Messages in this thread | | | Subject | Re: Device obligation to write into a DMA_FROM_DEVICE streaming DMA mapping | From | Robin Murphy <> | Date | Tue, 21 May 2019 18:27:49 +0100 |
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On 21/05/2019 18:14, Horia Geanta wrote: > Hi, > > Is it mandatory for a device to write data in an area DMA mapped DMA_FROM_DEVICE? > Can't the device just "ignore" that mapping - i.e. not write anything - and > driver should expect original data to be found in that location (since it was > not touched / written to by the device)? > [Let's leave cache coherency aside, and consider "original data" to be in RAM.] > > I am asking this since I am seeing what seems to be an inconsistent behavior / > semantics between cases when swiotlb bouncing is used and when it's not. > > Specifically, the context is: > 1. driver prepares a scatterlist with several entries and performs a > dma_map_sg() with direction FROM_DEVICE > 2. device decides there's no need to write into the buffer pointed by first > scatterlist entry and skips it (writing into subsequent buffers) > 3. driver is notified the device finished processing and dma unmaps the scatterlist > > When swiotlb bounce is used, the buffer pointed to by first scatterlist entry is > corrupted. That's because swiotlb implementation expects the device to write > something into that buffer, however the device logic is "whatever was previously > in that buffer should be used" (2. above). > > For FROM_DEVICE direction: > -swiotlb_tbl_map_single() does not copy data from original location to swiotlb > if (!(attrs & DMA_ATTR_SKIP_CPU_SYNC) && > (dir == DMA_TO_DEVICE || dir == DMA_BIDIRECTIONAL)) > swiotlb_bounce(orig_addr, tlb_addr, size, DMA_TO_DEVICE); > -swiotlb_tbl_unmap_single() copies data from swiotlb to original location > if (orig_addr != INVALID_PHYS_ADDR && > !(attrs & DMA_ATTR_SKIP_CPU_SYNC) && > ((dir == DMA_FROM_DEVICE) || (dir == DMA_BIDIRECTIONAL))) > swiotlb_bounce(orig_addr, tlb_addr, size, DMA_FROM_DEVICE); > and when device did not write anything (as in current situation), it overwrites > original data with zeros > > In case swiotlb bounce is not used and device does not write into the > FROM_DEVICE streaming DMA maping, the original data is available. > > Could you please clarify whether: > -I am missing something obvious OR > -the DMA API documentation should be updated - to mandate for device writes into > FROM_DEVICE mappings) OR > -the swiotlb implementation should be updated - to copy data from original > location irrespective of DMA mapping direction?
Hmm, that certainly feels like a bug in SWIOTLB - it seems reasonable in principle for a device to only partially update a mapped buffer before a sync/unmap, so I'd say it probably should be filling the bounce buffer with the original data at the start, regardless of direction.
Robin.
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