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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 <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
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 **DEVICE_ATTR_FUNCTIONS**
346 The function names used in DEVICE_ATTR is unusual.
347 Typically, the store and show functions are used with <attr>_store and
348 <attr>_show, where <attr> is a named attribute variable of the device.
349
350 Consider the following examples::
351
352 static DEVICE_ATTR(type, 0444, type_show, NULL);
353 static DEVICE_ATTR(power, 0644, power_show, power_store);
354
355 The function names should preferably follow the above pattern.
356
357 See: https://www.kernel.org/doc/html/latest/driver-api/driver-model/device.html#attributes
358
359 **DEVICE_ATTR_RO**
360 The DEVICE_ATTR_RO(name) helper macro can be used instead of
361 DEVICE_ATTR(name, 0444, name_show, NULL);
362
363 Note that the macro automatically appends _show to the named
364 attribute variable of the device for the show method.
365
366 See: https://www.kernel.org/doc/html/latest/driver-api/driver-model/device.html#attributes
367
368 **DEVICE_ATTR_RW**
369 The DEVICE_ATTR_RW(name) helper macro can be used instead of
370 DEVICE_ATTR(name, 0644, name_show, name_store);
371
372 Note that the macro automatically appends _show and _store to the
373 named attribute variable of the device for the show and store methods.
374
375 See: https://www.kernel.org/doc/html/latest/driver-api/driver-model/device.html#attributes
376
377 **DEVICE_ATTR_WO**
378 The DEVICE_AATR_WO(name) helper macro can be used instead of
379 DEVICE_ATTR(name, 0200, NULL, name_store);
380
381 Note that the macro automatically appends _store to the
382 named attribute variable of the device for the store method.
383
384 See: https://www.kernel.org/doc/html/latest/driver-api/driver-model/device.html#attributes
385
386 **DUPLICATED_SYSCTL_CONST**
387 Commit d91bff3011cf ("proc/sysctl: add shared variables for range
388 check") added some shared const variables to be used instead of a local
389 copy in each source file.
390
391 Consider replacing the sysctl range checking value with the shared
392 one in include/linux/sysctl.h. The following conversion scheme may
393 be used::
394
395 &zero -> SYSCTL_ZERO
396 &one -> SYSCTL_ONE
397 &int_max -> SYSCTL_INT_MAX
398
399 See:
400
401 1. https://lore.kernel.org/lkml/20190430180111.10688-1-mcroce@redhat.com/
402 2. https://lore.kernel.org/lkml/20190531131422.14970-1-mcroce@redhat.com/
403
404 **ENOSYS**
405 ENOSYS means that a nonexistent system call was called.
406 Earlier, it was wrongly used for things like invalid operations on
407 otherwise valid syscalls. This should be avoided in new code.
408
409 See: https://lore.kernel.org/lkml/5eb299021dec23c1a48fa7d9f2c8b794e967766d.1408730669.git.luto@amacapital.net/
410
411 **ENOTSUPP**
412 ENOTSUPP is not a standard error code and should be avoided in new patches.
413 EOPNOTSUPP should be used instead.
414
415 See: https://lore.kernel.org/netdev/20200510182252.GA411829@lunn.ch/
416
417 **EXPORT_SYMBOL**
418 EXPORT_SYMBOL should immediately follow the symbol to be exported.
419
420 **IN_ATOMIC**
421 in_atomic() is not for driver use so any such use is reported as an ERROR.
422 Also in_atomic() is often used to determine if sleeping is permitted,
423 but it is not reliable in this use model. Therefore its use is
424 strongly discouraged.
425
426 However, in_atomic() is ok for core kernel use.
427
428 See: https://lore.kernel.org/lkml/20080320201723.b87b3732.akpm@linux-foundation.org/
429
430 **LOCKDEP**
431 The lockdep_no_validate class was added as a temporary measure to
432 prevent warnings on conversion of device->sem to device->mutex.
