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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 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#commenting
469
470 **C99_COMMENTS**
471 C99 style single line comments (//) should not be used.
472 Prefer the block comment style instead.
473
474 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#commenting
475
476 **DATA_RACE**
477 Applications of data_race() should have a comment so as to document the
478 reasoning behind why it was deemed safe.
479
480 See: https://lore.kernel.org/lkml/20200401101714.44781-1-elver@google.com/
481
482 **FSF_MAILING_ADDRESS**
483 Kernel maintainers reject new instances of the GPL boilerplate paragraph
484 directing people to write to the FSF for a copy of the GPL, since the
485 FSF has moved in the past and may do so again.
486 So do not write paragraphs about writing to the Free Software Foundation's
487 mailing address.
488
489 See: https://lore.kernel.org/lkml/20131006222342.GT19510@leaf/
490
491 **UNCOMMENTED_RGMII_MODE**
492 Historically, the RGMII PHY modes specified in Device Trees have been
493 used inconsistently, often referring to the usage of delays on the PHY
494 side rather than describing the board.
495
496 PHY modes "rgmii", "rgmii-rxid" and "rgmii-txid" modes require the clock
497 signal to be delayed on the PCB; this unusual configuration should be
498 described in a comment. If they are not (meaning that the delay is realized
499 internally in the MAC or PHY), "rgmii-id" is the correct PHY mode.
500
501Commit message
502--------------
503
504 **BAD_SIGN_OFF**
505 The signed-off-by line does not fall in line with the standards
506 specified by the community.
507
508 See: https://www.kernel.org/doc/html/latest/process/submitting-patches.html#developer-s-certificate-of-origin-1-1
509
510 **BAD_STABLE_ADDRESS_STYLE**
511 The email format for stable is incorrect.
512 Some valid options for stable address are::
513
514 1. stable@vger.kernel.org
515 2. stable@kernel.org
516
517 For adding version info, the following comment style should be used::
518
519 stable@vger.kernel.org # version info
520
521 **COMMIT_COMMENT_SYMBOL**
522 Commit log lines starting with a '#' are ignored by git as
523 comments. To solve this problem addition of a single space
524 infront of the log line is enough.
525
526 **COMMIT_MESSAGE**
527 The patch is missing a commit description. A brief
528 description of the changes made by the patch should be added.
529
530 See: https://www.kernel.org/doc/html/latest/process/submitting-patches.html#describe-your-changes
531
532 **EMAIL_SUBJECT**
533 Naming the tool that found the issue is not very useful in the
534 subject line. A good subject line summarizes the change that
535 the patch brings.
536
537 See: https://www.kernel.org/doc/html/latest/process/submitting-patches.html#describe-your-changes
538
539 **FROM_SIGN_OFF_MISMATCH**
540 The author's email does not match with that in the Signed-off-by:
541 line(s). This can be sometimes caused due to an improperly configured
542 email client.
543
544 This message is emitted due to any of the following reasons::
545
546 - The email names do not match.
547 - The email addresses do not match.
548 - The email subaddresses do not match.
549 - The email comments do not match.
550
551 **MISSING_SIGN_OFF**
552 The patch is missing a Signed-off-by line. A signed-off-by
553 line should be added according to Developer's certificate of
554 Origin.
555
556 See: https://www.kernel.org/doc/html/latest/process/submitting-patches.html#sign-your-work-the-developer-s-certificate-of-origin
557
558 **NO_AUTHOR_SIGN_OFF**
559 The author of the patch has not signed off the patch. It is
560 required that a simple sign off line should be present at the
561 end of explanation of the patch to denote that the author has
562 written it or otherwise has the rights to pass it on as an open
563 source patch.
564
565 See: https://www.kernel.org/doc/html/latest/process/submitting-patches.html#sign-your-work-the-developer-s-certificate-of-origin
566
567 **DIFF_IN_COMMIT_MSG**
568 Avoid having diff content in commit message.
569 This causes problems when one tries to apply a file containing both
570 the changelog and the diff because patch(1) tries to apply the diff
571 which it found in the changelog.
