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". There is no need to wonder. "Frequent DMA mapping/Cached memory" is always faster than "No DMA mapping/Uncached memory". 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. Alan Stern
