xref: /linux-master/Documentation/dev-tools/checkpatch.rst

.. SPDX-License-Identifier: GPL-2.0-only

==========
Checkpatch
==========

Checkpatch (scripts/checkpatch.pl) is a perl script which checks for trivial
style violations in patches and optionally corrects them.  Checkpatch can
also be run on file contexts and without the kernel tree.

Checkpatch is not always right. Your judgement takes precedence over checkpatch
messages.  If your code looks better with the violations, then its probably
best left alone.


Options
=======

This section will describe the options checkpatch can be run with.

Usage::

  ./scripts/checkpatch.pl [OPTION]... [FILE]...

Available options:

 - -q,  --quiet

   Enable quiet mode.

 - -v,  --verbose
   Enable verbose mode.  Additional verbose test descriptions are output
   so as to provide information on why that particular message is shown.

 - --no-tree

   Run checkpatch without the kernel tree.

 - --no-signoff

   Disable the 'Signed-off-by' line check.  The sign-off is a simple line at
   the end of the explanation for the patch, which certifies that you wrote it
   or otherwise have the right to pass it on as an open-source patch.

   Example::

	 Signed-off-by: Random J Developer <random@developer.example.org>

   Setting this flag effectively stops a message for a missing signed-off-by
   line in a patch context.

 - --patch

   Treat FILE as a patch.  This is the default option and need not be
   explicitly specified.

 - --emacs

   Set output to emacs compile window format.  This allows emacs users to jump
   from the error in the compile window directly to the offending line in the
   patch.

 - --terse

   Output only one line per report.

 - --showfile

   Show the diffed file position instead of the input file position.

 - -g,  --git

   Treat FILE as a single commit or a git revision range.

   Single commit with:

   - <rev>
   - <rev>^
   - <rev>~n

   Multiple commits with:

   - <rev1>..<rev2>
   - <rev1>...<rev2>
   - <rev>-<count>

 - -f,  --file

   Treat FILE as a regular source file.  This option must be used when running
   checkpatch on source files in the kernel.

 - --subjective,  --strict

   Enable stricter tests in checkpatch.  By default the tests emitted as CHECK
   do not activate by default.  Use this flag to activate the CHECK tests.

 - --list-types

   Every message emitted by checkpatch has an associated TYPE.  Add this flag
   to display all the types in checkpatch.

   Note that when this flag is active, checkpatch does not read the input FILE,
   and no message is emitted.  Only a list of types in checkpatch is output.

 - --types TYPE(,TYPE2...)

   Only display messages with the given types.

   Example::

     ./scripts/checkpatch.pl mypatch.patch --types EMAIL_SUBJECT,BRACES

 - --ignore TYPE(,TYPE2...)

   Checkpatch will not emit messages for the specified types.

   Example::

     ./scripts/checkpatch.pl mypatch.patch --ignore EMAIL_SUBJECT,BRACES

 - --show-types

   By default checkpatch doesn't display the type associated with the messages.
   Set this flag to show the message type in the output.

 - --max-line-length=n

   Set the max line length (default 100).  If a line exceeds the specified
   length, a LONG_LINE message is emitted.


   The message level is different for patch and file contexts.  For patches,
   a WARNING is emitted.  While a milder CHECK is emitted for files.  So for
   file contexts, the --strict flag must also be enabled.

 - --min-conf-desc-length=n

   Set the Kconfig entry minimum description length, if shorter, warn.

 - --tab-size=n

   Set the number of spaces for tab (default 8).

 - --root=PATH

   PATH to the kernel tree root.

   This option must be specified when invoking checkpatch from outside
   the kernel root.

 - --no-summary

   Suppress the per file summary.

 - --mailback

   Only produce a report in case of Warnings or Errors.  Milder Checks are
   excluded from this.

 - --summary-file

   Include the filename in summary.

 - --debug KEY=[0|1]

   Turn on/off debugging of KEY, where KEY is one of 'values', 'possible',
   'type', and 'attr' (default is all off).

 - --fix

   This is an EXPERIMENTAL feature.  If correctable errors exist, a file
   <inputfile>.EXPERIMENTAL-checkpatch-fixes is created which has the
   automatically fixable errors corrected.

 - --fix-inplace

   EXPERIMENTAL - Similar to --fix but input file is overwritten with fixes.

   DO NOT USE this flag unless you are absolutely sure and you have a backup
   in place.

 - --ignore-perl-version

   Override checking of perl version.  Runtime errors may be encountered after
   enabling this flag if the perl version does not meet the minimum specified.

 - --codespell

   Use the codespell dictionary for checking spelling errors.

 - --codespellfile

   Use the specified codespell file.
   Default is '/usr/share/codespell/dictionary.txt'.

 - --typedefsfile

   Read additional types from this file.

 - --color[=WHEN]

   Use colors 'always', 'never', or only when output is a terminal ('auto').
   Default is 'auto'.