433 It should not be used for any other purpose.
434
435 See: https://lore.kernel.org/lkml/1268959062.9440.467.camel@laptop/
436
437 **MALFORMED_INCLUDE**
438 The #include statement has a malformed path. This has happened
439 because the author has included a double slash "//" in the pathname
440 accidentally.
441
442 **USE_LOCKDEP**
443 lockdep_assert_held() annotations should be preferred over
444 assertions based on spin_is_locked()
445
446 See: https://www.kernel.org/doc/html/latest/locking/lockdep-design.html#annotations
447
448 **UAPI_INCLUDE**
449 No #include statements in include/uapi should use a uapi/ path.
450
451 **USLEEP_RANGE**
452 usleep_range() should be preferred over udelay(). The proper way of
453 using usleep_range() is mentioned in the kernel docs.
454
455
456Comments
457--------
458
459 **BLOCK_COMMENT_STYLE**
460 The comment style is incorrect. The preferred style for multi-
461 line comments is::
462
463 /*
464 * This is the preferred style
465 * for multi line comments.
466 */
467
468 The networking comment style is a bit different, with the first line
469 not empty like the former::
470
471 /* This is the preferred comment style
472 * for files in net/ and drivers/net/
473 */
474
475 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#commenting
476
477 **C99_COMMENTS**
478 C99 style single line comments (//) should not be used.
479 Prefer the block comment style instead.
480
481 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#commenting
482
483 **DATA_RACE**
484 Applications of data_race() should have a comment so as to document the
485 reasoning behind why it was deemed safe.
486
487 See: https://lore.kernel.org/lkml/20200401101714.44781-1-elver@google.com/
488
489 **FSF_MAILING_ADDRESS**
490 Kernel maintainers reject new instances of the GPL boilerplate paragraph
491 directing people to write to the FSF for a copy of the GPL, since the
492 FSF has moved in the past and may do so again.
493 So do not write paragraphs about writing to the Free Software Foundation's
494 mailing address.
495
496 See: https://lore.kernel.org/lkml/20131006222342.GT19510@leaf/
497
498
499Commit message
500--------------
501
502 **BAD_SIGN_OFF**
503 The signed-off-by line does not fall in line with the standards
504 specified by the community.
505
506 See: https://www.kernel.org/doc/html/latest/process/submitting-patches.html#developer-s-certificate-of-origin-1-1
507
508 **BAD_STABLE_ADDRESS_STYLE**
509 The email format for stable is incorrect.
510 Some valid options for stable address are::
511
512 1. stable@vger.kernel.org
513 2. stable@kernel.org
514
515 For adding version info, the following comment style should be used::
516
517 stable@vger.kernel.org # version info
518
519 **COMMIT_COMMENT_SYMBOL**
520 Commit log lines starting with a '#' are ignored by git as
521 comments. To solve this problem addition of a single space
522 infront of the log line is enough.
523
524 **COMMIT_MESSAGE**
525 The patch is missing a commit description. A brief
526 description of the changes made by the patch should be added.
527
528 See: https://www.kernel.org/doc/html/latest/process/submitting-patches.html#describe-your-changes
529
530 **EMAIL_SUBJECT**
531 Naming the tool that found the issue is not very useful in the
532 subject line. A good subject line summarizes the change that
533 the patch brings.
534
535 See: https://www.kernel.org/doc/html/latest/process/submitting-patches.html#describe-your-changes
536
537 **FROM_SIGN_OFF_MISMATCH**
538 The author's email does not match with that in the Signed-off-by:
539 line(s). This can be sometimes caused due to an improperly configured
540 email client.
541
542 This message is emitted due to any of the following reasons::
543
544 - The email names do not match.
545 - The email addresses do not match.
546 - The email subaddresses do not match.
547 - The email comments do not match.
