - -g, --git
72
73 Treat FILE as a single commit or a git revision range.
74
75 Single commit with:
76
77 - <rev>
78 - <rev>^
79 - <rev>~n
80
81 Multiple commits with:
82
83 - <rev1>..<rev2>
84 - <rev1>...<rev2>
85 - <rev>-<count>
86
87 - -f, --file
88
89 Treat FILE as a regular source file. This option must be used when running
90 checkpatch on source files in the kernel.
91
92 - --subjective, --strict
93
94 Enable stricter tests in checkpatch. By default the tests emitted as CHECK
95 do not activate by default. Use this flag to activate the CHECK tests.
96
97 - --list-types
98
99 Every message emitted by checkpatch has an associated TYPE. Add this flag
100 to display all the types in checkpatch.
101
102 Note that when this flag is active, checkpatch does not read the input FILE,
103 and no message is emitted. Only a list of types in checkpatch is output.
104
105 - --types TYPE(,TYPE2...)
106
107 Only display messages with the given types.
108
109 Example::
110
111 ./scripts/checkpatch.pl mypatch.patch --types EMAIL_SUBJECT,BRACES
112
113 - --ignore TYPE(,TYPE2...)
114
115 Checkpatch will not emit messages for the specified types.
116
117 Example::
118
119 ./scripts/checkpatch.pl mypatch.patch --ignore EMAIL_SUBJECT,BRACES
120
121 - --show-types
122
123 By default checkpatch doesn't display the type associated with the messages.
124 Set this flag to show the message type in the output.
125
126 - --max-line-length=n
127
128 Set the max line length (default 100). If a line exceeds the specified
129 length, a LONG_LINE message is emitted.
130
131
132 The message level is different for patch and file contexts. For patches,
133 a WARNING is emitted. While a milder CHECK is emitted for files. So for
134 file contexts, the --strict flag must also be enabled.
135
136 - --min-conf-desc-length=n
137
138 Set the Kconfig entry minimum description length, if shorter, warn.
139
140 - --tab-size=n
141
142 Set the number of spaces for tab (default 8).
143
144 - --root=PATH
145
146 PATH to the kernel tree root.
147
148 This option must be specified when invoking checkpatch from outside
149 the kernel root.
150
151 - --no-summary
152
153 Suppress the per file summary.
154
155 - --mailback
156
157 Only produce a report in case of Warnings or Errors. Milder Checks are
158 excluded from this.
159
160 - --summary-file
161
162 Include the filename in summary.
163
164 - --debug KEY=[0|1]
165
166 Turn on/off debugging of KEY, where KEY is one of 'values', 'possible',
167 'type', and 'attr' (default is all off).
168
169 - --fix
170
171 This is an EXPERIMENTAL feature. If correctable errors exist, a file
172 <inputfile>.EXPERIMENTAL-checkpatch-fixes is created which has the
173 automatically fixable errors corrected.
174
175 - --fix-inplace
176
177 EXPERIMENTAL - Similar to --fix but input file is overwritten with fixes.
178
179 DO NOT USE this flag unless you are absolutely sure and you have a backup
180 in place.
181
182 - --ignore-perl-version
183
184 Override checking of perl version. Runtime errors may be encountered after
185 enabling this flag if the perl version does not meet the minimum specified.
186
187 - --codespell
188
189 Use the codespell dictionary for checking spelling errors.
190
191 - --codespellfile
192
193 Use the specified codespell file.
194 Default is '/usr/share/codespell/dictionary.txt'.
195
196 - --typedefsfile
197
198 Read additional types from this file.
199
200 - --color[=WHEN]
201
202 Use colors 'always', 'never', or only when output is a terminal ('auto').
203 Default is 'auto'.
204
205 - --kconfig-prefix=WORD
206
207 Use WORD as a prefix for Kconfig symbols (default is `CONFIG_`).
208
209 - -h, --help, --version
210
211 Display the help text.
212
213 Message Levels
214 ==============
215
216 Messages in checkpatch are divided into three levels. The levels of messages
217 in checkpatch denote the severity of the error. They are:
218
219 - ERROR
220
221 This is the most strict level. Messages of type ERROR must be taken
222 seriously as they denote things that are very likely to be wrong.
223
224 - WARNING
225
226 This is the next stricter level. Messages of type WARNING requires a
227 more careful review. But it is milder than an ERROR.
228
229 - CHECK
230
231 This is the mildest level. These are things which may require some thought.
232
233 Type Descriptions
234 =================
235
236 This section contains a description of all the message types in checkpatch.
237
238 .. Types in this section are also parsed by checkpatch.
239 .. The types are grouped into subsections based on use.
240
241
242 Allocation style
243 ----------------
244
245 **ALLOC_ARRAY_ARGS**
246 The first argument for kcalloc or kmalloc_array should be the
247 number of elements. sizeof() as the first argument is generally
248 wrong.
249
250 See: https://www.kernel.org/doc/html/latest/core-api/memory-allocation.html
251
252 **ALLOC_SIZEOF_STRUCT**
253 The allocation style is bad. In general for family of
254 allocation functions using sizeof() to get memory size,
255 constructs like::
256
257 p = alloc(sizeof(struct foo), ...)
