Am 12.04.2018 um 08:26 schrieb Christoph Hellwig: > 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. I agree that the common DRM functions should be able to deal with both, but we should let the driver side decide if it wants multiple addresses combined or not. > >>> 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. Sounds like my understanding of max_seg_size is incorrect, what exactly should that describe? Thanks, Christian.