548
549 **MISSING_SIGN_OFF**
550 The patch is missing a Signed-off-by line. A signed-off-by
551 line should be added according to Developer's certificate of
552 Origin.
553
554 See: https://www.kernel.org/doc/html/latest/process/submitting-patches.html#sign-your-work-the-developer-s-certificate-of-origin
555
556 **NO_AUTHOR_SIGN_OFF**
557 The author of the patch has not signed off the patch. It is
558 required that a simple sign off line should be present at the
559 end of explanation of the patch to denote that the author has
560 written it or otherwise has the rights to pass it on as an open
561 source patch.
562
563 See: https://www.kernel.org/doc/html/latest/process/submitting-patches.html#sign-your-work-the-developer-s-certificate-of-origin
564
565 **DIFF_IN_COMMIT_MSG**
566 Avoid having diff content in commit message.
567 This causes problems when one tries to apply a file containing both
568 the changelog and the diff because patch(1) tries to apply the diff
569 which it found in the changelog.
570
571 See: https://lore.kernel.org/lkml/20150611134006.9df79a893e3636019ad2759e@linux-foundation.org/
572
573 **GERRIT_CHANGE_ID**
574 To be picked up by gerrit, the footer of the commit message might
575 have a Change-Id like::
576
577 Change-Id: Ic8aaa0728a43936cd4c6e1ed590e01ba8f0fbf5b
578 Signed-off-by: A. U. Thor <author@example.com>
579
580 The Change-Id line must be removed before submitting.
581
582 **GIT_COMMIT_ID**
583 The proper way to reference a commit id is:
584 commit <12+ chars of sha1> ("<title line>")
585
586 An example may be::
587
588 Commit e21d2170f36602ae2708 ("video: remove unnecessary
589 platform_set_drvdata()") removed the unnecessary
590 platform_set_drvdata(), but left the variable "dev" unused,
591 delete it.
592
593 See: https://www.kernel.org/doc/html/latest/process/submitting-patches.html#describe-your-changes
594
595 **BAD_FIXES_TAG**
596 The Fixes: tag is malformed or does not follow the community conventions.
597 This can occur if the tag have been split into multiple lines (e.g., when
598 pasted in an email program with word wrapping enabled).
599
600 See: https://www.kernel.org/doc/html/latest/process/submitting-patches.html#describe-your-changes
601
602
603Comparison style
604----------------
605
606 **ASSIGN_IN_IF**
607 Do not use assignments in if condition.
608 Example::
609
610 if ((foo = bar(...)) < BAZ) {
611
612 should be written as::
613
614 foo = bar(...);
615 if (foo < BAZ) {
616
617 **BOOL_COMPARISON**
618 Comparisons of A to true and false are better written
619 as A and !A.
620
621 See: https://lore.kernel.org/lkml/1365563834.27174.12.camel@joe-AO722/
622
623 **COMPARISON_TO_NULL**
624 Comparisons to NULL in the form (foo == NULL) or (foo != NULL)
625 are better written as (!foo) and (foo).
626
627 **CONSTANT_COMPARISON**
628 Comparisons with a constant or upper case identifier on the left
629 side of the test should be avoided.
630
631
632Indentation and Line Breaks
633---------------------------
634
635 **CODE_INDENT**
636 Code indent should use tabs instead of spaces.
637 Outside of comments, documentation and Kconfig,
638 spaces are never used for indentation.
639
640 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#indentation
641
642 **DEEP_INDENTATION**
643 Indentation with 6 or more tabs usually indicate overly indented
644 code.
645
646 It is suggested to refactor excessive indentation of
647 if/else/for/do/while/switch statements.
648
649 See: https://lore.kernel.org/lkml/1328311239.21255.24.camel@joe2Laptop/
650
651 **SWITCH_CASE_INDENT_LEVEL**
652 switch should be at the same indent as case.
653 Example::
654
655 switch (suffix) {
656 case 'G':
657 case 'g':
658 mem <<= 30;
659 break;
660 case 'M':
661 case 'm':
662 mem <<= 20;
663 break;
664 case 'K':
665 case 'k':
666 mem <<= 10;
667 fallthrough;
668 default:
669 break;
670 }
671
672 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#indentation
673
674 **LONG_LINE**
675 The line has exceeded the specified maximum length.
676 To use a different maximum line length, the --max-line-length=n option
677 may be added while invoking checkpatch.
