[RFC PATCH 0/7] Introduce automatic DMA configuration for IOMMU masters

[Arnd Bergmann]

On Tuesday 02 September 2014 10:48:02 Marek Szyprowski wrote:
> 
> >   -- I have concerns that allocating one domain per master might be
> > too much, but it's hard to tell without an IOMMU driver ported over.
> 
> One domain per master is IMHO a sane default configuration. The only default
> alternative I see is to have only one domain (related with dma-mapping
> subsystem) and bind all devices to it. However I really don't see any
> disadvantage of having separate domain per each master and such 
> configuration
> gives devices better separation.

I was expecting that the dma-mapping implementation would by default use
one domain for all devices, since that is what the simpler IOMMUs without
domain support have to do anyway.

For isolation purposes, it can only help to have more domains, but
I would guess that there is some space overhead in maintaining lots
of page tables.

> However we also need to figure out how to let drivers to make their own
> configuration, like it is required by Exynos DRM subsystem, which consist
> of several devices, each having its own IOMMU controller, but for
> convenience those drivers assume that they all have been bound to the same,
> single domain.

IIRC with the way we ended up putting the mask into the iommu descriptor of
the ARM SMMU, you can put multiple devices into the same iommu group, and
have them automatically share a domain.

I don't know if the same would work for the Samsung implementation.

	Arnd