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SubjectRe: [RFC PATCH v1] media: uvcvideo: Cache URB header data before processing
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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