On Wed, 8 Aug 2018, Laurent Pinchart wrote: > Hello, > > 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. > > > There is no need to wonder. "Frequent DMA mapping/Cached memory" is > > always faster than "No DMA mapping/Uncached memory". > > Is it really, doesn't it depend on the CPU access pattern ? Well, if your access pattern involves transferring data in from the device and then throwing it away without reading it, you might get a different result. :-) But assuming you plan to read the data after transferring it, using uncached memory slows things down so much that the overhead of DMA mapping/unmapping is negligible by comparison. The only exception might be if you were talking about very small amounts of data. I don't know exactly where the crossover occurs, but bear in mind that Matwey's tests required ~50 us for mapping/unmapping and 3000 us for accessing uncached memory. He didn't say how large the transfers were, but that's still a pretty big difference. Alan Stern > > The only issue is that on some platform (such as x86) but not others, > > there is a third option: "No DMA mapping/Cached memory". On platforms > > which support it, this is the fastest option.
