Re: IOMMU/DMA API inquiry

[Mark Hounschell]

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.

Understood.

> > 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

Regards
Mark

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