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

[Laurent Pinchart]

Hi Matwey,

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 ?

> >> Does anybody happy with turning to streaming DMA or I'll introduce
> >> module-level switch as Ezequiel suggested?
> > 
> > A module-level switch isn't a good idea, it will just confuse users. We
> > need to establish a strategy and come up with a good heuristic that can
> > be applied at compile and/or runtime to automatically decide how to
> > allocate buffers.
> 
> I am agree in general, but I cannot understand why webcam driver
> should think about memory allocation heuristics.

I fully agree with you, this should be handled by either the USB core or the 
media core (possibly with a few static hints from the driver, such as buffer 
sizes, to help with heuristics, if needed at all).

-- 
Regards,

Laurent Pinchart