678
679 Earlier, the default line length was 80 columns. Commit bdc48fa11e46
680 ("checkpatch/coding-style: deprecate 80-column warning") increased the
681 limit to 100 columns. This is not a hard limit either and it's
682 preferable to stay within 80 columns whenever possible.
683
684 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#breaking-long-lines-and-strings
685
686 **LONG_LINE_STRING**
687 A string starts before but extends beyond the maximum line length.
688 To use a different maximum line length, the --max-line-length=n option
689 may be added while invoking checkpatch.
690
691 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#breaking-long-lines-and-strings
692
693 **LONG_LINE_COMMENT**
694 A comment starts before but extends beyond the maximum line length.
695 To use a different maximum line length, the --max-line-length=n option
696 may be added while invoking checkpatch.
697
698 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#breaking-long-lines-and-strings
699
700 **SPLIT_STRING**
701 Quoted strings that appear as messages in userspace and can be
702 grepped, should not be split across multiple lines.
703
704 See: https://lore.kernel.org/lkml/20120203052727.GA15035@leaf/
705
706 **MULTILINE_DEREFERENCE**
707 A single dereferencing identifier spanned on multiple lines like::
708
709 struct_identifier->member[index].
710 member = <foo>;
711
712 is generally hard to follow. It can easily lead to typos and so makes
713 the code vulnerable to bugs.
714
715 If fixing the multiple line dereferencing leads to an 80 column
716 violation, then either rewrite the code in a more simple way or if the
717 starting part of the dereferencing identifier is the same and used at
718 multiple places then store it in a temporary variable, and use that
719 temporary variable only at all the places. For example, if there are
720 two dereferencing identifiers::
721
722 member1->member2->member3.foo1;
723 member1->member2->member3.foo2;
724
725 then store the member1->member2->member3 part in a temporary variable.
726 It not only helps to avoid the 80 column violation but also reduces
727 the program size by removing the unnecessary dereferences.
728
729 But if none of the above methods work then ignore the 80 column
730 violation because it is much easier to read a dereferencing identifier
731 on a single line.
732
733 **TRAILING_STATEMENTS**
734 Trailing statements (for example after any conditional) should be
735 on the next line.
736 Statements, such as::
737
738 if (x == y) break;
739
740 should be::
741
742 if (x == y)
743 break;
744
745
746Macros, Attributes and Symbols
747------------------------------
748
749 **ARRAY_SIZE**
750 The ARRAY_SIZE(foo) macro should be preferred over
751 sizeof(foo)/sizeof(foo[0]) for finding number of elements in an
752 array.
753
754 The macro is defined in include/linux/kernel.h::
755
756 #define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
757
758 **AVOID_EXTERNS**
759 Function prototypes don't need to be declared extern in .h
760 files. It's assumed by the compiler and is unnecessary.
761
762 **AVOID_L_PREFIX**
763 Local symbol names that are prefixed with `.L` should be avoided,
764 as this has special meaning for the assembler; a symbol entry will
765 not be emitted into the symbol table. This can prevent `objtool`
766 from generating correct unwind info.
767
768 Symbols with STB_LOCAL binding may still be used, and `.L` prefixed
769 local symbol names are still generally usable within a function,
770 but `.L` prefixed local symbol names should not be used to denote
771 the beginning or end of code regions via
772 `SYM_CODE_START_LOCAL`/`SYM_CODE_END`
773
774 **BIT_MACRO**
775 Defines like: 1 << <digit> could be BIT(digit).
776 The BIT() macro is defined via include/linux/bits.h::
777
778 #define BIT(nr) (1UL << (nr))
779
780 **CONST_READ_MOSTLY**
781 When a variable is tagged with the __read_mostly annotation, it is a
782 signal to the compiler that accesses to the variable will be mostly
783 reads and rarely(but NOT never) a write.