 - --kconfig-prefix=WORD

   Use WORD as a prefix for Kconfig symbols (default is `CONFIG_`).

 - -h, --help, --version

   Display the help text.

Message Levels
==============

Messages in checkpatch are divided into three levels. The levels of messages
in checkpatch denote the severity of the error. They are:

 - ERROR

   This is the most strict level.  Messages of type ERROR must be taken
   seriously as they denote things that are very likely to be wrong.

 - WARNING

   This is the next stricter level.  Messages of type WARNING requires a
   more careful review.  But it is milder than an ERROR.

 - CHECK

   This is the mildest level.  These are things which may require some thought.

Type Descriptions
=================

This section contains a description of all the message types in checkpatch.

.. Types in this section are also parsed by checkpatch.
.. The types are grouped into subsections based on use.


Allocation style
----------------

  **ALLOC_ARRAY_ARGS**
    The first argument for kcalloc or kmalloc_array should be the
    number of elements.  sizeof() as the first argument is generally
    wrong.

    See: https://www.kernel.org/doc/html/latest/core-api/memory-allocation.html

  **ALLOC_SIZEOF_STRUCT**
    The allocation style is bad.  In general for family of
    allocation functions using sizeof() to get memory size,
    constructs like::

      p = alloc(sizeof(struct foo), ...)

    should be::

      p = alloc(sizeof(*p), ...)

    See: https://www.kernel.org/doc/html/latest/process/coding-style.html#allocating-memory

  **ALLOC_WITH_MULTIPLY**
    Prefer kmalloc_array/kcalloc over kmalloc/kzalloc with a
    sizeof multiply.

    See: https://www.kernel.org/doc/html/latest/core-api/memory-allocation.html


API usage
---------

  **ARCH_DEFINES**
    Architecture specific defines should be avoided wherever
    possible.

  **ARCH_INCLUDE_LINUX**
    Whenever asm/file.h is included and linux/file.h exists, a
    conversion can be made when linux/file.h includes asm/file.h.
    However this is not always the case (See signal.h).
    This message type is emitted only for includes from arch/.

  **AVOID_BUG**
    BUG() or BUG_ON() should be avoided totally.
    Use WARN() and WARN_ON() instead, and handle the "impossible"
    error condition as gracefully as possible.

    See: https://www.kernel.org/doc/html/latest/process/deprecated.html#bug-and-bug-on

  **CONSIDER_KSTRTO**
    The simple_strtol(), simple_strtoll(), simple_strtoul(), and
    simple_strtoull() functions explicitly ignore overflows, which
    may lead to unexpected results in callers.  The respective kstrtol(),
    kstrtoll(), kstrtoul(), and kstrtoull() functions tend to be the
    correct replacements.

    See: https://www.kernel.org/doc/html/latest/process/deprecated.html#simple-strtol-simple-strtoll-simple-strtoul-simple-strtoull

  **CONSTANT_CONVERSION**
    Use of __constant_<foo> form is discouraged for the following functions::

      __constant_cpu_to_be[x]
      __constant_cpu_to_le[x]
      __constant_be[x]_to_cpu
      __constant_le[x]_to_cpu
      __constant_htons
      __constant_ntohs

    Using any of these outside of include/uapi/ is not preferred as using the
    function without __constant_ is identical when the argument is a
    constant.

    In big endian systems, the macros like __constant_cpu_to_be32(x) and
    cpu_to_be32(x) expand to the same expression::

      #define __constant_cpu_to_be32(x) ((__force __be32)(__u32)(x))
      #define __cpu_to_be32(x)          ((__force __be32)(__u32)(x))

    In little endian systems, the macros __constant_cpu_to_be32(x) and
    cpu_to_be32(x) expand to __constant_swab32 and __swab32.  __swab32
    has a __builtin_constant_p check::

      #define __swab32(x)				\
        (__builtin_constant_p((__u32)(x)) ?	\
        ___constant_swab32(x) :			\
        __fswab32(x))

    So ultimately they have a special case for constants.
    Similar is the case with all of the macros in the list.  Thus
    using the __constant_... forms are unnecessarily verbose and
    not preferred outside of include/uapi.

    See: https://lore.kernel.org/lkml/1400106425.12666.6.camel@joe-AO725/

  **DEPRECATED_API**
    Usage of a deprecated RCU API is detected.  It is recommended to replace
    old flavourful RCU APIs by their new vanilla-RCU counterparts.

    The full list of available RCU APIs can be viewed from the kernel docs.

    See: https://www.kernel.org/doc/html/latest/RCU/whatisRCU.html#full-list-of-rcu-apis

  **DEPRECATED_VARIABLE**
    EXTRA_{A,C,CPP,LD}FLAGS are deprecated and should be replaced by the new
    flags added via commit f77bf01425b1 ("kbuild: introduce ccflags-y,
    asflags-y and ldflags-y").