258
259 should be::
260
261 p = alloc(sizeof(*p), ...)
262
263 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#allocating-memory
264
265 **ALLOC_WITH_MULTIPLY**
266 Prefer kmalloc_array/kcalloc over kmalloc/kzalloc with a
267 sizeof multiply.
268
269 See: https://www.kernel.org/doc/html/latest/core-api/memory-allocation.html
270
271
272 API usage
273 ---------
274
275 **ARCH_DEFINES**
276 Architecture specific defines should be avoided wherever
277 possible.
278
279 **ARCH_INCLUDE_LINUX**
280 Whenever asm/file.h is included and linux/file.h exists, a
281 conversion can be made when linux/file.h includes asm/file.h.
282 However this is not always the case (See signal.h).
283 This message type is emitted only for includes from arch/.
284
285 **AVOID_BUG**
286 BUG() or BUG_ON() should be avoided totally.
287 Use WARN() and WARN_ON() instead, and handle the "impossible"
288 error condition as gracefully as possible.
289
290 See: https://www.kernel.org/doc/html/latest/process/deprecated.html#bug-and-bug-on
291
292 **CONSIDER_KSTRTO**
293 The simple_strtol(), simple_strtoll(), simple_strtoul(), and
294 simple_strtoull() functions explicitly ignore overflows, which
295 may lead to unexpected results in callers. The respective kstrtol(),
296 kstrtoll(), kstrtoul(), and kstrtoull() functions tend to be the
297 correct replacements.
298
299 See: https://www.kernel.org/doc/html/latest/process/deprecated.html#simple-strtol-simple-strtoll-simple-strtoul-simple-strtoull
300
301 **CONSTANT_CONVERSION**
302 Use of __constant_<foo> form is discouraged for the following functions::
303
304 __constant_cpu_to_be[x]
305 __constant_cpu_to_le[x]
306 __constant_be[x]_to_cpu
307 __constant_le[x]_to_cpu
308 __constant_htons
309 __constant_ntohs
310
311 Using any of these outside of include/uapi/ is not preferred as using the
312 function without __constant_ is identical when the argument is a
313 constant.
314
315 In big endian systems, the macros like __constant_cpu_to_be32(x) and
316 cpu_to_be32(x) expand to the same expression::
317
318 #define __constant_cpu_to_be32(x) ((__force __be32)(__u32)(x))
319 #define __cpu_to_be32(x) ((__force __be32)(__u32)(x))
320
321 In little endian systems, the macros __constant_cpu_to_be32(x) and
322 cpu_to_be32(x) expand to __constant_swab32 and __swab32. __swab32
323 has a __builtin_constant_p check::
324
325 #define __swab32(x) \
326 (__builtin_constant_p((__u32)(x)) ? \
327 ___constant_swab32(x) : \
328 __fswab32(x))
329
330 So ultimately they have a special case for constants.
331 Similar is the case with all of the macros in the list. Thus
332 using the __constant_... forms are unnecessarily verbose and
333 not preferred outside of include/uapi.
334
335 See: https://lore.kernel.org/lkml/1400106425.12666.6.camel@joe-AO725/
336
337 **DEPRECATED_API**
338 Usage of a deprecated RCU API is detected. It is recommended to replace
339 old flavourful RCU APIs by their new vanilla-RCU counterparts.
340
341 The full list of available RCU APIs can be viewed from the kernel docs.
342
343 See: https://www.kernel.org/doc/html/latest/RCU/whatisRCU.html#full-list-of-rcu-apis
344
345 **DEPRECATED_VARIABLE**
346 EXTRA_{A,C,CPP,LD}FLAGS are deprecated and should be replaced by the new
347 flags added via commit f77bf01425b1 ("kbuild: introduce ccflags-y,
348 asflags-y and ldflags-y").
349
350 The following conversion scheme maybe used::
351
352 EXTRA_AFLAGS -> asflags-y
353 EXTRA_CFLAGS -> ccflags-y
354 EXTRA_CPPFLAGS -> cppflags-y
355 EXTRA_LDFLAGS -> ldflags-y
356
357 See:
358
359 1. https://lore.kernel.org/lkml/20070930191054.GA15876@uranus.ravnborg.org/
360 2. https://lore.kernel.org/lkml/1313384834-24433-12-git-send-email-lacombar@gmail.com/
361 3. https://www.kernel.org/doc/html/latest/kbuild/makefiles.html#compilation-flags
362
363 **DEVICE_ATTR_FUNCTIONS**
364 The function names used in DEVICE_ATTR is unusual.
365 Typically, the store and show functions are used with <attr>_store and
366 <attr>_show, where <attr> is a named attribute variable of the device.
367
368 Consider the following examples::
369
370 static DEVICE_ATTR(type, 0444, type_show, NULL);
371 static DEVICE_ATTR(power, 0644, power_show, power_store);
372
373 The function names should preferably follow the above pattern.
374
375 See: https://www.kernel.org/doc/html/latest/driver-api/driver-model/device.html#attributes
376
377 **DEVICE_ATTR_RO**
378 The DEVICE_ATTR_RO(name) helper macro can be used instead of
379 DEVICE_ATTR(name, 0444, name_show, NULL);
380
381 Note that the macro automatically appends _show to the named
382 attribute variable of the device for the show method.
