Re: Tearing down DMA transfer setup after DMA client has finished

[Mark Brown]

On Fri, Dec 09, 2016 at 07:23:17PM +0100, Mason wrote:
> On 09/12/2016 18:56, Vinod Koul wrote:

> > Right, but in that case the fallback would be PIO mode, and if that is
> > not availble (IIRC some f your devices don't) then reject the usage with
> > EAGAIN.

> Maybe I'm missing something, but I don't see how that would help.
> Take the NAND Flash controller driver, for instance. PIO is not
> an option, because the ECC engine is tied to DMA.

> And failing with -EAGAIN doesn't help the busy looping situation.
> The caller should be put on some kind of queue to wait for a
> "channel ready" event.

Even without the tie into ECC being an issue it seems like falling back
to PIO could produce poor results at the system level - it's likely that
the PIO would be very expensive and take longer than waiting would've
done, increasing the burden on the CPU and slowing things down overall
for userspace.

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