linux-kernel - Re: Question about iommu on x86

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Message-ID: <ae8450396be43e94c1ea724312428d4f@admin.gogi.tv>
Date:	Wed, 25 May 2011 21:21:21 +0100
From:	Daniel Haid <d.haid@...i.tv>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc:	Andi Kleen <andi@...stfloor.org>, <linux-kernel@...r.kernel.org>
Subject: Re: Question about iommu on x86_64 and radeon driver.

> 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?

 So if a driver wants to do DMA it requests DMA-memory and gets it from
 the lower than 4GB address space.

 But for some reason it is not done like this, why?
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