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1.. SPDX-License-Identifier: GPL-2.0-only
2
3==========
4Checkpatch
5==========
6
7Checkpatch (scripts/checkpatch.pl) is a perl script which checks for trivial
8style violations in patches and optionally corrects them. Checkpatch can
9also be run on file contexts and without the kernel tree.
10
11Checkpatch is not always right. Your judgement takes precedence over checkpatch
12messages. If your code looks better with the violations, then its probably
13best left alone.
14
15
16Options
17=======
18
19This section will describe the options checkpatch can be run with.
20
21Usage::
22
23 ./scripts/checkpatch.pl [OPTION]... [FILE]...
24
25Available 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 <[email protected]>
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 exists, 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 maybe 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
213Message Levels
214==============
215
216Messages in checkpatch are divided into three levels. The levels of messages
217in 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
233Type Descriptions
234=================
235
236This 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
242Allocation 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
272API 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/[email protected]/
360 2. https://lore.kernel.org/lkml/[email protected]/
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/[email protected]/
420 2. https://lore.kernel.org/lkml/[email protected]/
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/[email protected]/
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/[email protected]/
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
476Comments
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/[email protected]/
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
519Commit 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. [email protected]
533 2. [email protected]
534
535 For adding version info, the following comment style should be used::
536
537 [email protected] # 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/[email protected]/
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 <[email protected]>
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
616Comparison style
617----------------
618
619 **ASSIGN_IN_IF**
620 Do not use assignments in if condition.
621 Example::
622
623 if ((foo = bar(...)) < BAZ) {
624
625 should be written as::
626
627 foo = bar(...);
628 if (foo < BAZ) {
629
630 **BOOL_COMPARISON**
631 Comparisons of A to true and false are better written
632 as A and !A.
633
634 See: https://lore.kernel.org/lkml/1365563834.27174.12.camel@joe-AO722/
635
636 **COMPARISON_TO_NULL**
637 Comparisons to NULL in the form (foo == NULL) or (foo != NULL)
638 are better written as (!foo) and (foo).
639
640 **CONSTANT_COMPARISON**
641 Comparisons with a constant or upper case identifier on the left
642 side of the test should be avoided.
643
644
645Indentation and Line Breaks
646---------------------------
647
648 **CODE_INDENT**
649 Code indent should use tabs instead of spaces.
650 Outside of comments, documentation and Kconfig,
651 spaces are never used for indentation.
652
653 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#indentation
654
655 **DEEP_INDENTATION**
656 Indentation with 6 or more tabs usually indicate overly indented
657 code.
658
659 It is suggested to refactor excessive indentation of
660 if/else/for/do/while/switch statements.
661
662 See: https://lore.kernel.org/lkml/1328311239.21255.24.camel@joe2Laptop/
663
664 **SWITCH_CASE_INDENT_LEVEL**
665 switch should be at the same indent as case.
666 Example::
667
668 switch (suffix) {
669 case 'G':
670 case 'g':
671 mem <<= 30;
672 break;
673 case 'M':
674 case 'm':
675 mem <<= 20;
676 break;
677 case 'K':
678 case 'k':
679 mem <<= 10;
680 fallthrough;
681 default:
682 break;
683 }
684
685 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#indentation
686
687 **LONG_LINE**
688 The line has exceeded the specified maximum length.
689 To use a different maximum line length, the --max-line-length=n option
690 may be added while invoking checkpatch.
691
692 Earlier, the default line length was 80 columns. Commit bdc48fa11e46
693 ("checkpatch/coding-style: deprecate 80-column warning") increased the
694 limit to 100 columns. This is not a hard limit either and it's
695 preferable to stay within 80 columns whenever possible.
696
697 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#breaking-long-lines-and-strings
698
699 **LONG_LINE_STRING**
700 A string starts before but extends beyond the maximum line length.
701 To use a different maximum line length, the --max-line-length=n option
702 may be added while invoking checkpatch.
703
704 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#breaking-long-lines-and-strings
705
706 **LONG_LINE_COMMENT**
707 A comment 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 **SPLIT_STRING**
714 Quoted strings that appear as messages in userspace and can be
715 grepped, should not be split across multiple lines.
716
717 See: https://lore.kernel.org/lkml/20120203052727.GA15035@leaf/
718
719 **MULTILINE_DEREFERENCE**
720 A single dereferencing identifier spanned on multiple lines like::
721
722 struct_identifier->member[index].
