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

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Message-ID: <87lhci472f.fsf@steelpick.2x.cz>
Date:	Mon, 07 Sep 2015 16:50:32 +0200
From:	Michal Sojka <sojkam1@....cvut.cz>
To:	Christophe Leroy <christophe.leroy@....fr>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Michael Ellerman <mpe@...erman.id.au>, scottwood@...escale.com
Cc:	linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] powerpc32: memcpy: only use dcbz once cache is enabled

On Mon, Sep 07 2015, 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>
> ---
> @Michal, can you please test it ?

Yes, it works.

Tested-by: Michal Sojka <sojkam1@....cvut.cz>

-Michal
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