linux-kernel - Re: [PATCH v2] powerpc32: memcpy/memset: only use dcbz once cache is enabled

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Date:	Mon, 14 Sep 2015 19:59:10 +1000
From:	Michael Ellerman <mpe@...erman.id.au>
To:	christophe leroy <christophe.leroy@....fr>
Cc:	Scott Wood <scottwood@...escale.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>, sojkam1@....cvut.cz,
	linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2] powerpc32: memcpy/memset: only use dcbz once cache
 is enabled

On Sat, 2015-09-12 at 11:57 +0200, christophe leroy wrote:
> 
> Le 11/09/2015 03:24, Michael Ellerman a écrit :
> > On Thu, 2015-09-10 at 17:05 -0500, Scott Wood wrote:
> >>
> >> I don't think this duplication is what Michael meant by "the normal cpu
> >> feature sections".  What else is going to use this very specific
> >> infrastructure?
> > Yeah, sorry, I was hoping you could do it with the existing cpu feature
> > mechanism.
> >
> > It looks like the timing doesn't work, ie. you need to patch this stuff in
> > machine_init(), which is later than the regular patching which gets done in
> > early_init().
> >
> > This is one of the festering differences we have between the 32 and 64-bit
> > initialisation code, ie. on 64-bit we do the patching much later.
>
> I've just thought about maybe another alternative.
> Is there any issue with calling do_feature_fixups() twice for the same
> features ?

Not that I can think of, but you never know.

> If not, we could define a MMU_CACHE_NOW_ON dummy MMU feature, then
> call again do_feature_fixups() in machine_init() to patch memcpy/memset 
> stuff, something like:
> 
> In arch/powerpc/include/asm/mmu.h:
> +#define MMU_CACHE_NOW_ON                ASM_CONST(0x00008000)
> 
> In arch/powerpc/kernel/setup_32.c: @machine_init()
> 
>          udbg_early_init();
> 
> +        spec = identify_cpu(0, mfspr(SPRN_PVR));
> +        do_feature_fixups(spec->mmu_features | MMU_CACHE_NOW_ON,
> +                          &__start___mmu_ftr_fixup,
> +                          &__stop___mmu_ftr_fixup);


Did you try that? It would be cleaner, especially now that you have to do memset as well.

cheers


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