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

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Date:	Sat, 30 Apr 2011 08:53:40 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Thomas Hellstrom <thellstrom@...are.com>
Cc:	Arnd Bergmann <arnd@...db.de>, linaro-mm-sig@...ts.linaro.org,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [Linaro-mm-sig] [RFC] ARM DMA mapping TODO, v1

On Fri, 2011-04-29 at 14:06 +0200, Thomas Hellstrom wrote:
> I think Jerome has mentioned at one point that the Radeon graphics
> cards support non-coherent mappings.

If the card is PCI/PCI-X/PCIe then coherency is not its business, it's
the business of the host bridge. However, on PCIe at least, the card can
indeed set a "no snoop" attribute on DMA transactions to request "no
coherency". At least the systems have the latitude to just ignore that
bit (like we do on all ppc afaik) :-)

> Fwiw, the PowerVR SGX MMU also supports this mode of operation,
> although  it being functional I guess depends on the system
> implementation.

Right, it's not a GPU thing, it's really a system design thing.

Cheers,
Ben.


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