383
384 See: https://www.kernel.org/doc/html/latest/driver-api/driver-model/device.html#attributes
385
386 **DEVICE_ATTR_RW**
387 The DEVICE_ATTR_RW(name) helper macro can be used instead of
388 DEVICE_ATTR(name, 0644, name_show, name_store);
389
390 Note that the macro automatically appends _show and _store to the
391 named attribute variable of the device for the show and store methods.
392
393 See: https://www.kernel.org/doc/html/latest/driver-api/driver-model/device.html#attributes
394
395 **DEVICE_ATTR_WO**
396 The DEVICE_AATR_WO(name) helper macro can be used instead of
397 DEVICE_ATTR(name, 0200, NULL, name_store);
398
399 Note that the macro automatically appends _store to the
400 named attribute variable of the device for the store method.
401
402 See: https://www.kernel.org/doc/html/latest/driver-api/driver-model/device.html#attributes
403
404 **DUPLICATED_SYSCTL_CONST**
405 Commit d91bff3011cf ("proc/sysctl: add shared variables for range
406 check") added some shared const variables to be used instead of a local
407 copy in each source file.
408
409 Consider replacing the sysctl range checking value with the shared
410 one in include/linux/sysctl.h. The following conversion scheme may
411 be used::
412
413 &zero -> SYSCTL_ZERO
414 &one -> SYSCTL_ONE
415 &int_max -> SYSCTL_INT_MAX
416
417 See:
418
419 1. https://lore.kernel.org/lkml/20190430180111.10688-1-mcroce@redhat.com/
420 2. https://lore.kernel.org/lkml/20190531131422.14970-1-mcroce@redhat.com/
421
422 **ENOSYS**
423 ENOSYS means that a nonexistent system call was called.
424 Earlier, it was wrongly used for things like invalid operations on
425 otherwise valid syscalls. This should be avoided in new code.
426
427 See: https://lore.kernel.org/lkml/5eb299021dec23c1a48fa7d9f2c8b794e967766d.1408730669.git.luto@amacapital.net/
428
429 **ENOTSUPP**
430 ENOTSUPP is not a standard error code and should be avoided in new patches.
431 EOPNOTSUPP should be used instead.
432
433 See: https://lore.kernel.org/netdev/20200510182252.GA411829@lunn.ch/
434
435 **EXPORT_SYMBOL**
436 EXPORT_SYMBOL should immediately follow the symbol to be exported.
437
438 **IN_ATOMIC**
439 in_atomic() is not for driver use so any such use is reported as an ERROR.
440 Also in_atomic() is often used to determine if sleeping is permitted,
441 but it is not reliable in this use model. Therefore its use is
442 strongly discouraged.
443
444 However, in_atomic() is ok for core kernel use.
445
446 See: https://lore.kernel.org/lkml/20080320201723.b87b3732.akpm@linux-foundation.org/
447
448 **LOCKDEP**
449 The lockdep_no_validate class was added as a temporary measure to
450 prevent warnings on conversion of device->sem to device->mutex.
451 It should not be used for any other purpose.
452
453 See: https://lore.kernel.org/lkml/1268959062.9440.467.camel@laptop/
454
455 **MALFORMED_INCLUDE**
456 The #include statement has a malformed path. This has happened
457 because the author has included a double slash "//" in the pathname
458 accidentally.
459
460 **USE_LOCKDEP**
461 lockdep_assert_held() annotations should be preferred over
462 assertions based on spin_is_locked()
463
464 See: https://www.kernel.org/doc/html/latest/locking/lockdep-design.html#annotations
465
466 **UAPI_INCLUDE**
467 No #include statements in include/uapi should use a uapi/ path.
468
469 **USLEEP_RANGE**
470 usleep_range() should be preferred over udelay(). The proper way of
471 using usleep_range() is mentioned in the kernel docs.
472
473 See: https://www.kernel.org/doc/html/latest/timers/timers-howto.html#delays-information-on-the-various-kernel-delay-sleep-mechanisms
474
475
476 Comments
477 --------
478
479 **BLOCK_COMMENT_STYLE**
480 The comment style is incorrect. The preferred style for multi-
481 line comments is::
482
483 /*
484 * This is the preferred style
485 * for multi line comments.
486 */
487
488 The networking comment style is a bit different, with the first line
489 not empty like the former::
490
491 /* This is the preferred comment style
492 * for files in net/ and drivers/net/
493 */
494
495 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#commenting
496
497 **C99_COMMENTS**
498 C99 style single line comments (//) should not be used.
499 Prefer the block comment style instead.
500
501 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#commenting
502
503 **DATA_RACE**
504 Applications of data_race() should have a comment so as to document the
505 reasoning behind why it was deemed safe.
506
507 See: https://lore.kernel.org/lkml/20200401101714.44781-1-elver@google.com/
508
509 **FSF_MAILING_ADDRESS**
510 Kernel maintainers reject new instances of the GPL boilerplate paragraph
511 directing people to write to the FSF for a copy of the GPL, since the
512 FSF has moved in the past and may do so again.
513 So do not write paragraphs about writing to the Free Software Foundation's
514 mailing address.