784
785 const __read_mostly does not make any sense as const data is already
786 read-only. The __read_mostly annotation thus should be removed.
787
788 **DATE_TIME**
789 It is generally desirable that building the same source code with
790 the same set of tools is reproducible, i.e. the output is always
791 exactly the same.
792
793 The kernel does *not* use the ``__DATE__`` and ``__TIME__`` macros,
794 and enables warnings if they are used as they can lead to
795 non-deterministic builds.
796
797 See: https://www.kernel.org/doc/html/latest/kbuild/reproducible-builds.html#timestamps
798
799 **DEFINE_ARCH_HAS**
800 The ARCH_HAS_xyz and ARCH_HAVE_xyz patterns are wrong.
801
802 For big conceptual features use Kconfig symbols instead. And for
803 smaller things where we have compatibility fallback functions but
804 want architectures able to override them with optimized ones, we
805 should either use weak functions (appropriate for some cases), or
806 the symbol that protects them should be the same symbol we use.
807
808 See: https://lore.kernel.org/lkml/CA+55aFycQ9XJvEOsiM3txHL5bjUc8CeKWJNR_H+MiicaddB42Q@mail.gmail.com/
809
810 **DO_WHILE_MACRO_WITH_TRAILING_SEMICOLON**
811 do {} while(0) macros should not have a trailing semicolon.
812
813 **INIT_ATTRIBUTE**
814 Const init definitions should use __initconst instead of
815 __initdata.
816
817 Similarly init definitions without const require a separate
818 use of const.
819
820 **INLINE_LOCATION**
821 The inline keyword should sit between storage class and type.
822
823 For example, the following segment::
824
825 inline static int example_function(void)
826 {
827 ...
828 }
829
830 should be::
831
832 static inline int example_function(void)
833 {
834 ...
835 }
836
837 **MISPLACED_INIT**
838 It is possible to use section markers on variables in a way
839 which gcc doesn't understand (or at least not the way the
840 developer intended)::
841
842 static struct __initdata samsung_pll_clock exynos4_plls[nr_plls] = {
843
844 does not put exynos4_plls in the .initdata section. The __initdata
845 marker can be virtually anywhere on the line, except right after
846 "struct". The preferred location is before the "=" sign if there is
847 one, or before the trailing ";" otherwise.
848
849 See: https://lore.kernel.org/lkml/1377655732.3619.19.camel@joe-AO722/
850
851 **MULTISTATEMENT_MACRO_USE_DO_WHILE**
852 Macros with multiple statements should be enclosed in a
853 do - while block. Same should also be the case for macros
854 starting with `if` to avoid logic defects::
855
856 #define macrofun(a, b, c) \
857 do { \
858 if (a == 5) \
859 do_this(b, c); \
860 } while (0)
861
862 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#macros-enums-and-rtl
863
864 **PREFER_FALLTHROUGH**
865 Use the `fallthrough;` pseudo keyword instead of
866 `/* fallthrough */` like comments.
867
868 **TRAILING_SEMICOLON**
869 Macro definition should not end with a semicolon. The macro
870 invocation style should be consistent with function calls.
871 This can prevent any unexpected code paths::
872
873 #define MAC do_something;
874
875 If this macro is used within a if else statement, like::
876
877 if (some_condition)
878 MAC;
879
880 else
881 do_something;
882
883 Then there would be a compilation error, because when the macro is
884 expanded there are two trailing semicolons, so the else branch gets
885 orphaned.
886
887 See: https://lore.kernel.org/lkml/1399671106.2912.21.camel@joe-AO725/
888
889 **MACRO_ARG_UNUSED**
890 If function-like macros do not utilize a parameter, it might result
891 in a build warning. We advocate for utilizing static inline functions
892 to replace such macros.
