On Mon, 23 Jul 2018, Matwey V. Kornilov wrote: > I've tried to strategies: > > 1) Use dma_unmap and dma_map inside the handler (I suppose this is > similar to how USB core does when there is no URB_NO_TRANSFER_DMA_MAP) Yes. > 2) Use sync_cpu and sync_device inside the handler (and dma_map only > once at memory allocation) > > It is interesting that dma_unmap/dma_map pair leads to the lower > overhead (+1us) than sync_cpu/sync_device (+2us) at x86_64 platform. > At armv7l platform using dma_unmap/dma_map leads to ~50 usec in the > handler, and sync_cpu/sync_device - ~65 usec. > > However, I am not sure is it mandatory to call > dma_sync_single_for_device for FROM_DEVICE direction? According to Documentation/DMA-API-HOWTO.txt, the CPU should not write to a DMA_FROM_DEVICE-mapped area, so dma_sync_single_for_device() is not needed. Alan Stern