515
516 See: https://lore.kernel.org/lkml/20131006222342.GT19510@leaf/
517
518
519 Commit message
520 --------------
521
522 **BAD_SIGN_OFF**
523 The signed-off-by line does not fall in line with the standards
524 specified by the community.
525
526 See: https://www.kernel.org/doc/html/latest/process/submitting-patches.html#developer-s-certificate-of-origin-1-1
527
528 **BAD_STABLE_ADDRESS_STYLE**
529 The email format for stable is incorrect.
530 Some valid options for stable address are::
531
532 1. stable@vger.kernel.org
533 2. stable@kernel.org
534
535 For adding version info, the following comment style should be used::
536
537 stable@vger.kernel.org # version info
538
539 **COMMIT_COMMENT_SYMBOL**
540 Commit log lines starting with a '#' are ignored by git as
541 comments. To solve this problem addition of a single space
542 infront of the log line is enough.
543
544 **COMMIT_MESSAGE**
545 The patch is missing a commit description. A brief
546 description of the changes made by the patch should be added.
547
548 See: https://www.kernel.org/doc/html/latest/process/submitting-patches.html#describe-your-changes
549
550 **EMAIL_SUBJECT**
551 Naming the tool that found the issue is not very useful in the
552 subject line. A good subject line summarizes the change that
553 the patch brings.
554
555 See: https://www.kernel.org/doc/html/latest/process/submitting-patches.html#describe-your-changes
556
557 **FROM_SIGN_OFF_MISMATCH**
558 The author's email does not match with that in the Signed-off-by:
559 line(s). This can be sometimes caused due to an improperly configured
560 email client.
561
562 This message is emitted due to any of the following reasons::
563
564 - The email names do not match.
565 - The email addresses do not match.
566 - The email subaddresses do not match.
567 - The email comments do not match.
568
569 **MISSING_SIGN_OFF**
570 The patch is missing a Signed-off-by line. A signed-off-by
571 line should be added according to Developer's certificate of
572 Origin.
573
574 See: https://www.kernel.org/doc/html/latest/process/submitting-patches.html#sign-your-work-the-developer-s-certificate-of-origin
575
576 **NO_AUTHOR_SIGN_OFF**
577 The author of the patch has not signed off the patch. It is
578 required that a simple sign off line should be present at the
579 end of explanation of the patch to denote that the author has
580 written it or otherwise has the rights to pass it on as an open
581 source patch.
582
583 See: https://www.kernel.org/doc/html/latest/process/submitting-patches.html#sign-your-work-the-developer-s-certificate-of-origin
584
585 **DIFF_IN_COMMIT_MSG**
586 Avoid having diff content in commit message.
587 This causes problems when one tries to apply a file containing both
588 the changelog and the diff because patch(1) tries to apply the diff
589 which it found in the changelog.
590
591 See: https://lore.kernel.org/lkml/20150611134006.9df79a893e3636019ad2759e@linux-foundation.org/
592
593 **GERRIT_CHANGE_ID**
594 To be picked up by gerrit, the footer of the commit message might
595 have a Change-Id like::
596
597 Change-Id: Ic8aaa0728a43936cd4c6e1ed590e01ba8f0fbf5b
598 Signed-off-by: A. U. Thor <author@example.com>
599
600 The Change-Id line must be removed before submitting.
601
602 **GIT_COMMIT_ID**
603 The proper way to reference a commit id is:
604 commit <12+ chars of sha1> ("<title line>")
605
606 An example may be::
607
608 Commit e21d2170f36602ae2708 ("video: remove unnecessary
609 platform_set_drvdata()") removed the unnecessary
610 platform_set_drvdata(), but left the variable "dev" unused,
611 delete it.
612
613 See: https://www.kernel.org/doc/html/latest/process/submitting-patches.html#describe-your-changes
614
615 **BAD_FIXES_TAG**
616 The Fixes: tag is malformed or does not follow the community conventions.
617 This can occur if the tag have been split into multiple lines (e.g., when
618 pasted in an email program with word wrapping enabled).
619
620 See: https://www.kernel.org/doc/html/latest/process/submitting-patches.html#describe-your-changes
621
622
623 Comparison style
624 ----------------
625
626 **ASSIGN_IN_IF**
627 Do not use assignments in if condition.
628 Example::
629
630 if ((foo = bar(...)) < BAZ) {
631
632 should be written as::
633
634 foo = bar(...);
635 if (foo < BAZ) {
636
637 **BOOL_COMPARISON**
638 Comparisons of A to true and false are better written
639 as A and !A.
640
641 See: https://lore.kernel.org/lkml/1365563834.27174.12.camel@joe-AO722/
642
643 **COMPARISON_TO_NULL**
644 Comparisons to NULL in the form (foo == NULL) or (foo != NULL)
645 are better written as (!foo) and (foo).
646
647 **CONSTANT_COMPARISON**
648 Comparisons with a constant or upper case identifier on the left
649 side of the test should be avoided.
