Hi Matwey, On Tuesday, 24 July 2018 21:56:09 EEST Matwey V. Kornilov wrote: > 2018-07-23 21:57 GMT+03:00 Alan Stern: > > 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 I really don't understand why the sync option is slower. Could you please investigate ? Before doing anything we need to make sure we have a full understanding of the problem. > So, I suppose that unfortunately Tomasz suggestion doesn't work. There > is no performance improvement when dma_sync_single is used. > > At x86_64 the following happens: > > 1) coherent DMA: ~2 usec What do you mean by coherent DMA for x86_64 ? Is that usb_alloc_coherent() ? Could you trace it to see how memory is allocated exactly, and how it's mapped to the CPU ? I suspect that it will end up in dma_direct_alloc() but I'd like a confirmation. > 2) explicit dma_unmap and dma_map in the handler: ~3.5 usec > 3) explicit dma_sync_single_for_cpu (no dma_sync_single_for_device): ~4 usec > > So, whats to do next? Personally, I think that DMA streaming API > introduces not so great overhead. It might not be very large, but with USB3 cameras at high resolutions and framerates, it might still become noticeable. I wouldn't degrade performances on x86, especially if we can decide which option to use based on the platform (or perhaps even better based on Kconfig options such as DMA_NONCOHERENT). > Does anybody happy with turning to streaming DMA or I'll introduce > module-level switch as Ezequiel suggested? A module-level switch isn't a good idea, it will just confuse users. We need to establish a strategy and come up with a good heuristic that can be applied at compile and/or runtime to automatically decide how to allocate buffers. -- Regards, Laurent Pinchart
