dma_sync_single_for_cpu takes a really long time

Sylvain Munaut s.munaut at
Sun Jun 28 13:40:03 PDT 2015


I'm working on a DMA driver that uses the the streaming DMA API to
synchronize the access between host and device. The data flow is
exclusively from the device to the host (video grabber).

As such, I call dma_sync_single_for_cpu when the hardware is done
writing a frame to make sure that the cpu gets up to date data when
accessing the zone.

However this call takes a _long_ time to complete. For a 6 Megabytes
buffer, it takes about 13 ms which is just crazy ... at that rate it'd
be faster to just read random data from a random buffer to trash the
measly 512k of cache ...

Is there any alternative that's faster when dealing with large buffers ?

(The platform is a Zynq 7000 - Dual Cortex A9).



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