On Wed, Mar 06, 2024 at 11:05:18AM -0400, Jason Gunthorpe wrote: > > Yes. And this whole proposal also seems clearly confused (not just > > because of the gazillion reposts) but because it mixes up the case > > where we can coalesce CPU regions into a single dma_addr_t range > > (iommu and maybe in the future swiotlb) and one where we need a > > I had the broad expectation that the DMA API user would already be > providing a place to store the dma_addr_t as it has to feed that into > the HW. That memory should simply last up until we do dma unmap and > the cases that need dma_addr_t during unmap can go get it from there. Well. The dma_addr_t needs to be fed into the hardware somehow obviously. But for a the coalesced case we only need one such field, not Nranges. > We can't do much on the map side as single range doesn't imply > contiguous range, P2P and alignment create discontinuities in the > dma_addr_t that still have to be delt with. For alignment the right answer is almost always to require the upper layers to align to the iommu granularity. We've been a bit lax about that due to the way scatterlists are designed, but requiring the proper alignment actually benefits everyone.
