Re: [PATCH 2/2] media: usb: pwc: Don't use coherent DMA buffers for ISO transfer

Laurent Pinchart
2018-08-09T12:51:35Z

Hi Tomasz,

On Thursday, 9 August 2018 05:36:46 EEST Tomasz Figa wrote:
> On Thu, Aug 9, 2018 at 7:31 AM Laurent Pinchart wrote:
> > On Saturday, 4 August 2018 11:00:05 EEST Matwey V. Kornilov wrote:
> >> 2018-07-30 18:35 GMT+03:00 Laurent Pinchart:
> >>> On Tuesday, 24 July 2018 21:56:09 EEST Matwey V. Kornilov wrote:
> >>>> 2018-07-23 21:57 GMT+03:00 Alan Stern:
> >>>>> On Mon, 23 Jul 2018, Matwey V. Kornilov wrote:
> >>>>>> I've tried to strategies:
> >>>>>>
> >>>>>> 1) Use dma_unmap and dma_map inside the handler (I suppose this is
> >>>>>> similar to how USB core does when there is no
> >>>>>> URB_NO_TRANSFER_DMA_MAP)
> >>>>>
> >>>>> Yes.
> >>>>>
> >>>>>> 2) Use sync_cpu and sync_device inside the handler (and dma_map
> >>>>>> only once at memory allocation)
> >>>>>>
> >>>>>> It is interesting that dma_unmap/dma_map pair leads to the lower
> >>>>>> overhead (+1us) than sync_cpu/sync_device (+2us) at x86_64
> >>>>>> platform. At armv7l platform using dma_unmap/dma_map leads to ~50
> >>>>>> usec in the handler, and sync_cpu/sync_device - ~65 usec.
> >>>>>>
> >>>>>> However, I am not sure is it mandatory to call
> >>>>>> dma_sync_single_for_device for FROM_DEVICE direction?
> >>>>>
> >>>>> According to Documentation/DMA-API-HOWTO.txt, the CPU should not
> >>>>> write to a DMA_FROM_DEVICE-mapped area, so
> >>>>> dma_sync_single_for_device() is not needed.
> >>>>
> >>>> Well, I measured the following at armv7l. The handler execution time
> >>>> (URB_NO_TRANSFER_DMA_MAP is used for all cases):
> >>>>
> >>>> 1) coherent DMA: ~3000 usec (pwc is not functional)
> >>>> 2) explicit dma_unmap and dma_map in the handler: ~52 usec
> >>>> 3) explicit dma_sync_single_for_cpu (no dma_sync_single_for_device):
> >>>> ~56 usec
> >>>
> >>> I really don't understand why the sync option is slower. Could you
> >>> please investigate ? Before doing anything we need to make sure we have
> >>> a full understanding of the problem.
> >>
> >> Hi,
> >>
> >> I've found one drawback in my measurements. I forgot to fix CPU
> >> frequency at lowest state 300MHz. Now, I remeasured
> >>
> >> 2) dma_unmap and dma_map in the handler:
> >> 2A) dma_unmap_single call: 28.8 +- 1.5 usec
> >> 2B) memcpy and the rest: 58 +- 6 usec
> >> 2C) dma_map_single call: 22 +- 2 usec
> >> Total: 110 +- 7 usec
> >>
> >> 3) dma_sync_single_for_cpu
> >> 3A) dma_sync_single_for_cpu call: 29.4 +- 1.7 usec
> >> 3B) memcpy and the rest: 59 +- 6 usec
> >> 3C) noop (trace events overhead): 5 +- 2 usec
> >> Total: 93 +- 7 usec
> >>
> >> So, now we see that 2A and 3A (as well as 2B and 3B) agree good within
> >> error ranges.
> >
> > Thank you for the time you've spent on these measurements, the information
> > is useful and your work very appreciated.
> >
> >>>> So, I suppose that unfortunately Tomasz suggestion doesn't work.
> >>>> There is no performance improvement when dma_sync_single is used.
> >>>>
> >>>> At x86_64 the following happens:
> >>>>
> >>>> 1) coherent DMA: ~2 usec
> >>>
> >>> What do you mean by coherent DMA for x86_64 ? Is that
> >>> usb_alloc_coherent() ? Could you trace it to see how memory is allocated
> >>> exactly, and how it's mapped to the CPU ? I suspect that it will end up
> >>> in dma_direct_alloc() but I'd like a confirmation.
> >>
> >> usb_alloc_coherents() ends up inside hcd_buffer_alloc() where
> >> dma_alloc_coherent() is called. Keep in mind, that requested size is
> >> 9560 in our case and pool is not used.
> >>
> >>>> 2) explicit dma_unmap and dma_map in the handler: ~3.5 usec
> >>>> 3) explicit dma_sync_single_for_cpu (no dma_sync_single_for_device):
> >>>> ~4 usec
> >>>>
> >>>> So, whats to do next? Personally, I think that DMA streaming API
> >>>> introduces not so great overhead.
> >>>
> >>> It might not be very large, but with USB3 cameras at high resolutions
> >>> and framerates, it might still become noticeable. I wouldn't degrade
> >>> performances on x86, especially if we can decide which option to use
> >>> based on the platform (or perhaps even better based on Kconfig options
> >>> such as DMA_NONCOHERENT).
> >>
> >> PWC is discontinued chip, so there will not be any new USB3 cameras.
> >
> > You're right. I had in mind other USB cameras that would benefit from the
> > same change, and in particular the uvcvideo driver, which is used by USB3
> > cameras.
> >
> >> Kconfig won't work here, as I said before, DMA config is stored inside
> >> device tree blob on ARM architecture.
> >
> > But couldn't we skip it at least on x86 ?
>
> If we use the map-once, sync-repeatedly approach, would there be
> anything to gain on x86? I believe the sync ops there would be
> effectively no-ops, so the only overhead would be of a function call.

With that approach, and iff they're effectively no-ops, that should be fine.
We thus need to double-check.

--
Regards,

Laurent Pinchart