723 member = <foo>;
724
725 is generally hard to follow. It can easily lead to typos and so makes
726 the code vulnerable to bugs.
727
728 If fixing the multiple line dereferencing leads to an 80 column
729 violation, then either rewrite the code in a more simple way or if the
730 starting part of the dereferencing identifier is the same and used at
731 multiple places then store it in a temporary variable, and use that
732 temporary variable only at all the places. For example, if there are
733 two dereferencing identifiers::
734
735 member1->member2->member3.foo1;
736 member1->member2->member3.foo2;
737
738 then store the member1->member2->member3 part in a temporary variable.
739 It not only helps to avoid the 80 column violation but also reduces
740 the program size by removing the unnecessary dereferences.
741
742 But if none of the above methods work then ignore the 80 column
743 violation because it is much easier to read a dereferencing identifier
744 on a single line.
745
746 **TRAILING_STATEMENTS**
747 Trailing statements (for example after any conditional) should be
748 on the next line.
749 Statements, such as::
750
751 if (x == y) break;
752
753 should be::
754
755 if (x == y)
756 break;
757
758
759Macros, Attributes and Symbols
760------------------------------
761
762 **ARRAY_SIZE**
763 The ARRAY_SIZE(foo) macro should be preferred over
764 sizeof(foo)/sizeof(foo[0]) for finding number of elements in an
765 array.
766
767 The macro is defined in include/linux/kernel.h::
768
769 #define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
770
771 **AVOID_EXTERNS**
772 Function prototypes don't need to be declared extern in .h
773 files. It's assumed by the compiler and is unnecessary.
774
775 **AVOID_L_PREFIX**
776 Local symbol names that are prefixed with `.L` should be avoided,
777 as this has special meaning for the assembler; a symbol entry will
778 not be emitted into the symbol table. This can prevent `objtool`
779 from generating correct unwind info.
780
781 Symbols with STB_LOCAL binding may still be used, and `.L` prefixed
782 local symbol names are still generally usable within a function,
783 but `.L` prefixed local symbol names should not be used to denote
784 the beginning or end of code regions via
785 `SYM_CODE_START_LOCAL`/`SYM_CODE_END`
786
787 **BIT_MACRO**
788 Defines like: 1 << <digit> could be BIT(digit).
789 The BIT() macro is defined via include/linux/bits.h::
790
791 #define BIT(nr) (1UL << (nr))
792
793 **CONST_READ_MOSTLY**
794 When a variable is tagged with the __read_mostly annotation, it is a
795 signal to the compiler that accesses to the variable will be mostly
796 reads and rarely(but NOT never) a write.
797
798 const __read_mostly does not make any sense as const data is already
799 read-only. The __read_mostly annotation thus should be removed.
800
801 **DATE_TIME**
802 It is generally desirable that building the same source code with
803 the same set of tools is reproducible, i.e. the output is always
804 exactly the same.
805
806 The kernel does *not* use the ``__DATE__`` and ``__TIME__`` macros,
807 and enables warnings if they are used as they can lead to
808 non-deterministic builds.
809
810 See: https://www.kernel.org/doc/html/latest/kbuild/reproducible-builds.html#timestamps
811
812 **DEFINE_ARCH_HAS**
813 The ARCH_HAS_xyz and ARCH_HAVE_xyz patterns are wrong.
814
815 For big conceptual features use Kconfig symbols instead. And for
816 smaller things where we have compatibility fallback functions but
817 want architectures able to override them with optimized ones, we
818 should either use weak functions (appropriate for some cases), or
819 the symbol that protects them should be the same symbol we use.
820
821 See: https://lore.kernel.org/lkml/CA+55aFycQ9XJvEOsiM3txHL5bjUc8CeKWJNR_H+MiicaddB42Q@mail.gmail.com/
822
823 **DO_WHILE_MACRO_WITH_TRAILING_SEMICOLON**
824 do {} while(0) macros should not have a trailing semicolon.
825
826 **INIT_ATTRIBUTE**
827 Const init definitions should use __initconst instead of
828 __initdata.
829
830 Similarly init definitions without const require a separate
831 use of const.
832
833 **INLINE_LOCATION**
834 The inline keyword should sit between storage class and type.
835
836 For example, the following segment::
837
838 inline static int example_function(void)
839 {
840 ...
841 }
842
843 should be::
844
845 static inline int example_function(void)
846 {
847 ...
