linux-kernel - Re: [PATCH v2] xtensa: remove redundant flush_dcache_page and ARCH_IMPLEMENTS_FLUSH_DCACHE

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Message-ID: <CAMo8BfJ+AOOY8Sn-2=CHCJ9M9kzU=w6nnkqEg=8V=2F2uNhhqQ@mail.gmail.com>
Date: Mon, 18 Mar 2024 19:12:04 -0700
From: Max Filippov <jcmvbkbc@...il.com>
To: Barry Song <21cnbao@...il.com>
Cc: chris@...kel.net, akpm@...ux-foundation.org, linux-kernel@...r.kernel.org, 
	Guenter Roeck <linux@...ck-us.net>, willy@...radead.org, dennis@...nel.org, 
	alexghiti@...osinc.com, Barry Song <v-songbaohua@...o.com>, 
	Huacai Chen <chenhuacai@...ngson.cn>, Herbert Xu <herbert@...dor.apana.org.au>, 
	kernel test robot <lkp@...el.com>
Subject: Re: [PATCH v2] xtensa: remove redundant flush_dcache_page and
 ARCH_IMPLEMENTS_FLUSH_DCACHE_PAGE macros

On Mon, Mar 18, 2024 at 6:27 PM Barry Song <21cnbao@...il.com> wrote:
> On Tue, Mar 19, 2024 at 2:09 PM Barry Song <21cnbao@...il.com> wrote:
> > From: Barry Song <v-songbaohua@...o.com>
> >
> > xtensa's flush_dcache_page() can be a no-op sometimes. There is a
> > generic implementation for this case in include/asm-generic/
> > cacheflush.h.
> >  #ifndef ARCH_IMPLEMENTS_FLUSH_DCACHE_PAGE
> >  static inline void flush_dcache_page(struct page *page)
> >  {
> >  }
> >
> >  #define ARCH_IMPLEMENTS_FLUSH_DCACHE_PAGE 0
> >  #endif
> >
> > So remove the superfluous flush_dcache_page() definition, which also
> > helps silence potential build warnings complaining the page variable
> > passed to flush_dcache_page() is not used.
> >
> >    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 issue was originally reported on LoongArch by kernel test
> > robot (Huacai fixed it on LoongArch), then reported by Guenter
> > and me on xtensa.

I wonder why this warning is considered useful in the first place?
If I'm missing something and it's indeed useful, should there be a
rule in the Documentation/process/coding-style.rst saying that
function-like macros should evaluate all their arguments?

> > This patch also removes lots of redundant macros which have
> > been defined by asm-generic/cacheflush.h.
> >
> > Cc: Huacai Chen <chenhuacai@...ngson.cn>
> > Cc: Herbert Xu <herbert@...dor.apana.org.au>
> > Reported-by: kernel test robot <lkp@...el.com>
> > Closes: https://lore.kernel.org/oe-kbuild-all/202403091614.NeUw5zcv-lkp@intel.com/
> > Reported-by: Barry Song <v-songbaohua@...o.com>
> > Closes: https://lore.kernel.org/all/CAGsJ_4yDk1+axbte7FKQEwD7X2oxUCFrEc9M5YOS1BobfDFXPA@mail.gmail.com/
> > Reported-by: Guenter Roeck <linux@...ck-us.net>
>
> Hi Guenter,
> I am not a xtensa guy, so I will need your help for a full test. if
> turns out it is a too big(ambitious)
> fix, a minimal fix might be:
>
> diff --git a/arch/xtensa/include/asm/cacheflush.h
> b/arch/xtensa/include/asm/cacheflush.h
> index 38bcecb0e457..fdc692cf2b78 100644
> --- a/arch/xtensa/include/asm/cacheflush.h
> +++ b/arch/xtensa/include/asm/cacheflush.h
> @@ -145,7 +145,7 @@ void local_flush_cache_page(struct vm_area_struct *vma,
>  #define flush_cache_vunmap(start,end)                  do { } while (0)
>
>  #define ARCH_IMPLEMENTS_FLUSH_DCACHE_PAGE 0
> -#define flush_dcache_page(page)                                do { } while (0)
> +#define flush_dcache_page(page)                                do {
> (void)(page); } while (0)

This looks like a good fix, IMO better than adding __maybe_unused
to the variables reported in the warnings.

-- 
Thanks.
-- Max