650
651
652 Indentation and Line Breaks
653 ---------------------------
654
655 **CODE_INDENT**
656 Code indent should use tabs instead of spaces.
657 Outside of comments, documentation and Kconfig,
658 spaces are never used for indentation.
659
660 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#indentation
661
662 **DEEP_INDENTATION**
663 Indentation with 6 or more tabs usually indicate overly indented
664 code.
665
666 It is suggested to refactor excessive indentation of
667 if/else/for/do/while/switch statements.
668
669 See: https://lore.kernel.org/lkml/1328311239.21255.24.camel@joe2Laptop/
670
671 **SWITCH_CASE_INDENT_LEVEL**
672 switch should be at the same indent as case.
673 Example::
674
675 switch (suffix) {
676 case 'G':
677 case 'g':
678 mem <<= 30;
679 break;
680 case 'M':
681 case 'm':
682 mem <<= 20;
683 break;
684 case 'K':
685 case 'k':
686 mem <<= 10;
687 fallthrough;
688 default:
689 break;
690 }
691
692 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#indentation
693
694 **LONG_LINE**
695 The line has exceeded the specified maximum length.
696 To use a different maximum line length, the --max-line-length=n option
697 may be added while invoking checkpatch.
698
699 Earlier, the default line length was 80 columns. Commit bdc48fa11e46
700 ("checkpatch/coding-style: deprecate 80-column warning") increased the
701 limit to 100 columns. This is not a hard limit either and it's
702 preferable to stay within 80 columns whenever possible.
703
704 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#breaking-long-lines-and-strings
705
706 **LONG_LINE_STRING**
707 A string starts before but extends beyond the maximum line length.
708 To use a different maximum line length, the --max-line-length=n option
709 may be added while invoking checkpatch.
710
711 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#breaking-long-lines-and-strings
712
713 **LONG_LINE_COMMENT**
714 A comment starts before but extends beyond the maximum line length.
715 To use a different maximum line length, the --max-line-length=n option
716 may be added while invoking checkpatch.
717
718 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#breaking-long-lines-and-strings
719
720 **SPLIT_STRING**
721 Quoted strings that appear as messages in userspace and can be
722 grepped, should not be split across multiple lines.
723
724 See: https://lore.kernel.org/lkml/20120203052727.GA15035@leaf/
725
726 **MULTILINE_DEREFERENCE**
727 A single dereferencing identifier spanned on multiple lines like::
728
729 struct_identifier->member[index].
730 member = <foo>;
731
732 is generally hard to follow. It can easily lead to typos and so makes
733 the code vulnerable to bugs.
734
735 If fixing the multiple line dereferencing leads to an 80 column
736 violation, then either rewrite the code in a more simple way or if the
737 starting part of the dereferencing identifier is the same and used at
738 multiple places then store it in a temporary variable, and use that
739 temporary variable only at all the places. For example, if there are
740 two dereferencing identifiers::
741
742 member1->member2->member3.foo1;
743 member1->member2->member3.foo2;
744
745 then store the member1->member2->member3 part in a temporary variable.
746 It not only helps to avoid the 80 column violation but also reduces
747 the program size by removing the unnecessary dereferences.
748
749 But if none of the above methods work then ignore the 80 column
750 violation because it is much easier to read a dereferencing identifier
751 on a single line.
752
753 **TRAILING_STATEMENTS**
754 Trailing statements (for example after any conditional) should be
755 on the next line.
756 Statements, such as::
757
758 if (x == y) break;
759
760 should be::
761
762 if (x == y)
763 break;
764
765
766 Macros, Attributes and Symbols
767 ------------------------------
768
769 **ARRAY_SIZE**
770 The ARRAY_SIZE(foo) macro should be preferred over
771 sizeof(foo)/sizeof(foo[0]) for finding number of elements in an
772 array.
773
774 The macro is defined in include/linux/kernel.h::
775
776 #define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
777
778 **AVOID_EXTERNS**
779 Function prototypes don't need to be declared extern in .h
780 files. It's assumed by the compiler and is unnecessary.
781
782 **AVOID_L_PREFIX**
783 Local symbol names that are prefixed with `.L` should be avoided,
784 as this has special meaning for the assembler; a symbol entry will
785 not be emitted into the symbol table. This can prevent `objtool`
786 from generating correct unwind info.
787
788 Symbols with STB_LOCAL binding may still be used, and `.L` prefixed
789 local symbol names are still generally usable within a function,
790 but `.L` prefixed local symbol names should not be used to denote
791 the beginning or end of code regions via
792 `SYM_CODE_START_LOCAL`/`SYM_CODE_END`
793
794 **BIT_MACRO**
795 Defines like: 1 << <digit> could be BIT(digit).
796 The BIT() macro is defined via include/linux/bits.h::
797
798 #define BIT(nr) (1UL << (nr))
799
800 **CONST_READ_MOSTLY**
801 When a variable is tagged with the __read_mostly annotation, it is a
802 signal to the compiler that accesses to the variable will be mostly
803 reads and rarely(but NOT never) a write.
804
805 const __read_mostly does not make any sense as const data is already
806 read-only. The __read_mostly annotation thus should be removed.
