docs: checkpatch: add multiline, do/while, and multiple-assignment messages

Added and documented 3 new message types:
- MULTILINE_DEREFERENCE
- SINGLE_STATEMENT_DO_WHILE_MACRO
- MULTIPLE_ASSIGNMENTS

Signed-off-by: Utkarsh Verma <utkarshverma294@gmail.com>
Link: https://lore.kernel.org/r/20210925163852.12096-1-utkarshverma294@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

1 files changed, 43 insertions, 0 deletions

diff --git a/Documentation/dev-tools/checkpatch.rst b/Documentation/dev-tools/checkpatch.rst
index 4f89c363db08..5cbc8463b1f0 100644
--- a/Documentation/dev-tools/checkpatch.rst
+++ b/Documentation/dev-tools/checkpatch.rst
@@ -716,6 +716,33 @@ Indentation and Line Breaks
 
     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.
@@ -872,6 +899,17 @@ Macros, Attributes and Symbols
 
     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.
@@ -947,6 +985,11 @@ Functions and Variables
     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::