[PATCH v3] coding-style: Clarify the expectations around bool

[Jason Gunthorpe <jgg@ziepe.ca>]

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>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
---
 Documentation/process/coding-style.rst | 38 +++++++++++++++++++++++---
 1 file changed, 34 insertions(+), 4 deletions(-)

There hasn't been any negative feedback on the first thread that
spawned this discussion, so here is a cleanuped up patch, ready to be
applied. Acks?

v2:
 - Revise Joe's original suggestion with background and more details
v3:
 - s/C11/C99/ [Al Viro]
 - Add some remarks on the use of bool in function arguments [GalP]

I'm not sure what tree this should go through - Jonathan would you
pick it up?

Thanks,
Jason

diff --git a/Documentation/process/coding-style.rst b/Documentation/process/coding-style.rst
index b78dd680c03809..cbe6b01b05fa66 100644
--- a/Documentation/process/coding-style.rst
+++ b/Documentation/process/coding-style.rst
@@ -921,7 +921,37 @@ 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 uses the C99 standard for the 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 labels should be used instead
+of 0 and 1.
+
+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, 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 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
@@ -944,7 +974,7 @@ 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,
@@ -978,7 +1008,7 @@ 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
@@ -1010,7 +1040,7 @@ 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
-- 
2.20.1