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

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Message-ID: <ce352c9c-2052-4a23-809a-ef4cc9b7169c@roeck-us.net>
Date: Tue, 19 Mar 2024 07:17:58 -0700
From: Guenter Roeck <linux@...ck-us.net>
To: Barry Song <21cnbao@...il.com>, chris@...kel.net, jcmvbkbc@...il.com,
 akpm@...ux-foundation.org, linux-kernel@...r.kernel.org
Cc: 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 18:27, Barry Song 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.
>>
>> 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)

I sent a Tested-by: tag to your patch. As far as my testing goes, it works fine,
and I added your patch to my "testing" branch (which tries to be buildable
and testable for all architectures).

> fix, a minimal fix might be:
> 

FWIW, I think a minimal fix would have been to mark the variable as __maybe_unused,
but as others have pointed out it would be nice if there would be a guidance
about optional API functions like this one that specifies if it may be
implemented as macro and, if so, how it has to handle unused variables.

Thanks,
Guenter