807
808 **DATE_TIME**
809 It is generally desirable that building the same source code with
810 the same set of tools is reproducible, i.e. the output is always
811 exactly the same.
812
813 The kernel does *not* use the ``__DATE__`` and ``__TIME__`` macros,
814 and enables warnings if they are used as they can lead to
815 non-deterministic builds.
816
817 See: https://www.kernel.org/doc/html/latest/kbuild/reproducible-builds.html#timestamps
818
819 **DEFINE_ARCH_HAS**
820 The ARCH_HAS_xyz and ARCH_HAVE_xyz patterns are wrong.
821
822 For big conceptual features use Kconfig symbols instead. And for
823 smaller things where we have compatibility fallback functions but
824 want architectures able to override them with optimized ones, we
825 should either use weak functions (appropriate for some cases), or
826 the symbol that protects them should be the same symbol we use.
827
828 See: https://lore.kernel.org/lkml/CA+55aFycQ9XJvEOsiM3txHL5bjUc8CeKWJNR_H+MiicaddB42Q@mail.gmail.com/
829
830 **DO_WHILE_MACRO_WITH_TRAILING_SEMICOLON**
831 do {} while(0) macros should not have a trailing semicolon.
832
833 **INIT_ATTRIBUTE**
834 Const init definitions should use __initconst instead of
835 __initdata.
836
837 Similarly init definitions without const require a separate
838 use of const.
839
840 **INLINE_LOCATION**
841 The inline keyword should sit between storage class and type.
842
843 For example, the following segment::
844
845 inline static int example_function(void)
846 {
847 ...
848 }
849
850 should be::
851
852 static inline int example_function(void)
853 {
854 ...
855 }
856
857 **MISPLACED_INIT**
858 It is possible to use section markers on variables in a way
859 which gcc doesn't understand (or at least not the way the
860 developer intended)::
861
862 static struct __initdata samsung_pll_clock exynos4_plls[nr_plls] = {
863
864 does not put exynos4_plls in the .initdata section. The __initdata
865 marker can be virtually anywhere on the line, except right after
866 "struct". The preferred location is before the "=" sign if there is
867 one, or before the trailing ";" otherwise.
868
869 See: https://lore.kernel.org/lkml/1377655732.3619.19.camel@joe-AO722/
870
871 **MULTISTATEMENT_MACRO_USE_DO_WHILE**
872 Macros with multiple statements should be enclosed in a
873 do - while block. Same should also be the case for macros
874 starting with `if` to avoid logic defects::
875
876 #define macrofun(a, b, c) \
877 do { \
878 if (a == 5) \
879 do_this(b, c); \
880 } while (0)
881
882 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#macros-enums-and-rtl
883
884 **PREFER_FALLTHROUGH**
885 Use the `fallthrough;` pseudo keyword instead of
886 `/* fallthrough */` like comments.
887
888 **TRAILING_SEMICOLON**
889 Macro definition should not end with a semicolon. The macro
890 invocation style should be consistent with function calls.
891 This can prevent any unexpected code paths::
892
893 #define MAC do_something;
894
895 If this macro is used within a if else statement, like::
896
897 if (some_condition)
898 MAC;
899
900 else
901 do_something;
902
903 Then there would be a compilation error, because when the macro is
904 expanded there are two trailing semicolons, so the else branch gets
905 orphaned.
906
907 See: https://lore.kernel.org/lkml/1399671106.2912.21.camel@joe-AO725/
908
909 **MACRO_ARG_UNUSED**
910 If function-like macros do not utilize a parameter, it might result
911 in a build warning. We advocate for utilizing static inline functions
912 to replace such macros.
913 For example, for a macro such as the one below::
914
915 #define test(a) do { } while (0)
916
917 there would be a warning like below::
918
919 WARNING: Argument 'a' is not used in function-like macro.
920
921 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#macros-enums-and-rtl
922
923 **SINGLE_STATEMENT_DO_WHILE_MACRO**
924 For the multi-statement macros, it is necessary to use the do-while
925 loop to avoid unpredictable code paths. The do-while loop helps to
926 group the multiple statements into a single one so that a
927 function-like macro can be used as a function only.
928
929 But for the single statement macros, it is unnecessary to use the
930 do-while loop. Although the code is syntactically correct but using
931 the do-while loop is redundant. So remove the do-while loop for single
932 statement macros.
933
934 **WEAK_DECLARATION**
935 Using weak declarations like __attribute__((weak)) or __weak
936 can have unintended link defects. Avoid using them.
937
938
939 Functions and Variables
940 -----------------------
941
942 **CAMELCASE**
943 Avoid CamelCase Identifiers.
944
945 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#naming
946
947 **CONST_CONST**
948 Using `const <type> const *` is generally meant to be
949 written `const <type> * const`.
950
951 **CONST_STRUCT**
952 Using const is generally a good idea. Checkpatch reads
953 a list of frequently used structs that are always or
954 almost always constant.
955
956 The existing structs list can be viewed from
957 `scripts/const_structs.checkpatch`.
958
959 See: https://lore.kernel.org/lkml/alpine.DEB.2.10.1608281509480.3321@hadrien/
960
961 **EMBEDDED_FUNCTION_NAME**
962 Embedded function names are less appropriate to use as
963 refactoring can cause function renaming. Prefer the use of
964 "%s", __func__ to embedded function names.
