I think the recent discussions on linaro-mm-sig and the BoF last week at ELC have been quite productive, and at least my understanding of the missing pieces has improved quite a bit. This is a list of things that I think need to be done in the kernel. Please complain if any of these still seem controversial:
1. Fix the arm version of dma_alloc_coherent. It's in use today and is broken on modern CPUs because it results in both cached and uncached mappings. Rebecca suggested different approaches how to get there.
2. Implement dma_alloc_noncoherent on ARM. Marek pointed out that this is needed, and it currently is not implemented, with an outdated comment explaining why it used to not be possible to do it.
3. Convert ARM to use asm-generic/dma-mapping-common.h. We need both IOMMU and direct mapped DMA on some machines.
4. Implement an architecture independent version of dma_map_ops based on the iommu.h API. As Joerg mentioned, this has been missing for some time, and it would be better to do it once than for each IOMMU separately. This is probably a lot of work.
5. Find a way to define per-device IOMMUs, if that is not actually possible already. We had conflicting statements for this.
6. Implement iommu_ops for each of the ARM platforms that has an IOMMU. Needs some modifications for MSM and a rewrite for OMAP. Implementation for Samsung is under work.
7. Extend the dma_map_ops to have a way for mapping a buffer from dma_alloc_{non,}coherent into user space. We have not discussed that yet, but after thinking this for some time, I believe this would be the right approach to map buffers into user space from code that doesn't care about the underlying hardware.
After all these are in place, building anything on top of dma_alloc_{non,}coherent should be much easier. The question of passing buffers between V4L and DRM is still completely unsolved as far as I can tell, but that discussion might become more focused if we can agree on the above points and assume that it will be done.
I expect that I will have to update the list above as people point out mistakes in my assumptions.
Arnd