572
573 See: https://lore.kernel.org/lkml/20150611134006.9df79a893e3636019ad2759e@linux-foundation.org/
574
575 **GERRIT_CHANGE_ID**
576 To be picked up by gerrit, the footer of the commit message might
577 have a Change-Id like::
578
579 Change-Id: Ic8aaa0728a43936cd4c6e1ed590e01ba8f0fbf5b
580 Signed-off-by: A. U. Thor <author@example.com>
581
582 The Change-Id line must be removed before submitting.
583
584 **GIT_COMMIT_ID**
585 The proper way to reference a commit id is:
586 commit <12+ chars of sha1> ("<title line>")
587
588 An example may be::
589
590 Commit e21d2170f36602ae2708 ("video: remove unnecessary
591 platform_set_drvdata()") removed the unnecessary
592 platform_set_drvdata(), but left the variable "dev" unused,
593 delete it.
594
595 See: https://www.kernel.org/doc/html/latest/process/submitting-patches.html#describe-your-changes
596
597 **BAD_FIXES_TAG**
598 The Fixes: tag is malformed or does not follow the community conventions.
599 This can occur if the tag have been split into multiple lines (e.g., when
600 pasted in an email program with word wrapping enabled).
601
602 See: https://www.kernel.org/doc/html/latest/process/submitting-patches.html#describe-your-changes
603
604
605Comparison style
606----------------
607
608 **ASSIGN_IN_IF**
609 Do not use assignments in if condition.
610 Example::
611
612 if ((foo = bar(...)) < BAZ) {
613
614 should be written as::
615
616 foo = bar(...);
617 if (foo < BAZ) {
618
619 **BOOL_COMPARISON**
620 Comparisons of A to true and false are better written
621 as A and !A.
622
623 See: https://lore.kernel.org/lkml/1365563834.27174.12.camel@joe-AO722/
624
625 **COMPARISON_TO_NULL**
626 Comparisons to NULL in the form (foo == NULL) or (foo != NULL)
627 are better written as (!foo) and (foo).
628
629 **CONSTANT_COMPARISON**
630 Comparisons with a constant or upper case identifier on the left
631 side of the test should be avoided.
632
633
634Indentation and Line Breaks
635---------------------------
636
637 **CODE_INDENT**
638 Code indent should use tabs instead of spaces.
639 Outside of comments, documentation and Kconfig,
640 spaces are never used for indentation.
641
642 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#indentation
643
644 **DEEP_INDENTATION**
645 Indentation with 6 or more tabs usually indicate overly indented
646 code.
647
648 It is suggested to refactor excessive indentation of
649 if/else/for/do/while/switch statements.
650
651 See: https://lore.kernel.org/lkml/1328311239.21255.24.camel@joe2Laptop/
652
653 **SWITCH_CASE_INDENT_LEVEL**
654 switch should be at the same indent as case.
655 Example::
656
657 switch (suffix) {
658 case 'G':
659 case 'g':
660 mem <<= 30;
661 break;
662 case 'M':
663 case 'm':
664 mem <<= 20;
665 break;
666 case 'K':
667 case 'k':
668 mem <<= 10;
669 fallthrough;
670 default:
671 break;
672 }
673
674 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#indentation
675
676 **LONG_LINE**
677 The line has exceeded the specified maximum length.
678 To use a different maximum line length, the --max-line-length=n option
679 may be added while invoking checkpatch.
680
681 Earlier, the default line length was 80 columns. Commit bdc48fa11e46
682 ("checkpatch/coding-style: deprecate 80-column warning") increased the
683 limit to 100 columns. This is not a hard limit either and it's
684 preferable to stay within 80 columns whenever possible.
685
686 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#breaking-long-lines-and-strings
687
688 **LONG_LINE_STRING**
689 A string starts before but extends beyond the maximum line length.
690 To use a different maximum line length, the --max-line-length=n option
691 may be added while invoking checkpatch.