    The following conversion scheme maybe used::

      EXTRA_AFLAGS    ->  asflags-y
      EXTRA_CFLAGS    ->  ccflags-y
      EXTRA_CPPFLAGS  ->  cppflags-y
      EXTRA_LDFLAGS   ->  ldflags-y

    See:

      1. https://lore.kernel.org/lkml/20070930191054.GA15876@uranus.ravnborg.org/
      2. https://lore.kernel.org/lkml/1313384834-24433-12-git-send-email-lacombar@gmail.com/
      3. https://www.kernel.org/doc/html/latest/kbuild/makefiles.html#compilation-flags

  **DEVICE_ATTR_FUNCTIONS**
    The function names used in DEVICE_ATTR is unusual.
    Typically, the store and show functions are used with <attr>_store and
    <attr>_show, where <attr> is a named attribute variable of the device.

    Consider the following examples::

      static DEVICE_ATTR(type, 0444, type_show, NULL);
      static DEVICE_ATTR(power, 0644, power_show, power_store);

    The function names should preferably follow the above pattern.

    See: https://www.kernel.org/doc/html/latest/driver-api/driver-model/device.html#attributes

  **DEVICE_ATTR_RO**
    The DEVICE_ATTR_RO(name) helper macro can be used instead of
    DEVICE_ATTR(name, 0444, name_show, NULL);

    Note that the macro automatically appends _show to the named
    attribute variable of the device for the show method.

    See: https://www.kernel.org/doc/html/latest/driver-api/driver-model/device.html#attributes

  **DEVICE_ATTR_RW**
    The DEVICE_ATTR_RW(name) helper macro can be used instead of
    DEVICE_ATTR(name, 0644, name_show, name_store);

    Note that the macro automatically appends _show and _store to the
    named attribute variable of the device for the show and store methods.

    See: https://www.kernel.org/doc/html/latest/driver-api/driver-model/device.html#attributes

  **DEVICE_ATTR_WO**
    The DEVICE_AATR_WO(name) helper macro can be used instead of
    DEVICE_ATTR(name, 0200, NULL, name_store);

    Note that the macro automatically appends _store to the
    named attribute variable of the device for the store method.

    See: https://www.kernel.org/doc/html/latest/driver-api/driver-model/device.html#attributes

  **DUPLICATED_SYSCTL_CONST**
    Commit d91bff3011cf ("proc/sysctl: add shared variables for range
    check") added some shared const variables to be used instead of a local
    copy in each source file.

    Consider replacing the sysctl range checking value with the shared
    one in include/linux/sysctl.h.  The following conversion scheme may
    be used::

      &zero     ->  SYSCTL_ZERO
      &one      ->  SYSCTL_ONE
      &int_max  ->  SYSCTL_INT_MAX

    See:

      1. https://lore.kernel.org/lkml/20190430180111.10688-1-mcroce@redhat.com/
      2. https://lore.kernel.org/lkml/20190531131422.14970-1-mcroce@redhat.com/

  **ENOSYS**
    ENOSYS means that a nonexistent system call was called.
    Earlier, it was wrongly used for things like invalid operations on
    otherwise valid syscalls.  This should be avoided in new code.

    See: https://lore.kernel.org/lkml/5eb299021dec23c1a48fa7d9f2c8b794e967766d.1408730669.git.luto@amacapital.net/

  **ENOTSUPP**
    ENOTSUPP is not a standard error code and should be avoided in new patches.
    EOPNOTSUPP should be used instead.

    See: https://lore.kernel.org/netdev/20200510182252.GA411829@lunn.ch/

  **EXPORT_SYMBOL**
    EXPORT_SYMBOL should immediately follow the symbol to be exported.

  **IN_ATOMIC**
    in_atomic() is not for driver use so any such use is reported as an ERROR.
    Also in_atomic() is often used to determine if sleeping is permitted,
    but it is not reliable in this use model.  Therefore its use is
    strongly discouraged.

    However, in_atomic() is ok for core kernel use.

    See: https://lore.kernel.org/lkml/20080320201723.b87b3732.akpm@linux-foundation.org/

  **LOCKDEP**
    The lockdep_no_validate class was added as a temporary measure to
    prevent warnings on conversion of device->sem to device->mutex.
    It should not be used for any other purpose.

    See: https://lore.kernel.org/lkml/1268959062.9440.467.camel@laptop/

  **MALFORMED_INCLUDE**
    The #include statement has a malformed path.  This has happened
    because the author has included a double slash "//" in the pathname
    accidentally.

  **USE_LOCKDEP**
    lockdep_assert_held() annotations should be preferred over
    assertions based on spin_is_locked()

    See: https://www.kernel.org/doc/html/latest/locking/lockdep-design.html#annotations

  **UAPI_INCLUDE**
    No #include statements in include/uapi should use a uapi/ path.

  **USLEEP_RANGE**
    usleep_range() should be preferred over udelay(). The proper way of
    using usleep_range() is mentioned in the kernel docs.

