Re: [PATCH 2/3] iommu/arm-smmu: Add initial driver support for ARM SMMUv3 devices

[Joerg Roedel <joro-zLv9SwRftAIdnm+yROfE0A@public.gmane.org>]

On Fri, May 29, 2015 at 12:35:56PM +0100, Robin Murphy wrote:
> The trouble with this is, what about the CPU page size? Say you have
> some multimedia subsystem with its own integrated SMMU and for that
> they've only implemented the 16K granule scheme because it works
> best for the video hardware (and the GPU driver is making direct
> IOMMU API calls to remap carved-out RAM rather than using
> DMA-mapping). Now, the SMMU on the compute side of the SoC serving
> the general peripherals will be rendered useless by bumping the
> system-wide minimum page size up to 16K, because it then can't map
> that scatterlist of discontiguous 4K pages that the USB controller
> needs...
> 
> I think this really represents another push to get away from (or at
> least around) the page-at-a-time paradigm - if the IOMMU API itself
> wasn't too fussed about page sizes and could let drivers handle the
> full map/unmap requests however they see fit, I think we could
> bypass a lot of these issues. We've already got the Intel IOMMU
> driver doing horrible hacks with the pgsize_bitmap to cheat the
> system, I'm sure we don't want to add any more of that. How about
> something like the below diff as a first step?

Moving functionality out of the iommu core code into the drivers is the
wrong direction imo. It is better to solve it with something like

	struct iommu_domain *iommu_domain_alloc_for_group(struct iommu_group *group);

Which gets us a domain that can only be assigned to that particular
group. Since there is a clear one-to-many relationship between a
hardware iommu and the groups of devices behind it, we could propagate
the pgsize_bitmap from the iommu to the group and then to the domain.

Domains allocated via iommu_domain_alloc() would get the merged
pgsize_bitmap like I described in my previous mail.

But to make this happen we need a representation of single hardware
iommu instances in the iommu core first.


	Joerg