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

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Message-ID: <1304117514.2513.266.camel@pasglop>
Date:	Sat, 30 Apr 2011 08:51:54 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	Arnd Bergmann <arnd@...db.de>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	linaro-mm-sig@...ts.linaro.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC] ARM DMA mapping TODO, v1

On Fri, 2011-04-29 at 12:56 +0100, Alan Cox wrote:
> > I believe that the PC graphics cards that have noncoherent DMA mappings
> > are all of the unified memory (integrated into the northbridge) kind,
> > so they are not on the same host bridge as all regular PCI devices,
> > even if they appear as a PCI device.
> 
> The AGP GART is not coherent on a lot of systems - not necessarily
> unified memory though, it can be a plug in AGP card too.
> The GART is basically an IOMMU (and indeed in the later AMD case used
> exactly as that)

Right. Actually there's also the ability for PCIe devices to set a "no
snoop" bit on transactions and thus behave in a non-coherent manner.
Hopefully most sane PHBs ignore that bit ...

Cheers,
Ben.

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