linux-kernel - Re: State of RFC PATCH Remove CONFIG_DCACHE_WORD

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Date:   Tue, 13 Sep 2022 11:42:38 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Lukas Bulwahn <lukas.bulwahn@...il.com>,
        Joe Perches <joe@...ches.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        kernel-janitors <kernel-janitors@...r.kernel.org>
Subject: Re: State of RFC PATCH Remove CONFIG_DCACHE_WORD_ACCESS



Le 12/09/2022 à 17:22, Christophe Leroy a écrit :
> 
> 
> Le 12/09/2022 à 15:46, Lukas Bulwahn a écrit :
>> Hi Joe, hi Ben,
>>
>> While reviewing some kernel config, I came across
>> CONFIG_DCACHE_WORD_ACCESS and tried to understand its purpose.
>>
>> Then, I discovered this RFC patch from 2014 that seems never to have
>> been integrated:
>>
>> https://lore.kernel.org/all/1393964591.20435.58.camel@joe-AO722/
>> [RFC] Remove CONFIG_DCACHE_WORD_ACCESS
>>
>> The discussion seemed to just not continue and the patch was just not
>> integrated by anyone.
>>
>> In the meantime, the use of CONFIG_DCACHE_WORD_ACCESS has spread into
>> a few more files, but replacing it with
>> CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS still seems feasible.
>>
>> Are you aware of reasons that this patch from 2014 should not be 
>> integrated?
>>
>> I would spend some time to move the integration of this patch further
>> if you consider that the patch is not completely wrong.
>>
> 
> As far as I can see, for the time being this is not equivalent on powerpc:
> 
> select HAVE_EFFICIENT_UNALIGNED_ACCESS  if !(CPU_LITTLE_ENDIAN && 
> POWER7_CPU)
> 
> select DCACHE_WORD_ACCESS               if PPC64 && CPU_LITTLE_ENDIAN
> 
> This will need to be investigated I guess.
> 
> In the meantime I'll try to see if it makes any difference for ppc32.
> 

Selecting DCACHE_WORD_ACCESS on powerpc32 provides an improvement of 
approx 9% on a powerpc 8xx and about 2% on a powerpc 832x, using the 
benchmark test in commit a75c380c7129 ("powerpc: Enable 
DCACHE_WORD_ACCESS on ppc64le")

Christophe