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

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Date: Thu, 21 Mar 2024 20:42:38 +1300
From: Barry Song <21cnbao@...il.com>
To: Meiyong Yu <meiyong.yu@....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 Thu, Mar 21, 2024 at 5:40 PM Meiyong Yu <meiyong.yu@....com> wrote:
>
>
> 在 2024/3/21 8:11, Barry Song 写道:
> > On Thu, Mar 21, 2024 at 12:39 PM Meiyong Yu <meiyong.yu@....com> wrote:
> >>
> >>> 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.
> >>
> > Linux boasts support for numerous architectures, striving for
> > independence in its
> > drivers and core code implementation across these architectures. Consequently,
> > certain architectures may utilize parameters for the same APIs, while others may
> > not.
>
> So the probem is  designed api is not reasonable,  it use not essential
> paramter,
>
> you can change the api, but not avoid it.
>

Incorrect again. As an API, it must take into account various considerations.
Just because architecture A doesn't require flushing dcache doesn't imply
that architecture B doesn't need it.

> Anthor question, why you do not use the parameter, if not use it,  will
> trigger function/feature dismiss problem ?
>
> >> about the warning,  is  tool misreport,  the tool must make better
> >>
> > no. This is not the case.
> >
> >>> +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
> >>>
> >>
>
>