linux-kernel - Re: [PATCH] Documentation: coding-style: ask function-like macros to evaluate parameters

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Message-Id: <EFB48F08-F0B5-47C0-8C47-00A542344AC9@126.com>
Date: Thu, 21 Mar 2024 07:37:14 +0800
From: Meiyong Yu <meiyong.yu@....com>
To: Barry Song <21cnbao@...il.com>
Cc: corbet@....net, workflows@...r.kernel.org, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, Barry Song <v-songbaohua@...o.com>,
 Andrew Morton <akpm@...ux-foundation.org>, Chris Zankel <chris@...kel.net>,
 Huacai Chen <chenhuacai@...ngson.cn>,
 Herbert Xu <herbert@...dor.apana.org.au>,
 Guenter Roeck <linux@...ck-us.net>, Max Filippov <jcmvbkbc@...il.com>
Subject: Re: [PATCH] Documentation: coding-style: ask function-like macros to evaluate parameters


> On Mar 20, 2024, at 08:17, Barry Song <21cnbao@...il.com> wrote:
> 
> 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 have a guidance in codingstyle to ask for the evaluation
> of parameters.
> 
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> 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>
> Suggested-by: Max Filippov <jcmvbkbc@...il.com>
> Signed-off-by: Barry Song <v-songbaohua@...o.com>
> ---
> Documentation/process/coding-style.rst | 7 +++++++
> 1 file changed, 7 insertions(+)
> 
> diff --git a/Documentation/process/coding-style.rst b/Documentation/process/coding-style.rst
> index 9c7cf7347394..8065747fddff 100644
> --- a/Documentation/process/coding-style.rst
> +++ b/Documentation/process/coding-style.rst
> @@ -827,6 +827,13 @@ Macros with multiple statements should be enclosed in a do - while block:
>                do_this(b, c);        \
>        } while (0)
> 


> +Function-like macros should evaluate their parameters, for unused parameters,
I do not support this point, if the parameter is unused, why not to remove it.

about the warning,  is  tool misreport,  the tool must make better

> +cast them to void:
> +
> +.. code-block:: c
> +
> +    #define macrofun(a) do { (void) (a); } while (0)
> +
> Things to avoid when using macros:
> 
> 1) macros that affect control flow:
> --
> 2.34.1
>