* 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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