    See: https://www.kernel.org/doc/html/latest/timers/timers-howto.html#delays-information-on-the-various-kernel-delay-sleep-mechanisms


Comments
--------

  **BLOCK_COMMENT_STYLE**
    The comment style is incorrect.  The preferred style for multi-
    line comments is::

      /*
      * This is the preferred style
      * for multi line comments.
      */

    The networking comment style is a bit different, with the first line
    not empty like the former::

      /* This is the preferred comment style
      * for files in net/ and drivers/net/
      */

    See: https://www.kernel.org/doc/html/latest/process/coding-style.html#commenting

  **C99_COMMENTS**
    C99 style single line comments (//) should not be used.
    Prefer the block comment style instead.

    See: https://www.kernel.org/doc/html/latest/process/coding-style.html#commenting

  **DATA_RACE**
    Applications of data_race() should have a comment so as to document the
    reasoning behind why it was deemed safe.

    See: https://lore.kernel.org/lkml/20200401101714.44781-1-elver@google.com/

  **FSF_MAILING_ADDRESS**
    Kernel maintainers reject new instances of the GPL boilerplate paragraph
    directing people to write to the FSF for a copy of the GPL, since the
    FSF has moved in the past and may do so again.
    So do not write paragraphs about writing to the Free Software Foundation's
    mailing address.

    See: https://lore.kernel.org/lkml/20131006222342.GT19510@leaf/


Commit message
--------------

  **BAD_SIGN_OFF**
    The signed-off-by line does not fall in line with the standards
    specified by the community.

    See: https://www.kernel.org/doc/html/latest/process/submitting-patches.html#developer-s-certificate-of-origin-1-1

  **BAD_STABLE_ADDRESS_STYLE**
    The email format for stable is incorrect.
    Some valid options for stable address are::

      1. stable@vger.kernel.org
      2. stable@kernel.org

    For adding version info, the following comment style should be used::

      stable@vger.kernel.org # version info

  **COMMIT_COMMENT_SYMBOL**
    Commit log lines starting with a '#' are ignored by git as
    comments.  To solve this problem addition of a single space
    infront of the log line is enough.

  **COMMIT_MESSAGE**
    The patch is missing a commit description.  A brief
    description of the changes made by the patch should be added.

    See: https://www.kernel.org/doc/html/latest/process/submitting-patches.html#describe-your-changes

  **EMAIL_SUBJECT**
    Naming the tool that found the issue is not very useful in the
    subject line.  A good subject line summarizes the change that
    the patch brings.

    See: https://www.kernel.org/doc/html/latest/process/submitting-patches.html#describe-your-changes

  **FROM_SIGN_OFF_MISMATCH**
    The author's email does not match with that in the Signed-off-by:
    line(s). This can be sometimes caused due to an improperly configured
    email client.

    This message is emitted due to any of the following reasons::

      - The email names do not match.
      - The email addresses do not match.
      - The email subaddresses do not match.
      - The email comments do not match.

  **MISSING_SIGN_OFF**
    The patch is missing a Signed-off-by line.  A signed-off-by
    line should be added according to Developer's certificate of
    Origin.

    See: https://www.kernel.org/doc/html/latest/process/submitting-patches.html#sign-your-work-the-developer-s-certificate-of-origin

  **NO_AUTHOR_SIGN_OFF**
    The author of the patch has not signed off the patch.  It is
    required that a simple sign off line should be present at the
    end of explanation of the patch to denote that the author has
    written it or otherwise has the rights to pass it on as an open
    source patch.

    See: https://www.kernel.org/doc/html/latest/process/submitting-patches.html#sign-your-work-the-developer-s-certificate-of-origin

  **DIFF_IN_COMMIT_MSG**
    Avoid having diff content in commit message.
    This causes problems when one tries to apply a file containing both
    the changelog and the diff because patch(1) tries to apply the diff
    which it found in the changelog.

    See: https://lore.kernel.org/lkml/20150611134006.9df79a893e3636019ad2759e@linux-foundation.org/

  **GERRIT_CHANGE_ID**
    To be picked up by gerrit, the footer of the commit message might
    have a Change-Id like::

      Change-Id: Ic8aaa0728a43936cd4c6e1ed590e01ba8f0fbf5b
      Signed-off-by: A. U. Thor <author@example.com>

    The Change-Id line must be removed before submitting.

  **GIT_COMMIT_ID**
    The proper way to reference a commit id is:
    commit <12+ chars of sha1> ("<title line>")

    An example may be::

      Commit e21d2170f36602ae2708 ("video: remove unnecessary
      platform_set_drvdata()") removed the unnecessary
      platform_set_drvdata(), but left the variable "dev" unused,
      delete it.

    See: https://www.kernel.org/doc/html/latest/process/submitting-patches.html#describe-your-changes

  **BAD_FIXES_TAG**
    The Fixes: tag is malformed or does not follow the community conventions.
    This can occur if the tag have been split into multiple lines (e.g., when
    pasted in an email program with word wrapping enabled).

