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

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Date: Tue, 19 Mar 2024 15:16:08 +1300
From: Barry Song <21cnbao@...il.com>
To: Max Filippov <jcmvbkbc@...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 Tue, Mar 19, 2024 at 3:12 PM Max Filippov <jcmvbkbc@...il.com> wrote:
>
> 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?

Guenter reported it as an error not a warning
"

Error log:
crypto/scompress.c: In function 'scomp_acomp_comp_decomp':
crypto/scompress.c:174:38: error: unused variable 'dst_page'"

is it because xtensa's toolchain is different with others?



> 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.

i don't like adding __maybe_unused either as it is not the fault
of the drivers calling flush_dcache_page().

>
> --
> Thanks.
> -- Max