848 }
849
850 **MISPLACED_INIT**
851 It is possible to use section markers on variables in a way
852 which gcc doesn't understand (or at least not the way the
853 developer intended)::
854
855 static struct __initdata samsung_pll_clock exynos4_plls[nr_plls] = {
856
857 does not put exynos4_plls in the .initdata section. The __initdata
858 marker can be virtually anywhere on the line, except right after
859 "struct". The preferred location is before the "=" sign if there is
860 one, or before the trailing ";" otherwise.
861
862 See: https://lore.kernel.org/lkml/1377655732.3619.19.camel@joe-AO722/
863
864 **MULTISTATEMENT_MACRO_USE_DO_WHILE**
865 Macros with multiple statements should be enclosed in a
866 do - while block. Same should also be the case for macros
867 starting with `if` to avoid logic defects::
868
869 #define macrofun(a, b, c) \
870 do { \
871 if (a == 5) \
872 do_this(b, c); \
873 } while (0)
874
875 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#macros-enums-and-rtl
876
877 **PREFER_FALLTHROUGH**
878 Use the `fallthrough;` pseudo keyword instead of
879 `/* fallthrough */` like comments.
880
881 **TRAILING_SEMICOLON**
882 Macro definition should not end with a semicolon. The macro
883 invocation style should be consistent with function calls.
884 This can prevent any unexpected code paths::
885
886 #define MAC do_something;
887
888 If this macro is used within a if else statement, like::
889
890 if (some_condition)
891 MAC;
892
893 else
894 do_something;
895
896 Then there would be a compilation error, because when the macro is
897 expanded there are two trailing semicolons, so the else branch gets
898 orphaned.
899
900 See: https://lore.kernel.org/lkml/1399671106.2912.21.camel@joe-AO725/
901
902 **SINGLE_STATEMENT_DO_WHILE_MACRO**
903 For the multi-statement macros, it is necessary to use the do-while
904 loop to avoid unpredictable code paths. The do-while loop helps to
905 group the multiple statements into a single one so that a
906 function-like macro can be used as a function only.
907
908 But for the single statement macros, it is unnecessary to use the
909 do-while loop. Although the code is syntactically correct but using
910 the do-while loop is redundant. So remove the do-while loop for single
911 statement macros.
912
913 **WEAK_DECLARATION**
914 Using weak declarations like __attribute__((weak)) or __weak
915 can have unintended link defects. Avoid using them.
916
917
918Functions and Variables
919-----------------------
920
921 **CAMELCASE**
922 Avoid CamelCase Identifiers.
923
924 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#naming
925
926 **CONST_CONST**
927 Using `const <type> const *` is generally meant to be
928 written `const <type> * const`.
929
930 **CONST_STRUCT**
931 Using const is generally a good idea. Checkpatch reads
932 a list of frequently used structs that are always or
933 almost always constant.
934
935 The existing structs list can be viewed from
936 `scripts/const_structs.checkpatch`.
937
938 See: https://lore.kernel.org/lkml/alpine.DEB.2.10.1608281509480.3321@hadrien/
939
940 **EMBEDDED_FUNCTION_NAME**
941 Embedded function names are less appropriate to use as
942 refactoring can cause function renaming. Prefer the use of
943 "%s", __func__ to embedded function names.
944
945 Note that this does not work with -f (--file) checkpatch option
946 as it depends on patch context providing the function name.
947
948 **FUNCTION_ARGUMENTS**
949 This warning is emitted due to any of the following reasons:
950
951 1. Arguments for the function declaration do not follow
952 the identifier name. Example::
953
954 void foo
955 (int bar, int baz)
956
957 This should be corrected to::
958
959 void foo(int bar, int baz)
960
961 2. Some arguments for the function definition do not
962 have an identifier name. Example::
963
964 void foo(int)
965
966 All arguments should have identifier names.
967
968 **FUNCTION_WITHOUT_ARGS**
969 Function declarations without arguments like::
970
971 int foo()
972
973 should be::
974
975 int foo(void)
976
977 **GLOBAL_INITIALISERS**
978 Global variables should not be initialized explicitly to
979 0 (or NULL, false, etc.). Your compiler (or rather your
980 loader, which is responsible for zeroing out the relevant
981 sections) automatically does it for you.
982
983 **INITIALISED_STATIC**
984 Static variables should not be initialized explicitly to zero.
985 Your compiler (or rather your loader) automatically does
986 it for you.