    See: https://www.kernel.org/doc/html/latest/process/submitting-patches.html#describe-your-changes


Comparison style
----------------

  **ASSIGN_IN_IF**
    Do not use assignments in if condition.
    Example::

      if ((foo = bar(...)) < BAZ) {

    should be written as::

      foo = bar(...);
      if (foo < BAZ) {

  **BOOL_COMPARISON**
    Comparisons of A to true and false are better written
    as A and !A.

    See: https://lore.kernel.org/lkml/1365563834.27174.12.camel@joe-AO722/

  **COMPARISON_TO_NULL**
    Comparisons to NULL in the form (foo == NULL) or (foo != NULL)
    are better written as (!foo) and (foo).

  **CONSTANT_COMPARISON**
    Comparisons with a constant or upper case identifier on the left
    side of the test should be avoided.


Indentation and Line Breaks
---------------------------

  **CODE_INDENT**
    Code indent should use tabs instead of spaces.
    Outside of comments, documentation and Kconfig,
    spaces are never used for indentation.

    See: https://www.kernel.org/doc/html/latest/process/coding-style.html#indentation

  **DEEP_INDENTATION**
    Indentation with 6 or more tabs usually indicate overly indented
    code.

    It is suggested to refactor excessive indentation of
    if/else/for/do/while/switch statements.

    See: https://lore.kernel.org/lkml/1328311239.21255.24.camel@joe2Laptop/

  **SWITCH_CASE_INDENT_LEVEL**
    switch should be at the same indent as case.
    Example::

      switch (suffix) {
      case 'G':
      case 'g':
              mem <<= 30;
              break;
      case 'M':
      case 'm':
              mem <<= 20;
              break;
      case 'K':
      case 'k':
              mem <<= 10;
              fallthrough;
      default:
              break;
      }

    See: https://www.kernel.org/doc/html/latest/process/coding-style.html#indentation

  **LONG_LINE**
    The line has exceeded the specified maximum length.
    To use a different maximum line length, the --max-line-length=n option
    may be added while invoking checkpatch.

    Earlier, the default line length was 80 columns.  Commit bdc48fa11e46
    ("checkpatch/coding-style: deprecate 80-column warning") increased the
    limit to 100 columns.  This is not a hard limit either and it's
    preferable to stay within 80 columns whenever possible.

    See: https://www.kernel.org/doc/html/latest/process/coding-style.html#breaking-long-lines-and-strings

  **LONG_LINE_STRING**
    A string starts before but extends beyond the maximum line length.
    To use a different maximum line length, the --max-line-length=n option
    may be added while invoking checkpatch.

    See: https://www.kernel.org/doc/html/latest/process/coding-style.html#breaking-long-lines-and-strings

  **LONG_LINE_COMMENT**
    A comment starts before but extends beyond the maximum line length.
    To use a different maximum line length, the --max-line-length=n option
    may be added while invoking checkpatch.

    See: https://www.kernel.org/doc/html/latest/process/coding-style.html#breaking-long-lines-and-strings

  **SPLIT_STRING**
    Quoted strings that appear as messages in userspace and can be
    grepped, should not be split across multiple lines.

    See: https://lore.kernel.org/lkml/20120203052727.GA15035@leaf/

  **MULTILINE_DEREFERENCE**
    A single dereferencing identifier spanned on multiple lines like::

      struct_identifier->member[index].
      member = <foo>;

    is generally hard to follow. It can easily lead to typos and so makes
    the code vulnerable to bugs.

    If fixing the multiple line dereferencing leads to an 80 column
    violation, then either rewrite the code in a more simple way or if the
    starting part of the dereferencing identifier is the same and used at
    multiple places then store it in a temporary variable, and use that
    temporary variable only at all the places. For example, if there are
    two dereferencing identifiers::

      member1->member2->member3.foo1;
      member1->member2->member3.foo2;

    then store the member1->member2->member3 part in a temporary variable.
    It not only helps to avoid the 80 column violation but also reduces
    the program size by removing the unnecessary dereferences.

    But if none of the above methods work then ignore the 80 column
    violation because it is much easier to read a dereferencing identifier
    on a single line.

  **TRAILING_STATEMENTS**
    Trailing statements (for example after any conditional) should be
    on the next line.
    Statements, such as::

      if (x == y) break;

    should be::

      if (x == y)
              break;


Macros, Attributes and Symbols
------------------------------

  **ARRAY_SIZE**
    The ARRAY_SIZE(foo) macro should be preferred over
    sizeof(foo)/sizeof(foo[0]) for finding number of elements in an
    array.

    The macro is defined in include/linux/kernel.h::

      #define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))

  **AVOID_EXTERNS**
    Function prototypes don't need to be declared extern in .h
    files.  It's assumed by the compiler and is unnecessary.

