Hi Robin, On 2019-05-22 15:55, Robin Murphy wrote: > On 22/05/2019 14:34, Christoph Hellwig wrote: >> On Wed, May 22, 2019 at 02:25:38PM +0100, Robin Murphy wrote: >>> Sure, but that should be irrelevant since the effective problem here >>> is in >>> the sync_*_for_cpu direction, and it's the unmap which nobbles the >>> buffer. >>> If the driver does this: >>> >>> dma_map_single(whole buffer); >>> <device writes to part of buffer> >>> dma_unmap_single(whole buffer); >>> <contents of rest of buffer now undefined> >>> >>> then it could instead do this and be happy: >>> >>> dma_map_single(whole buffer, SKIP_CPU_SYNC); >>> <device writes to part of buffer> >>> dma_sync_single_for_cpu(updated part of buffer); >>> dma_unmap_single(whole buffer, SKIP_CPU_SYNC); >>> <contents of rest of buffer still valid> >> >> Assuming the driver knows how much was actually DMAed this would >> solve the issue. Horia, does this work for you? > > Ohhh, and now I've just twigged what you were suggesting - your > DMA_ATTR_PARTIAL flag would mean "treat this as a read-modify-write of > the buffer because we *don't* know exactly which parts the device may > write to". So indeed if we did go down that route we wouldn't need any > of the sync stuff I was worrying about (but I might suggest naming it > DMA_ATTR_UPDATE instead). Apologies for being slow :)
Don't we have DMA_BIDIRECTIONAL for such case? Maybe we should update documentation a bit to point that DMA_FROM_DEVICE expects the whole buffer to be filled by the device? Best regards -- Marek Szyprowski, PhD Samsung R&D Institute Poland _______________________________________________ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu