docs: add documentation for checkpatch

Add documentation for kernel script checkpatch.pl.
This documentation is also parsed by checkpatch to
enable a verbose mode.

The checkpatch message types are grouped by usage. Under
each group the types are described briefly. 34 of such
types are documented.
Signed-off-by: NDwaipayan Ray <dwaipayanray1@gmail.com>
Link: https://lore.kernel.org/r/20210226093827.12700-2-dwaipayanray1@gmail.comSigned-off-by: NJonathan Corbet <corbet@lwn.net>

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 exists, 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 maybe 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/.
+
+  **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_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
+
+  **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 in include/linux/bitops.h::
+
+      #define BIT(nr)         (1UL << (nr))
+
+  **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
+
+
+Comment style
+-------------
+
+  **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
+
+
+
+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
+
+  **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
+
+
+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.
+
+
+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 }
+
+  **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
+
+  **CONCATENATED_STRING**
+    Concatenated elements should have a space in between.
+    Example::
+
+      printk(KERN_INFO"bar");
+
+    should be::
+
+      printk(KERN_INFO "bar");
+
+  **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
+
+  **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
+
+
+Others
+------
+
+  **CAMELCASE**
+    Avoid CamelCase Identifiers.
+    See: https://www.kernel.org/doc/html/latest/process/coding-style.html#naming
+
+  **CONFIG_DESCRIPTION**
+    Kconfig symbols should have a help text which fully describes
+    it.

Documentation/dev-tools/index.rst

@@ -14,6 +14,7 @@ whole; patches welcome!
 .. toctree::
    :maxdepth: 2
 
+   checkpatch
    coccinelle
    sparse
    kcov