linux-kernel - Re: [PATCH v5 1/2] Documentation: coding-style: ask function-like macros to evaluate parameters

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Message-ID: <87msqbvj3g.fsf@meer.lwn.net>
Date: Tue, 02 Apr 2024 10:13:07 -0600
From: Jonathan Corbet <corbet@....net>
To: Barry Song <21cnbao@...il.com>, akpm@...ux-foundation.org,
 linux-doc@...r.kernel.org, workflows@...r.kernel.org
Cc: apw@...onical.com, broonie@...nel.org, chenhuacai@...ngson.cn,
 chris@...kel.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, Max Filippov <jcmvbkbc@...il.com>
Subject: Re: [PATCH v5 1/2] Documentation: coding-style: ask function-like
 macros to evaluate parameters

So I'm not sure what your desired path for getting this upstream is.  I
can take it, but I'm generally quite leery of taking coding-style
changes without some serious acks on them - nobody elected me as the
arbiter of proper coding style.

A nit below

Barry Song <21cnbao@...il.com> writes:

> 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>
> 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 | 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

I would just use the "::" notation here; the ..code-block:: just adds
noise IMO.

> +	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)
> +

1) If you're putting in examples of something *not* to do, it's probably
better to also put in something like:

   /* don't do this */

people don't always read closely.

2) Can we say *why* it's not recommended?

Thanks,

jon