Re: [PATCH v2] xtensa: remove redundant flush_dcache_page and ARCH_IMPLEMENTS_FLUSH_DCACHE_PAGE macros

[Guenter Roeck]

On 4/28/24 22:09, Max Filippov wrote:
> On Sat, Apr 27, 2024 at 12:05 PM Guenter Roeck <linux@xxxxxxxxxxxx> wrote:
> > Hi,
> >
> > On Tue, Mar 19, 2024 at 02:09:20PM +1300, Barry Song wrote:
> > > From: Barry Song <v-songbaohua@xxxxxxxx>
> > >
> > > 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.
> > >
> > > This patch also removes lots of redundant macros which have
> > > been defined by asm-generic/cacheflush.h.
> > >
> > > Cc: Huacai Chen <chenhuacai@xxxxxxxxxxx>
> > > Cc: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
> > > Reported-by: kernel test robot <lkp@xxxxxxxxx>
> > > Closes: https://lore.kernel.org/oe-kbuild-all/202403091614.NeUw5zcv-lkp@xxxxxxxxx/
> > > Reported-by: Barry Song <v-songbaohua@xxxxxxxx>
> > > Closes: https://lore.kernel.org/all/CAGsJ_4yDk1+axbte7FKQEwD7X2oxUCFrEc9M5YOS1BobfDFXPA@xxxxxxxxxxxxxx/
> > > Reported-by: Guenter Roeck <linux@xxxxxxxxxxxx>
> > > Closes: https://lore.kernel.org/all/aaa8b7d7-5abe-47bf-93f6-407942436472@xxxxxxxxxxxx/
> > > Fixes: 77292bb8ca69 ("crypto: scomp - remove memcpy if sg_nents is 1 and pages are lowmem")
> > > Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
> >
> > The mainline kernel still fails to build xtensa:allmodconfig.
> >
> > Building xtensa:allmodconfig ... failed
> > --------------
> > Error log:
> > crypto/scompress.c: In function 'scomp_acomp_comp_decomp':
> > crypto/scompress.c:174:38: error: unused variable 'dst_page' [-Werror=unused-variable]
> >    174 |                         struct page *dst_page = sg_page(req->dst);
> >
> > This patch fixes the problem. Is there a chance to get it applied to the
> > upstream kernel, or should I just stop build testing xtensa:allmodconfig ?
>
> Applied to my xtensa tree.
> I was still hoping to see rationale for why this is a useful warning.

Useful in general or here ? In general it helps a lot to identify unnecessary
or buggy code, and I think it is very useful. In this case, the other option
would have been to declare the variable as __maybe_unused. I think that
has been discussed. Personally I preferred the more comprehensive patch
because it makes the code more generic, but at this point in the release
cycle I really only want to know what to do with xtensa:allmodconfig test
builds.

Thanks,
Guenter