Hi Tomasz, On Wednesday, 8 August 2018 19:27:42 EEST Tomasz Figa wrote: > On Thu, Aug 9, 2018 at 1:21 AM Laurent Pinchart wrote: > > On Wednesday, 8 August 2018 17:20:21 EEST Alan Stern wrote: > >> On Wed, 8 Aug 2018, Keiichi Watanabe wrote: > >>> Hi Laurent, Kieran, Tomasz, > >>> > >>> Thank you for reviews and suggestions. > >>> I want to do additional measurements for improving the performance. > >>> > >>> Let me clarify my understanding: > >>> Currently, if the platform doesn't support coherent-DMA (e.g. ARM), > >>> urb_buffer is allocated by usb_alloc_coherent with > >>> URB_NO_TRANSFER_DMA_MAP flag instead of using kmalloc. > >> > >> Not exactly. You are mixing up allocation with mapping. The speed of > >> the allocation doesn't matter; all that matters is whether the memory > >> is cached and when it gets mapped/unmapped. > >> > >>> This is because we want to avoid frequent DMA mappings, which are > >>> generally expensive. However, memories allocated in this way are not > >>> cached. > >>> > >>> So, we wonder if using usb_alloc_coherent is really fast. > >>> In other words, we want to know which is better: > >>> "No DMA mapping/Uncached memory" v.s. "Frequent DMA mapping/Cached > >>> memory". > > > > The second option should also be split in two: > > > > - cached memory with DMA mapping/unmapping around each transfer > > - cached memory with DMA mapping/unmapping at allocation/free time, and > > DMA sync around each transfer > > > > The second option should in theory lead to at least slightly better > > performances, but tests with the pwc driver have reported contradictory > > results. I'd like to know whether that's also the case with the uvcvideo > > driver, and if so, why. > > I thought that the results from retesting on pwc, after making sure > that cpu frequency stays the same all the time, actually clarified > this and indeed map once, sync repeatedly was the fastest? I had missed that, thank you for the information. I've now caught up with the pwc thread. -- Regards, Laurent Pinchart
