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

[Matwey V. Kornilov]

2018-08-05 10:49 GMT+03:00 Christoph Hellwig <hch@xxxxxxxxxxxxx>:
> On Sat, Aug 04, 2018 at 10:46:35AM -0400, Alan Stern wrote:
>> > 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.
>>
>> Taken together, those measurements look like a pretty good argument for
>> always using dma_sync_single_for_cpu in the driver.  Provided results
>> on other platforms aren't too far out of line with these results.
>
> Logically speaking on no-mmio no-swiotlb platforms dma_sync_single_for_cpu
> and dma_unmap should always be identical.  With the migration towards
> everyone using dma-direct and dma-noncoherent this is actually going to
> be enforced, and I plan to move that enforcement to common code in the
> next merge window or two.
>

I think that Alan means that using dma_sync_single_for_cpu() we save
time required for subsequent dma_map() call (which is required when we
do dma_unmap()).

So, does everybody happy with dma_sync_single_for_cpu() for now? If
so, then I'll prepare new version of the patch series.

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