[RFC 4/4] drm: Add NVIDIA Tegra support

[Thierry Reding]

* Arnd Bergmann wrote:
> On Wednesday 11 April 2012, Thierry Reding wrote:
> > Daniel Vetter wrote:
> > > Well, you use the iommu api to map/unmap memory into the iommu for tegra,
> > > whereas usually device drivers just use the dma api to do that. The usual
> > > interface is dma_map_sg/dma_unmap_sg, but there are quite a few variants
> > > around. I'm just wondering why this you've choosen this.
> > 
> > I don't think this works on ARM. Maybe I'm not seeing the whole picture but
> > judging by a quick look through the kernel tree there aren't any users that
> > map DMA memory through an IOMMU.
> 
> dma_map_sg is certainly the right interface to use, and Marek Szyprowski has
> patches to make that work on ARM, hopefully going into v3.5, so you could
> use those.

I've looked at Marek's patches but I don't think they'll work for Tegra 2 or
Tegra 3. The corresponding iommu_map() functions only set one PTE, regardless
of the number of bytes passed to them. However, the Tegra TRM indicates that
mapping needs to be done on a per-page basis so contiguous regions cannot be
combined. I suppose the IOMMU driver would have to be fixed to program more
than a single page in that case.

Also this doesn't yet solve the vmap() problem that is needed for the kernel
virtual mapping. I did try using dma_alloc_writecombine(), but that only
works for chunks of 2 MB or smaller, unless I use init_consistent_dma_size()
during board setup, which isn't provided for in a DT setup. I couldn't find
a better alternative, but I admit I'm not very familiar with all the VM APIs.
Do you have any suggestions on how to solve this? Otherwise I'll try and dig
in some more.

Thierry
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