965
966 Note that this does not work with -f (--file) checkpatch option
967 as it depends on patch context providing the function name.
968
969 **FUNCTION_ARGUMENTS**
970 This warning is emitted due to any of the following reasons:
971
972 1. Arguments for the function declaration do not follow
973 the identifier name. Example::
974
975 void foo
976 (int bar, int baz)
977
978 This should be corrected to::
979
980 void foo(int bar, int baz)
981
982 2. Some arguments for the function definition do not
983 have an identifier name. Example::
984
985 void foo(int)
986
987 All arguments should have identifier names.
988
989 **FUNCTION_WITHOUT_ARGS**
990 Function declarations without arguments like::
991
992 int foo()
993
994 should be::
995
996 int foo(void)
997
998 **GLOBAL_INITIALISERS**
999 Global variables should not be initialized explicitly to
1000 0 (or NULL, false, etc.). Your compiler (or rather your
1001 loader, which is responsible for zeroing out the relevant
1002 sections) automatically does it for you.
1003
1004 **INITIALISED_STATIC**
1005 Static variables should not be initialized explicitly to zero.
1006 Your compiler (or rather your loader) automatically does
1007 it for you.
1008
1009 **MULTIPLE_ASSIGNMENTS**
1010 Multiple assignments on a single line makes the code unnecessarily
1011 complicated. So on a single line assign value to a single variable
1012 only, this makes the code more readable and helps avoid typos.
1013
1014 **RETURN_PARENTHESES**
1015 return is not a function and as such doesn't need parentheses::
1016
1017 return (bar);
1018
1019 can simply be::
1020
1021 return bar;
1022
1023
1024 Permissions
1025 -----------
1026
1027 **DEVICE_ATTR_PERMS**
1028 The permissions used in DEVICE_ATTR are unusual.
1029 Typically only three permissions are used - 0644 (RW), 0444 (RO)
1030 and 0200 (WO).
1031
1032 See: https://www.kernel.org/doc/html/latest/filesystems/sysfs.html#attributes
1033
1034 **EXECUTE_PERMISSIONS**
1035 There is no reason for source files to be executable. The executable
1036 bit can be removed safely.
1037
1038 **EXPORTED_WORLD_WRITABLE**
1039 Exporting world writable sysfs/debugfs files is usually a bad thing.
1040 When done arbitrarily they can introduce serious security bugs.
1041 In the past, some of the debugfs vulnerabilities would seemingly allow
1042 any local user to write arbitrary values into device registers - a
1043 situation from which little good can be expected to emerge.
1044
1045 See: https://lore.kernel.org/linux-arm-kernel/cover.1296818921.git.segoon@openwall.com/
1046
1047 **NON_OCTAL_PERMISSIONS**
1048 Permission bits should use 4 digit octal permissions (like 0700 or 0444).
1049 Avoid using any other base like decimal.
1050
1051 **SYMBOLIC_PERMS**
1052 Permission bits in the octal form are more readable and easier to
1053 understand than their symbolic counterparts because many command-line
1054 tools use this notation. Experienced kernel developers have been using
1055 these traditional Unix permission bits for decades and so they find it
1056 easier to understand the octal notation than the symbolic macros.
1057 For example, it is harder to read S_IWUSR|S_IRUGO than 0644, which
1058 obscures the developer's intent rather than clarifying it.
1059
1060 See: https://lore.kernel.org/lkml/CA+55aFw5v23T-zvDZp-MmD_EYxF8WbafwwB59934FV7g21uMGQ@mail.gmail.com/
1061
1062
1063 Spacing and Brackets
1064 --------------------
1065
1066 **ASSIGNMENT_CONTINUATIONS**
1067 Assignment operators should not be written at the start of a
1068 line but should follow the operand at the previous line.
1069
1070 **BRACES**
1071 The placement of braces is stylistically incorrect.
1072 The preferred way is to put the opening brace last on the line,
1073 and put the closing brace first::
1074
1075 if (x is true) {
1076 we do y
1077 }
1078
1079 This applies for all non-functional blocks.
1080 However, there is one special case, namely functions: they have the
1081 opening brace at the beginning of the next line, thus::
1082
1083 int function(int x)
1084 {
1085 body of function
1086 }
1087
1088 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#placing-braces-and-spaces
1089
1090 **BRACKET_SPACE**
1091 Whitespace before opening bracket '[' is prohibited.
1092 There are some exceptions:
1093
1094 1. With a type on the left::
1095
1096 int [] a;
1097
1098 2. At the beginning of a line for slice initialisers::
1099
1100 [0...10] = 5,
1101
1102 3. Inside a curly brace::
1103
1104 = { [0...10] = 5 }
1105
1106 **CONCATENATED_STRING**
1107 Concatenated elements should have a space in between.
1108 Example::
1109
1110 printk(KERN_INFO"bar");
1111
1112 should be::
1113
1114 printk(KERN_INFO "bar");
1115
1116 **ELSE_AFTER_BRACE**
1117 `else {` should follow the closing block `}` on the same line.
