coding-style: Clarify the expectations around bool

There has been some confusion since checkpatch started warning about bool
use in structures, and people have been avoiding using it.

Many people feel there is still a legitimate place for bool in structures,
so provide some guidance on bool usage derived from the entire thread that
spawned the checkpatch warning.

Link: https://lkml.kernel.org/r/CA+55aFwVZk1OfB9T2v014PTAKFhtVan_Zj2dOjnCy3x6E4UJfA@mail.gmail.com
Signed-off-by: Joe Perches <joe@perches.com>
Acked-by: Joe Perches <joe@perches.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Joey Pabalinas <joeypabalinas@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

1 files changed, 34 insertions, 4 deletions

diff --git a/Documentation/process/coding-style.rst b/Documentation/process/coding-style.rst
index 4e611a01adc6..8ea913e99fa1 100644
--- a/Documentation/process/coding-style.rst
+++ b/Documentation/process/coding-style.rst
@@ -938,7 +938,37 @@ result.  Typical examples would be functions that return pointers; they use
 NULL or the ERR_PTR mechanism to report failure.
 
 
-17) Don't re-invent the kernel macros
+17) Using bool
+--------------
+
+The Linux kernel bool type is an alias for the C99 _Bool type. bool values can
+only evaluate to 0 or 1, and implicit or explicit conversion to bool
+automatically converts the value to true or false. When using bool types the
+!! construction is not needed, which eliminates a class of bugs.
+
+When working with bool values the true and false definitions should be used
+instead of 1 and 0.
+
+bool function return types and stack variables are always fine to use whenever
+appropriate. Use of bool is encouraged to improve readability and is often a
+better option than 'int' for storing boolean values.
+
+Do not use bool if cache line layout or size of the value matters, as its size
+and alignment varies based on the compiled architecture. Structures that are
+optimized for alignment and size should not use bool.
+
+If a structure has many true/false values, consider consolidating them into a
+bitfield with 1 bit members, or using an appropriate fixed width type, such as
+u8.
+
+Similarly for function arguments, many true/false values can be consolidated
+into a single bitwise 'flags' argument and 'flags' can often be a more
+readable alternative if the call-sites have naked true/false constants.
+
+Otherwise limited use of bool in structures and arguments can improve
+readability.
+
+18) Don't re-invent the kernel macros
 -------------------------------------
 
 The header file include/linux/kernel.h contains a number of macros that
@@ -961,7 +991,7 @@ need them.  Feel free to peruse that header file to see what else is already
 defined that you shouldn't reproduce in your code.
 
 
-18) Editor modelines and other cruft
+19) Editor modelines and other cruft
 ------------------------------------
 
 Some editors can interpret configuration information embedded in source files,
@@ -995,7 +1025,7 @@ own custom mode, or may have some other magic method for making indentation
 work correctly.
 
 
-19) Inline assembly
+20) Inline assembly
 -------------------
 
 In architecture-specific code, you may need to use inline assembly to interface
@@ -1027,7 +1057,7 @@ the next instruction in the assembly output:
 	     : /* outputs */ : /* inputs */ : /* clobbers */);
 
 
-20) Conditional Compilation
+21) Conditional Compilation
 ---------------------------
 
 Wherever possible, don't use preprocessor conditionals (#if, #ifdef) in .c