  **AVOID_L_PREFIX**
    Local symbol names that are prefixed with `.L` should be avoided,
    as this has special meaning for the assembler; a symbol entry will
    not be emitted into the symbol table.  This can prevent `objtool`
    from generating correct unwind info.

    Symbols with STB_LOCAL binding may still be used, and `.L` prefixed
    local symbol names are still generally usable within a function,
    but `.L` prefixed local symbol names should not be used to denote
    the beginning or end of code regions via
    `SYM_CODE_START_LOCAL`/`SYM_CODE_END`

  **BIT_MACRO**
    Defines like: 1 << <digit> could be BIT(digit).
    The BIT() macro is defined via include/linux/bits.h::

      #define BIT(nr)         (1UL << (nr))

  **CONST_READ_MOSTLY**
    When a variable is tagged with the __read_mostly annotation, it is a
    signal to the compiler that accesses to the variable will be mostly
    reads and rarely(but NOT never) a write.

    const __read_mostly does not make any sense as const data is already
    read-only.  The __read_mostly annotation thus should be removed.

  **DATE_TIME**
    It is generally desirable that building the same source code with
    the same set of tools is reproducible, i.e. the output is always
    exactly the same.

    The kernel does *not* use the ``__DATE__`` and ``__TIME__`` macros,
    and enables warnings if they are used as they can lead to
    non-deterministic builds.

    See: https://www.kernel.org/doc/html/latest/kbuild/reproducible-builds.html#timestamps

  **DEFINE_ARCH_HAS**
    The ARCH_HAS_xyz and ARCH_HAVE_xyz patterns are wrong.

    For big conceptual features use Kconfig symbols instead.  And for
    smaller things where we have compatibility fallback functions but
    want architectures able to override them with optimized ones, we
    should either use weak functions (appropriate for some cases), or
    the symbol that protects them should be the same symbol we use.

    See: https://lore.kernel.org/lkml/CA+55aFycQ9XJvEOsiM3txHL5bjUc8CeKWJNR_H+MiicaddB42Q@mail.gmail.com/

  **DO_WHILE_MACRO_WITH_TRAILING_SEMICOLON**
    do {} while(0) macros should not have a trailing semicolon.

  **INIT_ATTRIBUTE**
    Const init definitions should use __initconst instead of
    __initdata.

    Similarly init definitions without const require a separate
    use of const.

  **INLINE_LOCATION**
    The inline keyword should sit between storage class and type.

    For example, the following segment::

      inline static int example_function(void)
      {
              ...
      }

    should be::

      static inline int example_function(void)
      {
              ...
      }

  **MISPLACED_INIT**
    It is possible to use section markers on variables in a way
    which gcc doesn't understand (or at least not the way the
    developer intended)::

      static struct __initdata samsung_pll_clock exynos4_plls[nr_plls] = {

    does not put exynos4_plls in the .initdata section. The __initdata
    marker can be virtually anywhere on the line, except right after
    "struct". The preferred location is before the "=" sign if there is
    one, or before the trailing ";" otherwise.

    See: https://lore.kernel.org/lkml/1377655732.3619.19.camel@joe-AO722/

  **MULTISTATEMENT_MACRO_USE_DO_WHILE**
    Macros with multiple statements should be enclosed in a
    do - while block.  Same should also be the case for macros
    starting with `if` to avoid logic defects::

      #define macrofun(a, b, c)                 \
        do {                                    \
                if (a == 5)                     \
                        do_this(b, c);          \
        } while (0)

    See: https://www.kernel.org/doc/html/latest/process/coding-style.html#macros-enums-and-rtl

  **PREFER_FALLTHROUGH**
    Use the `fallthrough;` pseudo keyword instead of
    `/* fallthrough */` like comments.

  **TRAILING_SEMICOLON**
    Macro definition should not end with a semicolon. The macro
    invocation style should be consistent with function calls.
    This can prevent any unexpected code paths::

      #define MAC do_something;

    If this macro is used within a if else statement, like::

      if (some_condition)
              MAC;

      else
              do_something;

    Then there would be a compilation error, because when the macro is
    expanded there are two trailing semicolons, so the else branch gets
    orphaned.

    See: https://lore.kernel.org/lkml/1399671106.2912.21.camel@joe-AO725/

  **SINGLE_STATEMENT_DO_WHILE_MACRO**
    For the multi-statement macros, it is necessary to use the do-while
    loop to avoid unpredictable code paths. The do-while loop helps to
    group the multiple statements into a single one so that a
    function-like macro can be used as a function only.

    But for the single statement macros, it is unnecessary to use the
    do-while loop. Although the code is syntactically correct but using
    the do-while loop is redundant. So remove the do-while loop for single
    statement macros.

  **WEAK_DECLARATION**
    Using weak declarations like __attribute__((weak)) or __weak
    can have unintended link defects.  Avoid using them.


