Messages in this thread | | | Date | Wed, 8 Aug 2018 10:20:21 -0400 (EDT) | From | Alan Stern <> | Subject | Re: [RFC PATCH v1] media: uvcvideo: Cache URB header data before processing |
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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
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