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

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Message-ID: <abf2b1db-3e13-4765-9034-712430e856d7@roeck-us.net>
Date: Mon, 18 Mar 2024 20:49:00 -0700
From: Guenter Roeck <linux@...ck-us.net>
To: Barry Song <21cnbao@...il.com>, Max Filippov <jcmvbkbc@...il.com>
Cc: chris@...kel.net, akpm@...ux-foundation.org,
 linux-kernel@...r.kernel.org, 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 3/18/24 19:16, Barry Song wrote:
> 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?
>

It is because -Werror is now default in allmodconfig builds.
Other test build systems may manually override that.

Guenter