linux-kernel - Re: [Linaro-mm-sig] [RFC] ARM DMA mapping TODO, v1

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Date:	Sat, 30 Apr 2011 08:55:06 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Jerome Glisse <j.glisse@...il.com>
Cc:	Thomas Hellstrom <thellstrom@...are.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org,
	linaro-mm-sig@...ts.linaro.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [Linaro-mm-sig] [RFC] ARM DMA mapping TODO, v1

On Fri, 2011-04-29 at 09:34 -0400, Jerome Glisse wrote:
> 
> Radeon memory controller can do non snooped pci transaction, as far as
> i have tested most of the x86 pci bridge don't try to be coherent then
> ie they don't analyze pci dma and ask for cpu flush they just perform
> the request (and i guess it's what all bridge will do), so it endup
> being noncoherent. I haven't done any benchmark of how faster it's for
> the GPU when it's not snooping but i guess it can give 50% boost as it
> likely drastictly reduce pci transaction overhead.
> 
> I am talking here about device that you plug into any pci or pcie
> slot, so it's not igp integrated into northbridge or into the cpu.

Right, the card has nothing to do with the snooping process, it's purely
a feature of the bridge, based on a flag optionally set by the card. As
I said earlier, bridges have the freedom to ignore it, which we do on
ppc, so that's a non issue.

Cheers,
Ben.

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