893 For example, for a macro such as the one below::
894
895 #define test(a) do { } while (0)
896
897 there would be a warning like below::
898
899 WARNING: Argument 'a' is not used in function-like macro.
900
901 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#macros-enums-and-rtl
902
903 **SINGLE_STATEMENT_DO_WHILE_MACRO**
904 For the multi-statement macros, it is necessary to use the do-while
905 loop to avoid unpredictable code paths. The do-while loop helps to
906 group the multiple statements into a single one so that a
907 function-like macro can be used as a function only.
908
909 But for the single statement macros, it is unnecessary to use the
910 do-while loop. Although the code is syntactically correct but using
911 the do-while loop is redundant. So remove the do-while loop for single
912 statement macros.
913
914 **WEAK_DECLARATION**
915 Using weak declarations like __attribute__((weak)) or __weak
916 can have unintended link defects. Avoid using them.
917
918
919Functions and Variables
920-----------------------
921
922 **CAMELCASE**
923 Avoid CamelCase Identifiers.
924
925 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#naming
926
927 **CONST_CONST**
928 Using `const <type> const *` is generally meant to be
929 written `const <type> * const`.
930
931 **CONST_STRUCT**
932 Using const is generally a good idea. Checkpatch reads
933 a list of frequently used structs that are always or
934 almost always constant.
935
936 The existing structs list can be viewed from
937 `scripts/const_structs.checkpatch`.
938
939 See: https://lore.kernel.org/lkml/alpine.DEB.2.10.1608281509480.3321@hadrien/
940
941 **EMBEDDED_FUNCTION_NAME**
942 Embedded function names are less appropriate to use as
943 refactoring can cause function renaming. Prefer the use of
944 "%s", __func__ to embedded function names.
945
946 Note that this does not work with -f (--file) checkpatch option
947 as it depends on patch context providing the function name.
948
949 **FUNCTION_ARGUMENTS**
950 This warning is emitted due to any of the following reasons:
951
952 1. Arguments for the function declaration do not follow
953 the identifier name. Example::
954
955 void foo
956 (int bar, int baz)
957
958 This should be corrected to::
959
960 void foo(int bar, int baz)
961
962 2. Some arguments for the function definition do not
963 have an identifier name. Example::
964
965 void foo(int)
966
967 All arguments should have identifier names.
968
969 **FUNCTION_WITHOUT_ARGS**
970 Function declarations without arguments like::
971
972 int foo()
973
974 should be::
975
976 int foo(void)
977
978 **GLOBAL_INITIALISERS**
979 Global variables should not be initialized explicitly to
980 0 (or NULL, false, etc.). Your compiler (or rather your
981 loader, which is responsible for zeroing out the relevant
982 sections) automatically does it for you.
983
984 **INITIALISED_STATIC**
985 Static variables should not be initialized explicitly to zero.
986 Your compiler (or rather your loader) automatically does
987 it for you.
988
989 **MULTIPLE_ASSIGNMENTS**
990 Multiple assignments on a single line makes the code unnecessarily
991 complicated. So on a single line assign value to a single variable
992 only, this makes the code more readable and helps avoid typos.
993
994 **RETURN_PARENTHESES**
995 return is not a function and as such doesn't need parentheses::
996
997 return (bar);
998
999 can simply be::
1000
1001 return bar;
1002
1003
1004Permissions
1005-----------
1006
1007 **DEVICE_ATTR_PERMS**
1008 The permissions used in DEVICE_ATTR are unusual.
1009 Typically only three permissions are used - 0644 (RW), 0444 (RO)
1010 and 0200 (WO).
1011
1012 See: https://www.kernel.org/doc/html/latest/filesystems/sysfs.html#attributes
1013
1014 **EXECUTE_PERMISSIONS**
1015 There is no reason for source files to be executable. The executable
1016 bit can be removed safely.
