Documentation: coding-style: ask function-like macros to evaluate parameters

Patch series "codingstyle: avoid unused parameters for a function-like
macro", v7.

A function-like macro could result in build warnings such as "unused
variable." This patchset updates the guidance to recommend always using a
static inline function instead and also provides checkpatch support for
this new rule.


This patch (of 2):

Recent commit 77292bb ("crypto: scomp - remove memcpy if
sg_nents is 1 and pages are lowmem") leads to warnings on xtensa
and loongarch,
   In file included from crypto/scompress.c:12:
   include/crypto/scatterwalk.h: In function 'scatterwalk_pagedone':
   include/crypto/scatterwalk.h:76:30: warning: variable 'page' set but not used [-Wunused-but-set-variable]
      76 |                 struct page *page;
         |                              ^~~~
   crypto/scompress.c: In function 'scomp_acomp_comp_decomp':
>> crypto/scompress.c:174:38: warning: unused variable 'dst_page' [-Wunused-variable]
     174 |                         struct page *dst_page = sg_page(req->dst);
         |

The reason is that flush_dcache_page() is implemented as a noop
macro on these platforms as below,

 #define flush_dcache_page(page) do { } while (0)

The driver code, for itself, seems be quite innocent and placing
maybe_unused seems pointless,

 struct page *dst_page = sg_page(req->dst);

 for (i = 0; i < nr_pages; i++)
 	flush_dcache_page(dst_page + i);

And it should be independent of architectural implementation
differences.

Let's provide guidance on coding style for requesting parameter
evaluation or proposing the migration to a static inline
function.

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Barry Song <[email protected]>
Suggested-by: Max Filippov <[email protected]>
Reviewed-by: Mark Brown <[email protected]>
Acked-by: Joe Perches <[email protected]>
Cc: Chris Zankel <[email protected]>
Cc: Huacai Chen <[email protected]>
Cc: Herbert Xu <[email protected]>
Cc: Guenter Roeck <[email protected]>
Cc: Stephen Rothwell <[email protected]>
Cc: Andy Whitcroft <[email protected]>
Cc: Dwaipayan Ray <[email protected]>
Cc: Joe Perches <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Lukas Bulwahn <[email protected]>
Cc: Xining Xu <[email protected]>
Cc: Charlemagne Lasse <[email protected]>
Cc: Jeff Johnson <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Barry Song

Documentation/process/coding-style.rst

@@ -827,6 +827,29 @@ Macros with multiple statements should be enclosed in a do - while block:
 				do_this(b, c);		\
 		} while (0)
 
+Function-like macros with unused parameters should be replaced by static
+inline functions to avoid the issue of unused variables:
+
+.. code-block:: c
+
+	static inline void fun(struct foo *foo)
+	{
+	}
+
+Due to historical practices, many files still employ the "cast to (void)"
+approach to evaluate parameters. However, this method is not advisable.
+Inline functions address the issue of "expression with side effects
+evaluated more than once", circumvent unused-variable problems, and
+are generally better documented than macros for some reason.
+
+.. code-block:: c
+
+	/*
+	 * Avoid doing this whenever possible and instead opt for static
+	 * inline functions
+	 */
+	#define macrofun(foo) do { (void) (foo); } while (0)
+
 Things to avoid when using macros:
 
 1) macros that affect control flow: