linux-kernel - Re: Question about iommu on x86

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Message-ID: <756073831c36e0404199a4e8239bcb70@admin.gogi.tv>
Date:	Tue, 24 May 2011 00:45:47 +0100
From:	Daniel Haid <d.haid@...i.tv>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc:	<linux-kernel@...r.kernel.org>
Subject: Re: Question about iommu on x86_64 and radeon driver.

> There had to be more than 'Map around memory hole'? Was it called
> GART or IOMMU?
 I do not think that there was "IOMMU" or "GART" written there,
 but I do not think that the mainboard in question has an IOMMU
 (Am I correct that it would be a feature of the mainboard while
 the AMD GART is a feature of the CPU?).
 I will look again as soon as I have physical access to the system.

> The problem you are hitting (I think) is that the AMD GART poor-man
> IOMMU is turned off
> and the SWIOTLB is used instead. If you would like some technical
> details, take a look at:
> 
> http://lists.freedesktop.org/archives/dri-devel/2011-January/006885.html
> (the point #2 is what you are hitting).

 You are correct. In all the cases where the radeon card does not work
 I see that SWIOTLB has been enabled in the kernel log.

 So this is a bug? I suppose that all hardware should be working with
 SWIOTLB? Will a patch that fixes this somewhen be included?
 (The bug where your link points to was closed with WONTFIX)

> Not sure why the AMD GART IOMMU gets disabled on VIA chipsets. You 
> might
> want to use 'git gui blame arch/x86/kernel/early-quirks.c' and look
> at the code in question to figure that out.
> Well, if everything works.... but you might just want to use
> the git gui blame to take a look at the back-story of why the quirk
> was added.

 Unfortunately I am getting crashes with "iommu=allowed". I will look
 at git blame.

 Thank you for your answers.

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