On Wed, Jul 25, 2018 at 10:47 PM Matwey V. Kornilov <matwey@xxxxxxxxxx> wrote: > > 2018-07-24 23:55 GMT+03:00 Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>: > > On Tue, 24 Jul 2018, Matwey V. Kornilov wrote: > > > >> 2018-07-23 21:57 GMT+03:00 Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>: > >> > 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. > >> > >> Well, I measured the following at armv7l. The handler execution time > >> (URB_NO_TRANSFER_DMA_MAP is used for all cases): > >> > >> 1) coherent DMA: ~3000 usec (pwc is not functional) > >> 2) explicit dma_unmap and dma_map in the handler: ~52 usec > >> 3) explicit dma_sync_single_for_cpu (no dma_sync_single_for_device): ~56 usec That's very strange because on ARM dma_unmap_single() does exactly the same thing as dma_sync_single_for_cpu(): arm_dma_map_page() https://elixir.bootlin.com/linux/v4.18-rc7/source/arch/arm/mm/dma-mapping.c#L129 arm_dma_unmap_page() https://elixir.bootlin.com/linux/v4.18-rc7/source/arch/arm/mm/dma-mapping.c#L159 arm_dma_sync_single_for_cpu() https://elixir.bootlin.com/linux/v4.18-rc7/source/arch/arm/mm/dma-mapping.c#L167 Could you post the code you used for testing of cases 2) and 3)? Best regards, Tomasz