692
693 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#breaking-long-lines-and-strings
694
695 **LONG_LINE_COMMENT**
696 A comment starts before but extends beyond the maximum line length.
697 To use a different maximum line length, the --max-line-length=n option
698 may be added while invoking checkpatch.
699
700 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#breaking-long-lines-and-strings
701
702 **SPLIT_STRING**
703 Quoted strings that appear as messages in userspace and can be
704 grepped, should not be split across multiple lines.
705
706 See: https://lore.kernel.org/lkml/20120203052727.GA15035@leaf/
707
708 **MULTILINE_DEREFERENCE**
709 A single dereferencing identifier spanned on multiple lines like::
710
711 struct_identifier->member[index].
712 member = <foo>;
713
714 is generally hard to follow. It can easily lead to typos and so makes
715 the code vulnerable to bugs.
716
717 If fixing the multiple line dereferencing leads to an 80 column
718 violation, then either rewrite the code in a more simple way or if the
719 starting part of the dereferencing identifier is the same and used at
720 multiple places then store it in a temporary variable, and use that
721 temporary variable only at all the places. For example, if there are
722 two dereferencing identifiers::
723
724 member1->member2->member3.foo1;
725 member1->member2->member3.foo2;
726
727 then store the member1->member2->member3 part in a temporary variable.
728 It not only helps to avoid the 80 column violation but also reduces
729 the program size by removing the unnecessary dereferences.
730
731 But if none of the above methods work then ignore the 80 column
732 violation because it is much easier to read a dereferencing identifier
733 on a single line.
734
735 **TRAILING_STATEMENTS**
736 Trailing statements (for example after any conditional) should be
737 on the next line.
738 Statements, such as::
739
740 if (x == y) break;
741
742 should be::
743
744 if (x == y)
745 break;
746
747
748Macros, Attributes and Symbols
749------------------------------
750
751 **ARRAY_SIZE**
752 The ARRAY_SIZE(foo) macro should be preferred over
753 sizeof(foo)/sizeof(foo[0]) for finding number of elements in an
754 array.
755
756 The macro is defined in include/linux/kernel.h::
757
758 #define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
759
760 **AVOID_EXTERNS**
761 Function prototypes don't need to be declared extern in .h
762 files. It's assumed by the compiler and is unnecessary.
763
764 **AVOID_L_PREFIX**
765 Local symbol names that are prefixed with `.L` should be avoided,
766 as this has special meaning for the assembler; a symbol entry will
767 not be emitted into the symbol table. This can prevent `objtool`
768 from generating correct unwind info.
769
770 Symbols with STB_LOCAL binding may still be used, and `.L` prefixed
771 local symbol names are still generally usable within a function,
772 but `.L` prefixed local symbol names should not be used to denote
773 the beginning or end of code regions via
774 `SYM_CODE_START_LOCAL`/`SYM_CODE_END`
775
776 **BIT_MACRO**
777 Defines like: 1 << <digit> could be BIT(digit).
778 The BIT() macro is defined via include/linux/bits.h::
779
780 #define BIT(nr) (1UL << (nr))
781
782 **CONST_READ_MOSTLY**
783 When a variable is tagged with the __read_mostly annotation, it is a
784 signal to the compiler that accesses to the variable will be mostly
785 reads and rarely(but NOT never) a write.
786
787 const __read_mostly does not make any sense as const data is already
788 read-only. The __read_mostly annotation thus should be removed.
789
790 **DATE_TIME**
791 It is generally desirable that building the same source code with
792 the same set of tools is reproducible, i.e. the output is always
793 exactly the same.
794
795 The kernel does *not* use the ``__DATE__`` and ``__TIME__`` macros,
796 and enables warnings if they are used as they can lead to
797 non-deterministic builds.
798
799 See: https://www.kernel.org/doc/html/latest/kbuild/reproducible-builds.html#timestamps
800
801 **DEFINE_ARCH_HAS**
802 The ARCH_HAS_xyz and ARCH_HAVE_xyz patterns are wrong.
803
804 For big conceptual features use Kconfig symbols instead. And for
805 smaller things where we have compatibility fallback functions but
806 want architectures able to override them with optimized ones, we
807 should either use weak functions (appropriate for some cases), or
808 the symbol that protects them should be the same symbol we use.