1017
1018 **EXPORTED_WORLD_WRITABLE**
1019 Exporting world writable sysfs/debugfs files is usually a bad thing.
1020 When done arbitrarily they can introduce serious security bugs.
1021 In the past, some of the debugfs vulnerabilities would seemingly allow
1022 any local user to write arbitrary values into device registers - a
1023 situation from which little good can be expected to emerge.
1024
1025 See: https://lore.kernel.org/linux-arm-kernel/cover.1296818921.git.segoon@openwall.com/
1026
1027 **NON_OCTAL_PERMISSIONS**
1028 Permission bits should use 4 digit octal permissions (like 0700 or 0444).
1029 Avoid using any other base like decimal.
1030
1031 **SYMBOLIC_PERMS**
1032 Permission bits in the octal form are more readable and easier to
1033 understand than their symbolic counterparts because many command-line
1034 tools use this notation. Experienced kernel developers have been using
1035 these traditional Unix permission bits for decades and so they find it
1036 easier to understand the octal notation than the symbolic macros.
1037 For example, it is harder to read S_IWUSR|S_IRUGO than 0644, which
1038 obscures the developer's intent rather than clarifying it.
1039
1040 See: https://lore.kernel.org/lkml/CA+55aFw5v23T-zvDZp-MmD_EYxF8WbafwwB59934FV7g21uMGQ@mail.gmail.com/
1041
1042
1043Spacing and Brackets
1044--------------------
1045
1046 **ASSIGNMENT_CONTINUATIONS**
1047 Assignment operators should not be written at the start of a
1048 line but should follow the operand at the previous line.
1049
1050 **BRACES**
1051 The placement of braces is stylistically incorrect.
1052 The preferred way is to put the opening brace last on the line,
1053 and put the closing brace first::
1054
1055 if (x is true) {
1056 we do y
1057 }
1058
1059 This applies for all non-functional blocks.
1060 However, there is one special case, namely functions: they have the
1061 opening brace at the beginning of the next line, thus::
1062
1063 int function(int x)
1064 {
1065 body of function
1066 }
1067
1068 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#placing-braces-and-spaces
1069
1070 **BRACKET_SPACE**
1071 Whitespace before opening bracket '[' is prohibited.
1072 There are some exceptions:
1073
1074 1. With a type on the left::
1075
1076 int [] a;
1077
1078 2. At the beginning of a line for slice initialisers::
1079
1080 [0...10] = 5,
1081
1082 3. Inside a curly brace::
1083
1084 = { [0...10] = 5 }
1085
1086 **CONCATENATED_STRING**
1087 Concatenated elements should have a space in between.
1088 Example::
1089
1090 printk(KERN_INFO"bar");
1091
1092 should be::
1093
1094 printk(KERN_INFO "bar");
1095
1096 **ELSE_AFTER_BRACE**
1097 `else {` should follow the closing block `}` on the same line.
1098
1099 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#placing-braces-and-spaces
1100
1101 **LINE_SPACING**
1102 Vertical space is wasted given the limited number of lines an
1103 editor window can display when multiple blank lines are used.
1104
1105 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#spaces
1106
1107 **OPEN_BRACE**
1108 The opening brace should be following the function definitions on the
1109 next line. For any non-functional block it should be on the same line
1110 as the last construct.
1111
1112 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#placing-braces-and-spaces
1113
1114 **POINTER_LOCATION**
1115 When using pointer data or a function that returns a pointer type,
1116 the preferred use of * is adjacent to the data name or function name
1117 and not adjacent to the type name.
1118 Examples::
1119
1120 char *linux_banner;
1121 unsigned long long memparse(char *ptr, char **retptr);
1122 char *match_strdup(substring_t *s);
1123
1124 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#spaces
1125
1126 **SPACING**
1127 Whitespace style used in the kernel sources is described in kernel docs.
1128
1129 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#spaces
1130
1131 **TRAILING_WHITESPACE**
1132 Trailing whitespace should always be removed.
1133 Some editors highlight the trailing whitespace and cause visual
1134 distractions when editing files.
