linux-kernel - [PATCH v7 1/2] Documentation: coding-style: ask function-like macros to evaluate parameters

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Message-Id: <20240507032757.146386-2-21cnbao@gmail.com>
Date: Tue,  7 May 2024 15:27:56 +1200
From: Barry Song <21cnbao@...il.com>
To: akpm@...ux-foundation.org,
	linux-doc@...r.kernel.org
Cc: apw@...onical.com,
	broonie@...nel.org,
	chenhuacai@...ngson.cn,
	chris@...kel.net,
	corbet@....net,
	dwaipayanray1@...il.com,
	herbert@...dor.apana.org.au,
	joe@...ches.com,
	linux-kernel@...r.kernel.org,
	linux@...ck-us.net,
	lukas.bulwahn@...il.com,
	mac.xxn@...look.com,
	sfr@...b.auug.org.au,
	v-songbaohua@...o.com,
	workflows@...r.kernel.org,
	Max Filippov <jcmvbkbc@...il.com>
Subject: [PATCH v7 1/2] Documentation: coding-style: ask function-like macros to evaluate parameters

From: Barry Song <v-songbaohua@...o.com>

Recent commit 77292bb8ca69c80 ("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.

Signed-off-by: Barry Song <v-songbaohua@...o.com>
Suggested-by: Max Filippov <jcmvbkbc@...il.com>
Reviewed-by: Mark Brown <broonie@...nel.org>
Acked-by: Joe Perches <joe@...ches.com>
Cc: Chris Zankel <chris@...kel.net>
Cc: Huacai Chen <chenhuacai@...ngson.cn>
Cc: Herbert Xu <herbert@...dor.apana.org.au>
Cc: Guenter Roeck <linux@...ck-us.net>
Cc: Stephen Rothwell <sfr@...b.auug.org.au>
Cc: Andy Whitcroft <apw@...onical.com>
Cc: Dwaipayan Ray <dwaipayanray1@...il.com>
Cc: Joe Perches <joe@...ches.com>
Cc: Jonathan Corbet <corbet@....net>
Cc: Lukas Bulwahn <lukas.bulwahn@...il.com>
Cc: Xining Xu <mac.xxn@...look.com>
---
 Documentation/process/coding-style.rst | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/Documentation/process/coding-style.rst b/Documentation/process/coding-style.rst
index 9c7cf7347394..7e768c65aa92 100644
--- a/Documentation/process/coding-style.rst
+++ b/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:
-- 
2.34.1