linux-kernel - Re: Problem with DMA on x86

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Date:	Tue, 21 Nov 2006 18:27:26 +0000
From:	Alan <alan@...rguk.ukuu.org.uk>
To:	Andi Kleen <ak@...e.de>
Cc:	Larry Finger <Larry.Finger@...inger.net>,
	Ray Lee <ray-lk@...rabbit.org>, linux-kernel@...r.kernel.org
Subject: Re: Problem with DMA on x86_64 with 3 GB RAM

On Tue, 21 Nov 2006 17:46:39 +0100
Andi Kleen <ak@...e.de> wrote:

> 
> > Shouldn't this problem be mentioned somewhere in the documentation, or did I miss something?

The documentation is correct, the implementation is broken. The
documented behaviour works for all platforms except one whose maintainer
has a problem with it and refuses to follow the spec.

> Possibly, but devices that cannot address at least 4GB are normally
> categorized as "hardware bugs" (or less polite descriptions) and those don't 
> tend to get much airtime in documentation.

The rest of the kernel deals with hardware limitations, 30bit DMA works
on the other platforms. This is an x86-64 platform problem. It
misimplements the basic pci_ functionality. If it doesn't wish to
implement the stuff (and there btw Andi I do think your view has
considerable merit) it should fail the set_mask requests.

Alan
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