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

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

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