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