Re: Question about iommu on x86_64 and radeon driver.

From: Daniel Haid
Date: Mon May 23 2011 - 19:45:55 EST

There had to be more than 'Map around memory hole'? Was it called
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:
(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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