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

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Date:	Sat, 12 Sep 2015 11:57:21 +0200
From:	christophe leroy <christophe.leroy@....fr>
To:	Michael Ellerman <mpe@...erman.id.au>,
	Scott Wood <scottwood@...escale.com>
Cc:	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



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 ?
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);

         /* Do some early initialization based on the flat device tree */
         early_init_devtree(__va(dt_ptr));


Christophe

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