809
810 See: https://lore.kernel.org/lkml/CA+55aFycQ9XJvEOsiM3txHL5bjUc8CeKWJNR_H+MiicaddB42Q@mail.gmail.com/
811
812 **DO_WHILE_MACRO_WITH_TRAILING_SEMICOLON**
813 do {} while(0) macros should not have a trailing semicolon.
814
815 **INIT_ATTRIBUTE**
816 Const init definitions should use __initconst instead of
817 __initdata.
818
819 Similarly init definitions without const require a separate
820 use of const.
821
822 **INLINE_LOCATION**
823 The inline keyword should sit between storage class and type.
824
825 For example, the following segment::
826
827 inline static int example_function(void)
828 {
829 ...
830 }
831
832 should be::
833
834 static inline int example_function(void)
835 {
836 ...
837 }
838
839 **MISPLACED_INIT**
840 It is possible to use section markers on variables in a way
841 which gcc doesn't understand (or at least not the way the
842 developer intended)::
843
844 static struct __initdata samsung_pll_clock exynos4_plls[nr_plls] = {
845
846 does not put exynos4_plls in the .initdata section. The __initdata
847 marker can be virtually anywhere on the line, except right after
848 "struct". The preferred location is before the "=" sign if there is
849 one, or before the trailing ";" otherwise.
850
851 See: https://lore.kernel.org/lkml/1377655732.3619.19.camel@joe-AO722/
852
853 **MULTISTATEMENT_MACRO_USE_DO_WHILE**
854 Macros with multiple statements should be enclosed in a
855 do - while block. Same should also be the case for macros
856 starting with `if` to avoid logic defects::
857
858 #define macrofun(a, b, c) \
859 do { \
860 if (a == 5) \
861 do_this(b, c); \
862 } while (0)
863
864 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#macros-enums-and-rtl
865
866 **PREFER_FALLTHROUGH**
867 Use the `fallthrough;` pseudo keyword instead of
868 `/* fallthrough */` like comments.
869
870 **TRAILING_SEMICOLON**
871 Macro definition should not end with a semicolon. The macro
872 invocation style should be consistent with function calls.
873 This can prevent any unexpected code paths::
874
875 #define MAC do_something;
876
877 If this macro is used within a if else statement, like::
878
879 if (some_condition)
880 MAC;
881
882 else
883 do_something;
884
885 Then there would be a compilation error, because when the macro is
886 expanded there are two trailing semicolons, so the else branch gets
887 orphaned.
888
889 See: https://lore.kernel.org/lkml/1399671106.2912.21.camel@joe-AO725/
890
891 **MACRO_ARG_UNUSED**
892 If function-like macros do not utilize a parameter, it might result
893 in a build warning. We advocate for utilizing static inline functions
894 to replace such macros.
895 For example, for a macro such as the one below::
896
897 #define test(a) do { } while (0)
898
899 there would be a warning like below::
900
901 WARNING: Argument 'a' is not used in function-like macro.
902
903 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#macros-enums-and-rtl
904
905 **SINGLE_STATEMENT_DO_WHILE_MACRO**
906 For the multi-statement macros, it is necessary to use the do-while
907 loop to avoid unpredictable code paths. The do-while loop helps to
908 group the multiple statements into a single one so that a
909 function-like macro can be used as a function only.
910
911 But for the single statement macros, it is unnecessary to use the
912 do-while loop. Although the code is syntactically correct but using
913 the do-while loop is redundant. So remove the do-while loop for single
914 statement macros.
915
916 **WEAK_DECLARATION**
917 Using weak declarations like __attribute__((weak)) or __weak
918 can have unintended link defects. Avoid using them.
919
920
921Functions and Variables
922-----------------------
923
924 **CAMELCASE**
925 Avoid CamelCase Identifiers.
926
927 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#naming
928
929 **CONST_CONST**
930 Using `const <type> const *` is generally meant to be
931 written `const <type> * const`.
