linux-kernel - Re: [PATCH] powerpc32: memcpy: only use dcbz once cache is enabled

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Message-ID: <1441684596.14597.5.camel@ellerman.id.au>
Date:	Tue, 08 Sep 2015 13:56:36 +1000
From:	Michael Ellerman <mpe@...erman.id.au>
To:	Christophe Leroy <christophe.leroy@....fr>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>, scottwood@...escale.com,
	sojkam1@....cvut.cz, linux-kernel@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] powerpc32: memcpy: only use dcbz once cache is enabled

On Mon, 2015-09-07 at 16:24 +0200, Christophe Leroy wrote:
> memcpy() uses instruction dcbz to speed up copy by not wasting time
> loading cache line with data that will be overwritten.
> Some platform like mpc52xx do no have cache active at startup and
> can therefore not use memcpy(). Allthough no part of the code
> explicitly uses memcpy(), GCC makes calls to it.
> 
> This patch modifies memcpy() such that at startup, the 'dcbz'
> instruction is replaced by 'dcbt' which is harmless if cache is not
> enabled, and which helps a bit (allthough not as much as dcbz) if
> cache is already enabled.
> 
> Once the initial MMU is setup, in machine_init() we patch memcpy()
> by replacing the temporary 'dcbt' by 'dcbz'
> 
> Reported-by: Michal Sojka <sojkam1@....cvut.cz>
> Signed-off-by: Christophe Leroy <christophe.leroy@....fr>


Is there some reason you can't use the normal cpu feature sections? See
arch/powerpc/lib/memcpy_64.S for an example of what I mean.

cheers


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