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Date: Mon, 23 Jul 2012 20:37:11 -0500
From: Scott Wood <scottwood@freescale.com>
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To: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Subject: Re: [PATCH] powerpc/mm: add ZONE_NORMAL zone for 64 bit kernel
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Cc: Wood Scott-B07421 <B07421@freescale.com>,
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On 07/23/2012 06:30 PM, Benjamin Herrenschmidt wrote:
> On Tue, 2012-07-24 at 09:29 +1000, Benjamin Herrenschmidt wrote:
> 
>> The layers in between, not the well behaved drivers. Again, we have
>> ZONE_DMA32 specifically for the purpose, why use something else ?
>>
>> In any case, make the whole thing at the very least a config option, I
>> don't want sane HW to have to deal with split zones.
> 
> Or if possible a flag set by machine probe()

I suggested making the threshold configurable by platform code.  Sane
hardware would leave it at its default of infinity (~0ULL), and you
wouldn't have a split zone.  Our hardware would set it at 31-bit.

It looks like this is already sort-of done for ISA_DMA_THRESHOLD for
32-bit non-coherent-DMA platforms (amigaone sets a 24-bit threshold),
though I don't see where ZONE_DMA is limited accordingly.

We'd add an additional threshold for ZONE_DMA32, make it actually affect
the zone definition, and make the standard alloc_coherent() honor it.

-Scott