On Thu, Aug 9, 2018 at 7:31 AM Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx> wrote: > > Hi Matwey, > > On Saturday, 4 August 2018 11:00:05 EEST Matwey V. Kornilov wrote: > > 2018-07-30 18:35 GMT+03:00 Laurent Pinchart: > > > 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. > > > > Hi, > > > > I've found one drawback in my measurements. I forgot to fix CPU > > frequency at lowest state 300MHz. Now, I remeasured > > > > 2) dma_unmap and dma_map in the handler: > > 2A) dma_unmap_single call: 28.8 +- 1.5 usec > > 2B) memcpy and the rest: 58 +- 6 usec > > 2C) dma_map_single call: 22 +- 2 usec > > Total: 110 +- 7 usec > > > > 3) dma_sync_single_for_cpu > > 3A) dma_sync_single_for_cpu call: 29.4 +- 1.7 usec > > 3B) memcpy and the rest: 59 +- 6 usec > > 3C) noop (trace events overhead): 5 +- 2 usec > > Total: 93 +- 7 usec > > > > So, now we see that 2A and 3A (as well as 2B and 3B) agree good within > > error ranges. > > Thank you for the time you've spent on these measurements, the information is > useful and your work very appreciated. > > > >> 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. > > > > usb_alloc_coherents() ends up inside hcd_buffer_alloc() where > > dma_alloc_coherent() is called. Keep in mind, that requested size is > > 9560 in our case and pool is not used. > > > > >> 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). > > > > PWC is discontinued chip, so there will not be any new USB3 cameras. > > You're right. I had in mind other USB cameras that would benefit from the same > change, and in particular the uvcvideo driver, which is used by USB3 cameras. > > > Kconfig won't work here, as I said before, DMA config is stored inside > > device tree blob on ARM architecture. > > But couldn't we skip it at least on x86 ? If we use the map-once, sync-repeatedly approach, would there be anything to gain on x86? I believe the sync ops there would be effectively no-ops, so the only overhead would be of a function call. Best regards, Tomasz