Functions and Variables
-----------------------

  **CAMELCASE**
    Avoid CamelCase Identifiers.

    See: https://www.kernel.org/doc/html/latest/process/coding-style.html#naming

  **CONST_CONST**
    Using `const <type> const *` is generally meant to be
    written `const <type> * const`.

  **CONST_STRUCT**
    Using const is generally a good idea.  Checkpatch reads
    a list of frequently used structs that are always or
    almost always constant.

    The existing structs list can be viewed from
    `scripts/const_structs.checkpatch`.

    See: https://lore.kernel.org/lkml/alpine.DEB.2.10.1608281509480.3321@hadrien/

  **EMBEDDED_FUNCTION_NAME**
    Embedded function names are less appropriate to use as
    refactoring can cause function renaming.  Prefer the use of
    "%s", __func__ to embedded function names.

    Note that this does not work with -f (--file) checkpatch option
    as it depends on patch context providing the function name.

  **FUNCTION_ARGUMENTS**
    This warning is emitted due to any of the following reasons:

      1. Arguments for the function declaration do not follow
         the identifier name.  Example::

           void foo
           (int bar, int baz)

         This should be corrected to::

           void foo(int bar, int baz)

      2. Some arguments for the function definition do not
         have an identifier name.  Example::

           void foo(int)

         All arguments should have identifier names.

  **FUNCTION_WITHOUT_ARGS**
    Function declarations without arguments like::

      int foo()

    should be::

      int foo(void)

  **GLOBAL_INITIALISERS**
    Global variables should not be initialized explicitly to
    0 (or NULL, false, etc.).  Your compiler (or rather your
    loader, which is responsible for zeroing out the relevant
    sections) automatically does it for you.

  **INITIALISED_STATIC**
    Static variables should not be initialized explicitly to zero.
    Your compiler (or rather your loader) automatically does
    it for you.

  **MULTIPLE_ASSIGNMENTS**
    Multiple assignments on a single line makes the code unnecessarily
    complicated. So on a single line assign value to a single variable
    only, this makes the code more readable and helps avoid typos.

  **RETURN_PARENTHESES**
    return is not a function and as such doesn't need parentheses::

      return (bar);

    can simply be::

      return bar;


Permissions
-----------

  **DEVICE_ATTR_PERMS**
    The permissions used in DEVICE_ATTR are unusual.
    Typically only three permissions are used - 0644 (RW), 0444 (RO)
    and 0200 (WO).

    See: https://www.kernel.org/doc/html/latest/filesystems/sysfs.html#attributes

  **EXECUTE_PERMISSIONS**
    There is no reason for source files to be executable.  The executable
    bit can be removed safely.

  **EXPORTED_WORLD_WRITABLE**
    Exporting world writable sysfs/debugfs files is usually a bad thing.
    When done arbitrarily they can introduce serious security bugs.
    In the past, some of the debugfs vulnerabilities would seemingly allow
    any local user to write arbitrary values into device registers - a
    situation from which little good can be expected to emerge.

    See: https://lore.kernel.org/linux-arm-kernel/cover.1296818921.git.segoon@openwall.com/

  **NON_OCTAL_PERMISSIONS**
    Permission bits should use 4 digit octal permissions (like 0700 or 0444).
    Avoid using any other base like decimal.

  **SYMBOLIC_PERMS**
    Permission bits in the octal form are more readable and easier to
    understand than their symbolic counterparts because many command-line
    tools use this notation. Experienced kernel developers have been using
    these traditional Unix permission bits for decades and so they find it
    easier to understand the octal notation than the symbolic macros.
    For example, it is harder to read S_IWUSR|S_IRUGO than 0644, which
    obscures the developer's intent rather than clarifying it.

    See: https://lore.kernel.org/lkml/CA+55aFw5v23T-zvDZp-MmD_EYxF8WbafwwB59934FV7g21uMGQ@mail.gmail.com/


Spacing and Brackets
--------------------

  **ASSIGNMENT_CONTINUATIONS**
    Assignment operators should not be written at the start of a
    line but should follow the operand at the previous line.

  **BRACES**
    The placement of braces is stylistically incorrect.
    The preferred way is to put the opening brace last on the line,
    and put the closing brace first::

      if (x is true) {
              we do y
      }

    This applies for all non-functional blocks.
    However, there is one special case, namely functions: they have the
    opening brace at the beginning of the next line, thus::

      int function(int x)
      {
              body of function
      }

    See: https://www.kernel.org/doc/html/latest/process/coding-style.html#placing-braces-and-spaces

  **BRACKET_SPACE**
    Whitespace before opening bracket '[' is prohibited.
    There are some exceptions:

    1. With a type on the left::

        int [] a;

    2. At the beginning of a line for slice initialisers::

        [0...10] = 5,

    3. Inside a curly brace::

        = { [0...10] = 5 }

  **CONCATENATED_STRING**
    Concatenated elements should have a space in between.
    Example::

      printk(KERN_INFO"bar");

    should be::

      printk(KERN_INFO "bar");

  **ELSE_AFTER_BRACE**
    `else {` should follow the closing block `}` on the same line.

    See: https://www.kernel.org/doc/html/latest/process/coding-style.html#placing-braces-and-spaces

  **LINE_SPACING**
    Vertical space is wasted given the limited number of lines an
    editor window can display when multiple blank lines are used.

