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Subject[PATCH v5 1/2] Documentation: coding-style: ask function-like macros to evaluate parameters
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From: Barry Song <v-songbaohua@oppo.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@oppo.com>
Suggested-by: Max Filippov <jcmvbkbc@gmail.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Cc: Chris Zankel <chris@zankel.net>
Cc: Huacai Chen <chenhuacai@loongson.cn>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Andy Whitcroft <apw@canonical.com>
Cc: Dwaipayan Ray <dwaipayanray1@gmail.com>
Cc: Joe Perches <joe@perches.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Cc: Xining Xu <mac.xxn@outlook.com>
---
Documentation/process/coding-style.rst | 16 ++++++++++++++++
1 file changed, 16 insertions(+)

diff --git a/Documentation/process/coding-style.rst b/Documentation/process/coding-style.rst
index 9c7cf7347394..791d333a57fd 100644
--- a/Documentation/process/coding-style.rst
+++ b/Documentation/process/coding-style.rst
@@ -827,6 +827,22 @@ 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)
+ {
+ }
+
+For historical reasons, many files still use the cast to (void) to evaluate
+parameters, but this method is not recommended:
+
+.. code-block:: c
+
+ #define macrofun(foo) do { (void) (foo); } while (0)
+
Things to avoid when using macros:

1) macros that affect control flow:
--
2.34.1

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