Re: IOMMU/DMA API inquiry

From: Mark Hounschell
Date: Thu Feb 19 2015 - 14:41:44 EST

Hi Joerg, Thanks for the response.

On 02/18/2015 01:19 PM, joro@xxxxxxxxxx >> Joerg Roedel wrote:
Hi Mark,

On Tue, Feb 17, 2015 at 02:48:03PM -0500, Mark Hounschell wrote:
I understand that AMD IOMMU support is not available for 32-bit
kernels. I believe the INTEL IOMMU is supported there. Not knowing
why, I was curious if that is going to remain that way?

Yes, I have no plan on making the AMD IOMMU driver available on 32bit.
But I would not be resistant to patches enabling the driver there.

OK. I might take a look. Should it not "just work" more or less?

I've learned that the AMD IOMMU does not play well with the kernels
"Contiguous Memory Allocator" (CMA). I also believe, but could be
mistaken, that the INTEL IOMMU does. Again, not knowing why, I was
curious if that is going to remain that way also?

No, I will queue a patch for the next merge window to enable CMA use in
the AMD IOMMU driver. So there will be support for this.

Great! That's a couple of kernels away then. I would be happy to test if you have a need.

Is the fact that the AMD IOMMU is not supported on 32 bit kernels
the reason that dma_map_page always returns 0 on 32 bit kernels.

There is no particular reason for not supporting it on 32 bit kernels,
it just didn't seem to be important yet. At least not important enough
to justify the work.


I've read the DMA-API-HOWTO.txt concerning dma_map_sg and at first I
thought that maybe dma_map_sg could be used to get around the fact
that AMD IOMMU doesn't work with CMA. But it looks as though I was
mistaken and I would actually have to do a DMA for_each_sg(sglist,
sg, count, i). Is that correct or can dma_map_sg somehow enable you
to do a single DMA using a single address for the entire sglist?

The map_sg functions can't be used with the AMD IOMMU driver to work
around missing CMA support. Depending on what you want it might work
with the Intel IOMMU driver, as this one allocates a single IOVA region
for the entire sg_list.

Thanks Joerg


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