1118
1119 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#placing-braces-and-spaces
1120
1121 **LINE_SPACING**
1122 Vertical space is wasted given the limited number of lines an
1123 editor window can display when multiple blank lines are used.
1124
1125 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#spaces
1126
1127 **OPEN_BRACE**
1128 The opening brace should be following the function definitions on the
1129 next line. For any non-functional block it should be on the same line
1130 as the last construct.
1131
1132 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#placing-braces-and-spaces
1133
1134 **POINTER_LOCATION**
1135 When using pointer data or a function that returns a pointer type,
1136 the preferred use of * is adjacent to the data name or function name
1137 and not adjacent to the type name.
1138 Examples::
1139
1140 char *linux_banner;
1141 unsigned long long memparse(char *ptr, char **retptr);
1142 char *match_strdup(substring_t *s);
1143
1144 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#spaces
1145
1146 **SPACING**
1147 Whitespace style used in the kernel sources is described in kernel docs.
1148
1149 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#spaces
1150
1151 **TRAILING_WHITESPACE**
1152 Trailing whitespace should always be removed.
1153 Some editors highlight the trailing whitespace and cause visual
1154 distractions when editing files.
1155
1156 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#spaces
1157
1158 **UNNECESSARY_PARENTHESES**
1159 Parentheses are not required in the following cases:
1160
1161 1. Function pointer uses::
1162
1163 (foo->bar)();
1164
1165 could be::
1166
1167 foo->bar();
1168
1169 2. Comparisons in if::
1170
1171 if ((foo->bar) && (foo->baz))
1172 if ((foo == bar))
1173
1174 could be::
1175
1176 if (foo->bar && foo->baz)
1177 if (foo == bar)
1178
1179 3. addressof/dereference single Lvalues::
1180
1181 &(foo->bar)
1182 *(foo->bar)
1183
1184 could be::
1185
1186 &foo->bar
1187 *foo->bar
1188
1189 **WHILE_AFTER_BRACE**
1190 while should follow the closing bracket on the same line::
1191
1192 do {
1193 ...
1194 } while(something);
1195
1196 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#placing-braces-and-spaces
1197
1198
1199 Others
1200 ------
1201
1202 **CONFIG_DESCRIPTION**
1203 Kconfig symbols should have a help text which fully describes
1204 it.
1205
1206 **CORRUPTED_PATCH**
1207 The patch seems to be corrupted or lines are wrapped.
1208 Please regenerate the patch file before sending it to the maintainer.
1209
1210 **CVS_KEYWORD**
1211 Since linux moved to git, the CVS markers are no longer used.
1212 So, CVS style keywords ($Id$, $Revision$, $Log$) should not be
1213 added.
1214
1215 **DEFAULT_NO_BREAK**
1216 switch default case is sometimes written as "default:;". This can
1217 cause new cases added below default to be defective.
1218
1219 A "break;" should be added after empty default statement to avoid
1220 unwanted fallthrough.
1221
1222 **DOS_LINE_ENDINGS**
1223 For DOS-formatted patches, there are extra ^M symbols at the end of
1224 the line. These should be removed.
1225
1226 **DT_SCHEMA_BINDING_PATCH**
1227 DT bindings moved to a json-schema based format instead of
1228 freeform text.
1229
1230 See: https://www.kernel.org/doc/html/latest/devicetree/bindings/writing-schema.html
1231
1232 **DT_SPLIT_BINDING_PATCH**
1233 Devicetree bindings should be their own patch. This is because
1234 bindings are logically independent from a driver implementation,
1235 they have a different maintainer (even though they often
1236 are applied via the same tree), and it makes for a cleaner history in the
1237 DT only tree created with git-filter-branch.
1238
1239 See: https://www.kernel.org/doc/html/latest/devicetree/bindings/submitting-patches.html#i-for-patch-submitters
1240
1241 **EMBEDDED_FILENAME**
1242 Embedding the complete filename path inside the file isn't particularly
1243 useful as often the path is moved around and becomes incorrect.
1244
1245 **FILE_PATH_CHANGES**
1246 Whenever files are added, moved, or deleted, the MAINTAINERS file
1247 patterns can be out of sync or outdated.
1248
1249 So MAINTAINERS might need updating in these cases.
1250
1251 **MEMSET**
1252 The memset use appears to be incorrect. This may be caused due to
1253 badly ordered parameters. Please recheck the usage.
1254
1255 **NOT_UNIFIED_DIFF**
1256 The patch file does not appear to be in unified-diff format. Please
1257 regenerate the patch file before sending it to the maintainer.
1258
1259 **PRINTF_0XDECIMAL**
1260 Prefixing 0x with decimal output is defective and should be corrected.
1261
1262 **SPDX_LICENSE_TAG**
1263 The source file is missing or has an improper SPDX identifier tag.
1264 The Linux kernel requires the precise SPDX identifier in all source files,
1265 and it is thoroughly documented in the kernel docs.
1266
1267 See: https://www.kernel.org/doc/html/latest/process/license-rules.html
1268
1269 **TYPO_SPELLING**
1270 Some words may have been misspelled. Consider reviewing them.
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