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

Matwey V. Kornilov
2018-08-04T10:00:13Z

2018-07-30 18:35 GMT+03:00 Laurent Pinchart <laurent.pinchart@ideasonboard.com>:
> Hi Matwey,
>
> 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.

>
>> 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.

Kconfig won't work here, as I said before, DMA config is stored inside
device tree blob on ARM architecture.

>
>> 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.

>
> --
> Regards,
>
> Laurent Pinchart
>
>
>

--
With best regards,
Matwey V. Kornilov.
Sternberg Astronomical Institute, Lomonosov Moscow State University, Russia
119234, Moscow, Universitetsky pr-k 13, +7 (495) 9392382