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Date: Sun, 5 Aug 2018 00:49:46 -0700
From: Christoph Hellwig <hch@infradead.org>
To: Alan Stern <stern@rowland.harvard.edu>
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Subject: Re: [PATCH 2/2] media: usb: pwc: Don't use coherent DMA buffers for
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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.