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

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Message-ID: <59ea2e9e-cff1-4d99-a586-8df289964f21@roeck-us.net>
Date: Mon, 29 Apr 2024 06:58:10 -0700
From: Guenter Roeck <linux@...ck-us.net>
To: Max Filippov <jcmvbkbc@...il.com>
Cc: Barry Song <21cnbao@...il.com>, 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 4/28/24 22:09, Max Filippov wrote:
> On Sat, Apr 27, 2024 at 12:05 PM Guenter Roeck <linux@...ck-us.net> wrote:
>>
>> Hi,
>>
>> On Tue, Mar 19, 2024 at 02:09:20PM +1300, Barry Song 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.
>>>
>>> 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>
>>> Closes: https://lore.kernel.org/all/aaa8b7d7-5abe-47bf-93f6-407942436472@roeck-us.net/
>>> Fixes: 77292bb8ca69 ("crypto: scomp - remove memcpy if sg_nents is 1 and pages are lowmem")
>>> Signed-off-by: Barry Song <v-songbaohua@...o.com>
>>
>> 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