987
988 **MULTIPLE_ASSIGNMENTS**
989 Multiple assignments on a single line makes the code unnecessarily
990 complicated. So on a single line assign value to a single variable
991 only, this makes the code more readable and helps avoid typos.
992
993 **RETURN_PARENTHESES**
994 return is not a function and as such doesn't need parentheses::
995
996 return (bar);
997
998 can simply be::
999
1000 return bar;
1001
1002
1003Permissions
1004-----------
1005
1006 **DEVICE_ATTR_PERMS**
1007 The permissions used in DEVICE_ATTR are unusual.
1008 Typically only three permissions are used - 0644 (RW), 0444 (RO)
1009 and 0200 (WO).
1010
1011 See: https://www.kernel.org/doc/html/latest/filesystems/sysfs.html#attributes
1012
1013 **EXECUTE_PERMISSIONS**
1014 There is no reason for source files to be executable. The executable
1015 bit can be removed safely.
1016
1017 **EXPORTED_WORLD_WRITABLE**
1018 Exporting world writable sysfs/debugfs files is usually a bad thing.
1019 When done arbitrarily they can introduce serious security bugs.
1020 In the past, some of the debugfs vulnerabilities would seemingly allow
1021 any local user to write arbitrary values into device registers - a
1022 situation from which little good can be expected to emerge.
1023
1024 See: https://lore.kernel.org/linux-arm-kernel/[email protected]/
1025
1026 **NON_OCTAL_PERMISSIONS**
1027 Permission bits should use 4 digit octal permissions (like 0700 or 0444).
1028 Avoid using any other base like decimal.
1029
1030 **SYMBOLIC_PERMS**
1031 Permission bits in the octal form are more readable and easier to
1032 understand than their symbolic counterparts because many command-line
1033 tools use this notation. Experienced kernel developers have been using
1034 these traditional Unix permission bits for decades and so they find it
1035 easier to understand the octal notation than the symbolic macros.
1036 For example, it is harder to read S_IWUSR|S_IRUGO than 0644, which
1037 obscures the developer's intent rather than clarifying it.
1038
1039 See: https://lore.kernel.org/lkml/CA+55aFw5v23T-zvDZp-MmD_EYxF8WbafwwB59934FV7g21uMGQ@mail.gmail.com/
1040
1041
1042Spacing and Brackets
1043--------------------
1044
1045 **ASSIGNMENT_CONTINUATIONS**
1046 Assignment operators should not be written at the start of a
1047 line but should follow the operand at the previous line.
1048
1049 **BRACES**
1050 The placement of braces is stylistically incorrect.
1051 The preferred way is to put the opening brace last on the line,
1052 and put the closing brace first::
1053
1054 if (x is true) {
1055 we do y
1056 }
1057
1058 This applies for all non-functional blocks.
1059 However, there is one special case, namely functions: they have the
1060 opening brace at the beginning of the next line, thus::
1061
1062 int function(int x)
1063 {
1064 body of function
1065 }
1066
1067 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#placing-braces-and-spaces
1068
1069 **BRACKET_SPACE**
1070 Whitespace before opening bracket '[' is prohibited.
1071 There are some exceptions:
1072
1073 1. With a type on the left::
1074
1075 int [] a;
1076
1077 2. At the beginning of a line for slice initialisers::
1078
1079 [0...10] = 5,
1080
1081 3. Inside a curly brace::
1082
1083 = { [0...10] = 5 }
1084
1085 **CONCATENATED_STRING**
1086 Concatenated elements should have a space in between.
1087 Example::
1088
1089 printk(KERN_INFO"bar");
1090
1091 should be::
1092
1093 printk(KERN_INFO "bar");
1094
1095 **ELSE_AFTER_BRACE**
1096 `else {` should follow the closing block `}` on the same line.
1097
1098 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#placing-braces-and-spaces
1099
1100 **LINE_SPACING**
1101 Vertical space is wasted given the limited number of lines an
1102 editor window can display when multiple blank lines are used.
1103
1104 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#spaces
1105
1106 **OPEN_BRACE**
1107 The opening brace should be following the function definitions on the
1108 next line. For any non-functional block it should be on the same line
1109 as the last construct.
1110
1111 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#placing-braces-and-spaces
1112
1113 **POINTER_LOCATION**
1114 When using pointer data or a function that returns a pointer type,
1115 the preferred use of * is adjacent to the data name or function name
1116 and not adjacent to the type name.