    See: https://www.kernel.org/doc/html/latest/process/coding-style.html#spaces

  **OPEN_BRACE**
    The opening brace should be following the function definitions on the
    next line.  For any non-functional block it should be on the same line
    as the last construct.

    See: https://www.kernel.org/doc/html/latest/process/coding-style.html#placing-braces-and-spaces

  **POINTER_LOCATION**
    When using pointer data or a function that returns a pointer type,
    the preferred use of * is adjacent to the data name or function name
    and not adjacent to the type name.
    Examples::

      char *linux_banner;
      unsigned long long memparse(char *ptr, char **retptr);
      char *match_strdup(substring_t *s);

    See: https://www.kernel.org/doc/html/latest/process/coding-style.html#spaces

  **SPACING**
    Whitespace style used in the kernel sources is described in kernel docs.

    See: https://www.kernel.org/doc/html/latest/process/coding-style.html#spaces

  **TRAILING_WHITESPACE**
    Trailing whitespace should always be removed.
    Some editors highlight the trailing whitespace and cause visual
    distractions when editing files.

    See: https://www.kernel.org/doc/html/latest/process/coding-style.html#spaces

  **UNNECESSARY_PARENTHESES**
    Parentheses are not required in the following cases:

      1. Function pointer uses::

          (foo->bar)();

        could be::

          foo->bar();

      2. Comparisons in if::

          if ((foo->bar) && (foo->baz))
          if ((foo == bar))

        could be::

          if (foo->bar && foo->baz)
          if (foo == bar)

      3. addressof/dereference single Lvalues::

          &(foo->bar)
          *(foo->bar)

        could be::

          &foo->bar
          *foo->bar

  **WHILE_AFTER_BRACE**
    while should follow the closing bracket on the same line::

      do {
              ...
      } while(something);

    See: https://www.kernel.org/doc/html/latest/process/coding-style.html#placing-braces-and-spaces


Others
------

  **CONFIG_DESCRIPTION**
    Kconfig symbols should have a help text which fully describes
    it.

  **CORRUPTED_PATCH**
    The patch seems to be corrupted or lines are wrapped.
    Please regenerate the patch file before sending it to the maintainer.

  **CVS_KEYWORD**
    Since linux moved to git, the CVS markers are no longer used.
    So, CVS style keywords ($Id$, $Revision$, $Log$) should not be
    added.

  **DEFAULT_NO_BREAK**
    switch default case is sometimes written as "default:;".  This can
    cause new cases added below default to be defective.

    A "break;" should be added after empty default statement to avoid
    unwanted fallthrough.

  **DOS_LINE_ENDINGS**
    For DOS-formatted patches, there are extra ^M symbols at the end of
    the line.  These should be removed.

  **DT_SCHEMA_BINDING_PATCH**
    DT bindings moved to a json-schema based format instead of
    freeform text.

    See: https://www.kernel.org/doc/html/latest/devicetree/bindings/writing-schema.html

  **DT_SPLIT_BINDING_PATCH**
    Devicetree bindings should be their own patch.  This is because
    bindings are logically independent from a driver implementation,
    they have a different maintainer (even though they often
    are applied via the same tree), and it makes for a cleaner history in the
    DT only tree created with git-filter-branch.

    See: https://www.kernel.org/doc/html/latest/devicetree/bindings/submitting-patches.html#i-for-patch-submitters

  **EMBEDDED_FILENAME**
    Embedding the complete filename path inside the file isn't particularly
    useful as often the path is moved around and becomes incorrect.

  **FILE_PATH_CHANGES**
    Whenever files are added, moved, or deleted, the MAINTAINERS file
    patterns can be out of sync or outdated.

    So MAINTAINERS might need updating in these cases.

  **MEMSET**
    The memset use appears to be incorrect.  This may be caused due to
    badly ordered parameters.  Please recheck the usage.

  **NOT_UNIFIED_DIFF**
    The patch file does not appear to be in unified-diff format.  Please
    regenerate the patch file before sending it to the maintainer.

  **PRINTF_0XDECIMAL**
    Prefixing 0x with decimal output is defective and should be corrected.

  **SPDX_LICENSE_TAG**
    The source file is missing or has an improper SPDX identifier tag.
    The Linux kernel requires the precise SPDX identifier in all source files,
    and it is thoroughly documented in the kernel docs.

    See: https://www.kernel.org/doc/html/latest/process/license-rules.html

  **TYPO_SPELLING**
    Some words may have been misspelled.  Consider reviewing them.