On Tue, Oct 13, 2020 at 6:42 AM Robin Murphy <robin.murphy@xxxxxxx> wrote: > > On 2020-10-07 07:25, Christoph Hellwig wrote: > > On Tue, Oct 06, 2020 at 09:19:32AM -0400, Jonathan Marek wrote: > >> One example why drm/msm can't use DMA API is multiple page table support > >> (that is landing in 5.10), which is something that definitely couldn't work > >> with DMA API. > >> > >> Another one is being able to choose the address for mappings, which AFAIK > >> DMA API can't do (somewhat related to this: qcom hardware often has ranges > >> of allowed addresses, which the dma_mask mechanism fails to represent, what > >> I see is drivers using dma_mask as a "maximum address", and since addresses > >> are allocated from the top it generally works) > > > > That sounds like a good enough rason to use the IOMMU API. I just > > wanted to make sure this really makes sense. > > I still think this situation would be best handled with a variant of > dma_ops_bypass that also guarantees to bypass SWIOTLB, and can be set > automatically when attaching to an unmanaged IOMMU domain. That way the > device driver can make DMA API calls in the appropriate places that do > the right thing either way, and only needs logic to decide whether to > use the returned DMA addresses directly or ignore them if it knows > they're overridden by its own IOMMU mapping. > That would be pretty ideal from my PoV BR, -R