1117 Examples::
1118
1119 char *linux_banner;
1120 unsigned long long memparse(char *ptr, char **retptr);
1121 char *match_strdup(substring_t *s);
1122
1123 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#spaces
1124
1125 **SPACING**
1126 Whitespace style used in the kernel sources is described in kernel docs.
1127
1128 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#spaces
1129
1130 **TRAILING_WHITESPACE**
1131 Trailing whitespace should always be removed.
1132 Some editors highlight the trailing whitespace and cause visual
1133 distractions when editing files.
1134
1135 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#spaces
1136
1137 **UNNECESSARY_PARENTHESES**
1138 Parentheses are not required in the following cases:
1139
1140 1. Function pointer uses::
1141
1142 (foo->bar)();
1143
1144 could be::
1145
1146 foo->bar();
1147
1148 2. Comparisons in if::
1149
1150 if ((foo->bar) && (foo->baz))
1151 if ((foo == bar))
1152
1153 could be::
1154
1155 if (foo->bar && foo->baz)
1156 if (foo == bar)
1157
1158 3. addressof/dereference single Lvalues::
1159
1160 &(foo->bar)
1161 *(foo->bar)
1162
1163 could be::
1164
1165 &foo->bar
1166 *foo->bar
1167
1168 **WHILE_AFTER_BRACE**
1169 while should follow the closing bracket on the same line::
1170
1171 do {
1172 ...
1173 } while(something);
1174
1175 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#placing-braces-and-spaces
1176
1177
1178Others
1179------
1180
1181 **CONFIG_DESCRIPTION**
1182 Kconfig symbols should have a help text which fully describes
1183 it.
1184
1185 **CORRUPTED_PATCH**
1186 The patch seems to be corrupted or lines are wrapped.
1187 Please regenerate the patch file before sending it to the maintainer.
1188
1189 **CVS_KEYWORD**
1190 Since linux moved to git, the CVS markers are no longer used.
1191 So, CVS style keywords ($Id$, $Revision$, $Log$) should not be
1192 added.
1193
1194 **DEFAULT_NO_BREAK**
1195 switch default case is sometimes written as "default:;". This can
1196 cause new cases added below default to be defective.
1197
1198 A "break;" should be added after empty default statement to avoid
1199 unwanted fallthrough.
1200
1201 **DOS_LINE_ENDINGS**
1202 For DOS-formatted patches, there are extra ^M symbols at the end of
1203 the line. These should be removed.
1204
1205 **DT_SCHEMA_BINDING_PATCH**
1206 DT bindings moved to a json-schema based format instead of
1207 freeform text.
1208
1209 See: https://www.kernel.org/doc/html/latest/devicetree/bindings/writing-schema.html
1210
1211 **DT_SPLIT_BINDING_PATCH**
1212 Devicetree bindings should be their own patch. This is because
1213 bindings are logically independent from a driver implementation,
1214 they have a different maintainer (even though they often
1215 are applied via the same tree), and it makes for a cleaner history in the
1216 DT only tree created with git-filter-branch.
1217
1218 See: https://www.kernel.org/doc/html/latest/devicetree/bindings/submitting-patches.html#i-for-patch-submitters
1219
1220 **EMBEDDED_FILENAME**
1221 Embedding the complete filename path inside the file isn't particularly
1222 useful as often the path is moved around and becomes incorrect.
1223
1224 **FILE_PATH_CHANGES**
1225 Whenever files are added, moved, or deleted, the MAINTAINERS file
1226 patterns can be out of sync or outdated.
1227
1228 So MAINTAINERS might need updating in these cases.
1229
1230 **MEMSET**
1231 The memset use appears to be incorrect. This may be caused due to
1232 badly ordered parameters. Please recheck the usage.
1233
1234 **NOT_UNIFIED_DIFF**
1235 The patch file does not appear to be in unified-diff format. Please
1236 regenerate the patch file before sending it to the maintainer.
1237
1238 **PRINTF_0XDECIMAL**
1239 Prefixing 0x with decimal output is defective and should be corrected.
1240
1241 **SPDX_LICENSE_TAG**
1242 The source file is missing or has an improper SPDX identifier tag.
1243 The Linux kernel requires the precise SPDX identifier in all source files,
1244 and it is thoroughly documented in the kernel docs.
1245
1246 See: https://www.kernel.org/doc/html/latest/process/license-rules.html
1247
1248 **TYPO_SPELLING**
1249 Some words may have been misspelled. Consider reviewing them.