932
933 **CONST_STRUCT**
934 Using const is generally a good idea. Checkpatch reads
935 a list of frequently used structs that are always or
936 almost always constant.
937
938 The existing structs list can be viewed from
939 `scripts/const_structs.checkpatch`.
940
941 See: https://lore.kernel.org/lkml/alpine.DEB.2.10.1608281509480.3321@hadrien/
942
943 **EMBEDDED_FUNCTION_NAME**
944 Embedded function names are less appropriate to use as
945 refactoring can cause function renaming. Prefer the use of
946 "%s", __func__ to embedded function names.
947
948 Note that this does not work with -f (--file) checkpatch option
949 as it depends on patch context providing the function name.
950
951 **FUNCTION_ARGUMENTS**
952 This warning is emitted due to any of the following reasons:
953
954 1. Arguments for the function declaration do not follow
955 the identifier name. Example::
956
957 void foo
958 (int bar, int baz)
959
960 This should be corrected to::
961
962 void foo(int bar, int baz)
963
964 2. Some arguments for the function definition do not
965 have an identifier name. Example::
966
967 void foo(int)
968
969 All arguments should have identifier names.
970
971 **FUNCTION_WITHOUT_ARGS**
972 Function declarations without arguments like::
973
974 int foo()
975
976 should be::
977
978 int foo(void)
979
980 **GLOBAL_INITIALISERS**
981 Global variables should not be initialized explicitly to
982 0 (or NULL, false, etc.). Your compiler (or rather your
983 loader, which is responsible for zeroing out the relevant
984 sections) automatically does it for you.
985
986 **INITIALISED_STATIC**
987 Static variables should not be initialized explicitly to zero.
988 Your compiler (or rather your loader) automatically does
989 it for you.
990
991 **MULTIPLE_ASSIGNMENTS**
992 Multiple assignments on a single line makes the code unnecessarily
993 complicated. So on a single line assign value to a single variable
994 only, this makes the code more readable and helps avoid typos.
995
996 **RETURN_PARENTHESES**
997 return is not a function and as such doesn't need parentheses::
998
999 return (bar);
1000
1001 can simply be::
1002
1003 return bar;
1004
1005 **UNINITIALIZED_PTR_WITH_FREE**
1006 Pointers with __free attribute should be declared at the place of use
1007 and initialized (see include/linux/cleanup.h). In this case
1008 declarations at the top of the function rule can be relaxed. Not doing
1009 so may lead to undefined behavior as the memory assigned (garbage,
1010 in case not initialized) to the pointer is freed automatically when
1011 the pointer goes out of scope.
1012
1013 Also see: https://lore.kernel.org/lkml/58fd478f408a34b578ee8d949c5c4b4da4d4f41d.camel@HansenPartnership.com/
1014
1015 Example::
1016
1017 type var __free(free_func);
1018 ... // var not used, but, in future someone might add a return here
1019 var = malloc(var_size);
1020 ...
1021
1022 should be initialized as::
1023
1024 ...
1025 type var __free(free_func) = malloc(var_size);
1026 ...
1027
1028
1029Permissions
1030-----------
1031
1032 **DEVICE_ATTR_PERMS**
1033 The permissions used in DEVICE_ATTR are unusual.
1034 Typically only three permissions are used - 0644 (RW), 0444 (RO)
1035 and 0200 (WO).
1036
1037 See: https://www.kernel.org/doc/html/latest/filesystems/sysfs.html#attributes
1038
1039 **EXECUTE_PERMISSIONS**
1040 There is no reason for source files to be executable. The executable
1041 bit can be removed safely.
1042
1043 **EXPORTED_WORLD_WRITABLE**
1044 Exporting world writable sysfs/debugfs files is usually a bad thing.
1045 When done arbitrarily they can introduce serious security bugs.
1046 In the past, some of the debugfs vulnerabilities would seemingly allow
1047 any local user to write arbitrary values into device registers - a
1048 situation from which little good can be expected to emerge.
