linux-kernel - Re: Question about iommu on x86

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Message-ID: <20110525230544.GA4065@one.firstfloor.org>
Date:	Thu, 26 May 2011 01:05:44 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Daniel Haid <d.haid@...i.tv>
Cc:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org
Subject: Re: Question about iommu on x86_64 and radeon driver.

On Wed, May 25, 2011 at 09:21:21PM +0100, Daniel Haid wrote:
> >Only if you allow 3GB or less in the machine. So you would have to do
> >mem=3G as well.
> >
> >The reason is that (and you can see that yourself by looking at the
> >E820), is that 1GB is actually _above_ the 4GB.
> 
> Just another question on this one:
> 
> Why can a driver not simply ask for DMA-capable memory, is this not
> what memory zones are for?

In many cases the memory gets passed into the driver.

If it's not already in the right boundaries it would need to copy through
a bounce buffer. That is what swiotlb does, if there's no IOMMU.

-Andi
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