1135
1136 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#spaces
1137
1138 **UNNECESSARY_PARENTHESES**
1139 Parentheses are not required in the following cases:
1140
1141 1. Function pointer uses::
1142
1143 (foo->bar)();
1144
1145 could be::
1146
1147 foo->bar();
1148
1149 2. Comparisons in if::
1150
1151 if ((foo->bar) && (foo->baz))
1152 if ((foo == bar))
1153
1154 could be::
1155
1156 if (foo->bar && foo->baz)
1157 if (foo == bar)
1158
1159 3. addressof/dereference single Lvalues::
1160
1161 &(foo->bar)
1162 *(foo->bar)
1163
1164 could be::
1165
1166 &foo->bar
1167 *foo->bar
1168
1169 **WHILE_AFTER_BRACE**
1170 while should follow the closing bracket on the same line::
1171
1172 do {
1173 ...
1174 } while(something);
1175
1176 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#placing-braces-and-spaces
1177
1178
1179Others
1180------
1181
1182 **CONFIG_DESCRIPTION**
1183 Kconfig symbols should have a help text which fully describes
1184 it.
1185
1186 **CORRUPTED_PATCH**
1187 The patch seems to be corrupted or lines are wrapped.
1188 Please regenerate the patch file before sending it to the maintainer.
1189
1190 **CVS_KEYWORD**
1191 Since linux moved to git, the CVS markers are no longer used.
1192 So, CVS style keywords ($Id$, $Revision$, $Log$) should not be
1193 added.
1194
1195 **DEFAULT_NO_BREAK**
1196 switch default case is sometimes written as "default:;". This can
1197 cause new cases added below default to be defective.
1198
1199 A "break;" should be added after empty default statement to avoid
1200 unwanted fallthrough.
1201
1202 **DOS_LINE_ENDINGS**
1203 For DOS-formatted patches, there are extra ^M symbols at the end of
1204 the line. These should be removed.
1205
1206 **DT_SCHEMA_BINDING_PATCH**
1207 DT bindings moved to a json-schema based format instead of
1208 freeform text.
1209
1210 See: https://www.kernel.org/doc/html/latest/devicetree/bindings/writing-schema.html
1211
1212 **DT_SPLIT_BINDING_PATCH**
1213 Devicetree bindings should be their own patch. This is because
1214 bindings are logically independent from a driver implementation,
1215 they have a different maintainer (even though they often
1216 are applied via the same tree), and it makes for a cleaner history in the
1217 DT only tree created with git-filter-branch.
1218
1219 See: https://www.kernel.org/doc/html/latest/devicetree/bindings/submitting-patches.html#i-for-patch-submitters
1220
1221 **EMBEDDED_FILENAME**
1222 Embedding the complete filename path inside the file isn't particularly
1223 useful as often the path is moved around and becomes incorrect.
1224
1225 **FILE_PATH_CHANGES**
1226 Whenever files are added, moved, or deleted, the MAINTAINERS file
1227 patterns can be out of sync or outdated.
1228
1229 So MAINTAINERS might need updating in these cases.
1230
1231 **MEMSET**
1232 The memset use appears to be incorrect. This may be caused due to
1233 badly ordered parameters. Please recheck the usage.
1234
1235 **NOT_UNIFIED_DIFF**
1236 The patch file does not appear to be in unified-diff format. Please
1237 regenerate the patch file before sending it to the maintainer.
1238
1239 **PRINTF_0XDECIMAL**
1240 Prefixing 0x with decimal output is defective and should be corrected.
1241
1242 **SPDX_LICENSE_TAG**
1243 The source file is missing or has an improper SPDX identifier tag.
1244 The Linux kernel requires the precise SPDX identifier in all source files,
1245 and it is thoroughly documented in the kernel docs.
1246
1247 See: https://www.kernel.org/doc/html/latest/process/license-rules.html
1248
1249 **TYPO_SPELLING**
1250 Some words may have been misspelled. Consider reviewing them.