1049
1050 See: https://lore.kernel.org/linux-arm-kernel/cover.1296818921.git.segoon@openwall.com/
1051
1052 **NON_OCTAL_PERMISSIONS**
1053 Permission bits should use 4 digit octal permissions (like 0700 or 0444).
1054 Avoid using any other base like decimal.
1055
1056 **SYMBOLIC_PERMS**
1057 Permission bits in the octal form are more readable and easier to
1058 understand than their symbolic counterparts because many command-line
1059 tools use this notation. Experienced kernel developers have been using
1060 these traditional Unix permission bits for decades and so they find it
1061 easier to understand the octal notation than the symbolic macros.
1062 For example, it is harder to read S_IWUSR|S_IRUGO than 0644, which
1063 obscures the developer's intent rather than clarifying it.
1064
1065 See: https://lore.kernel.org/lkml/CA+55aFw5v23T-zvDZp-MmD_EYxF8WbafwwB59934FV7g21uMGQ@mail.gmail.com/
1066
1067
1068Spacing and Brackets
1069--------------------
1070
1071 **ASSIGNMENT_CONTINUATIONS**
1072 Assignment operators should not be written at the start of a
1073 line but should follow the operand at the previous line.
1074
1075 **BRACES**
1076 The placement of braces is stylistically incorrect.
1077 The preferred way is to put the opening brace last on the line,
1078 and put the closing brace first::
1079
1080 if (x is true) {
1081 we do y
1082 }
1083
1084 This applies for all non-functional blocks.
1085 However, there is one special case, namely functions: they have the
1086 opening brace at the beginning of the next line, thus::
1087
1088 int function(int x)
1089 {
1090 body of function
1091 }
1092
1093 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#placing-braces-and-spaces
1094
1095 **BRACKET_SPACE**
1096 Whitespace before opening bracket '[' is prohibited.
1097 There are some exceptions:
1098
1099 1. With a type on the left::
1100
1101 int [] a;
1102
1103 2. At the beginning of a line for slice initialisers::
1104
1105 [0...10] = 5,
1106
1107 3. Inside a curly brace::
1108
1109 = { [0...10] = 5 }
1110
1111 **CONCATENATED_STRING**
1112 Concatenated elements should have a space in between.
1113 Example::
1114
1115 printk(KERN_INFO"bar");
1116
1117 should be::
1118
1119 printk(KERN_INFO "bar");
1120
1121 **ELSE_AFTER_BRACE**
1122 `else {` should follow the closing block `}` on the same line.
1123
1124 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#placing-braces-and-spaces
1125
1126 **LINE_SPACING**
1127 Vertical space is wasted given the limited number of lines an
1128 editor window can display when multiple blank lines are used.
1129
1130 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#spaces
1131
1132 **OPEN_BRACE**
1133 The opening brace should be following the function definitions on the
1134 next line. For any non-functional block it should be on the same line
1135 as the last construct.
1136
1137 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#placing-braces-and-spaces
1138
1139 **POINTER_LOCATION**
1140 When using pointer data or a function that returns a pointer type,
1141 the preferred use of * is adjacent to the data name or function name
1142 and not adjacent to the type name.
1143 Examples::
1144
1145 char *linux_banner;
1146 unsigned long long memparse(char *ptr, char **retptr);
1147 char *match_strdup(substring_t *s);
1148
1149 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#spaces
1150
1151 **SPACING**
1152 Whitespace style used in the kernel sources is described in kernel docs.
1153
1154 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#spaces
1155
1156 **TRAILING_WHITESPACE**
1157 Trailing whitespace should always be removed.
1158 Some editors highlight the trailing whitespace and cause visual
1159 distractions when editing files.
