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

Matwey V. Kornilov
2018-07-24T20:04:21Z

2018-07-23 21:57 GMT+03:00 Alan Stern <stern@rowland.harvard.edu>:
> 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

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
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.
Does anybody happy with turning to streaming DMA or I'll introduce
module-level switch as Ezequiel suggested?

>
> Alan Stern
>

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