On Wed, Apr 11, 2018 at 01:03:59PM +0100, Robin Murphy wrote: > On 10/04/18 21:59, Sinan Kaya wrote: >> Code is expecing to observe the same number of buffers returned from >> dma_map_sg() function compared to sg_alloc_table_from_pages(). This >> doesn't hold true universally especially for systems with IOMMU. > > So why not fix said code? It's clearly not a real hardware limitation, and > the map_sg() APIs have potentially returned fewer than nents since forever, > so there's really no excuse.
Yes, relying on dma_map_sg returning the same number of entries as passed it is completely bogus. >> IOMMU driver tries to combine buffers into a single DMA address as much >> as it can. The right thing is to tell the DMA layer how much combining >> IOMMU can do. > > Disagree; this is a dodgy hack, since you'll now end up passing > scatterlists into dma_map_sg() which already violate max_seg_size to begin > with, and I think a conscientious DMA API implementation would be at rights > to fail the mapping for that reason (I know arm64 happens not to, but that > was a deliberate design decision to make my life easier at the time). Agreed.