1160
1161 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#spaces
1162
1163 **UNNECESSARY_PARENTHESES**
1164 Parentheses are not required in the following cases:
1165
1166 1. Function pointer uses::
1167
1168 (foo->bar)();
1169
1170 could be::
1171
1172 foo->bar();
1173
1174 2. Comparisons in if::
1175
1176 if ((foo->bar) && (foo->baz))
1177 if ((foo == bar))
1178
1179 could be::
1180
1181 if (foo->bar && foo->baz)
1182 if (foo == bar)
1183
1184 3. addressof/dereference single Lvalues::
1185
1186 &(foo->bar)
1187 *(foo->bar)
1188
1189 could be::
1190
1191 &foo->bar
1192 *foo->bar
1193
1194 **WHILE_AFTER_BRACE**
1195 while should follow the closing bracket on the same line::
1196
1197 do {
1198 ...
1199 } while(something);
1200
1201 See: https://www.kernel.org/doc/html/latest/process/coding-style.html#placing-braces-and-spaces
1202
1203
1204Others
1205------
1206
1207 **CONFIG_DESCRIPTION**
1208 Kconfig symbols should have a help text which fully describes
1209 it.
1210
1211 **CORRUPTED_PATCH**
1212 The patch seems to be corrupted or lines are wrapped.
1213 Please regenerate the patch file before sending it to the maintainer.
1214
1215 **CVS_KEYWORD**
1216 Since linux moved to git, the CVS markers are no longer used.
1217 So, CVS style keywords ($Id$, $Revision$, $Log$) should not be
1218 added.
1219
1220 **DEFAULT_NO_BREAK**
1221 switch default case is sometimes written as "default:;". This can
1222 cause new cases added below default to be defective.
1223
1224 A "break;" should be added after empty default statement to avoid
1225 unwanted fallthrough.
1226
1227 **DOS_LINE_ENDINGS**
1228 For DOS-formatted patches, there are extra ^M symbols at the end of
1229 the line. These should be removed.
1230
1231 **DT_SCHEMA_BINDING_PATCH**
1232 DT bindings moved to a json-schema based format instead of
1233 freeform text.
1234
1235 See: https://www.kernel.org/doc/html/latest/devicetree/bindings/writing-schema.html
1236
1237 **DT_SPLIT_BINDING_PATCH**
1238 Devicetree bindings should be their own patch. This is because
1239 bindings are logically independent from a driver implementation,
1240 they have a different maintainer (even though they often
1241 are applied via the same tree), and it makes for a cleaner history in the
1242 DT only tree created with git-filter-branch.
1243
1244 See: https://www.kernel.org/doc/html/latest/devicetree/bindings/submitting-patches.html#i-for-patch-submitters
1245
1246 **EMBEDDED_FILENAME**
1247 Embedding the complete filename path inside the file isn't particularly
1248 useful as often the path is moved around and becomes incorrect.
1249
1250 **FILE_PATH_CHANGES**
1251 Whenever files are added, moved, or deleted, the MAINTAINERS file
1252 patterns can be out of sync or outdated.
1253
1254 So MAINTAINERS might need updating in these cases.
1255
1256 **MEMSET**
1257 The memset use appears to be incorrect. This may be caused due to
1258 badly ordered parameters. Please recheck the usage.
1259
1260 **NOT_UNIFIED_DIFF**
1261 The patch file does not appear to be in unified-diff format. Please
1262 regenerate the patch file before sending it to the maintainer.
1263
1264 **PLACEHOLDER_USE**
1265 Detects unhandled placeholder text left in cover letters or commit headers/logs.
1266 Common placeholders include lines like::
1267
1268 *** SUBJECT HERE ***
1269 *** BLURB HERE ***
1270
1271 These typically come from autogenerated templates. Replace them with a proper
1272 subject and description before sending.
1273
1274 **PRINTF_0XDECIMAL**
1275 Prefixing 0x with decimal output is defective and should be corrected.
1276
1277 **SPDX_LICENSE_TAG**
1278 The source file is missing or has an improper SPDX identifier tag.
1279 The Linux kernel requires the precise SPDX identifier in all source files,
1280 and it is thoroughly documented in the kernel docs.
1281
1282 See: https://www.kernel.org/doc/html/latest/process/license-rules.html
1283
1284 **TYPO_SPELLING**
1285 Some words may have been misspelled. Consider reviewing them.