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

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Message-ID: <91e6cf8b-f66a-3ea1-daa0-2ea875b7e7e8@csgroup.eu>
Date:   Mon, 12 Sep 2022 15:22:25 +0000
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 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.

Christophe