[RFC] dmaengine: omap-dma: Allow DMA controller to prefetch data

[Mark A. Greer]

On Fri, Oct 19, 2012 at 02:45:55PM +0200, Péter Ujfalusi wrote:
> Hi,
> 
> On 10/19/2012 01:33 AM, Russell King - ARM Linux wrote:
> > I would suggest getting some feedback from the ASoC people first, before
> > trying to invent new APIs to work around this stuff.  If they can live
> > with having prefetch enabled on OMAP then there isn't an issue here.  If
> > not, we need a solution to this.
> > 
> > I do not believe that precisely stopping and starting playback across a
> > suspend/resume event is really necessary (it's desirable but the world
> > doesn't collapse if you miss a few samples.)  It could be more of an
> > issue for pause/resume though, but as I say, that's for ASoC people to
> > comment on.
> 
> There is another issue with the prefetch in audio:
> we tend to like to know the position of the DMA and also to know how much data
> we have stored in buffers, FIFOs. This information is used by userspace to do
> echo cancellation and also used by PA for example to do runtime mixing
> directly in the audio buffer. We have means to extract this information from
> McBSP for example (and from tlv320dac33 codec) but AFAIK this information can
> not be retrieved from sDMA.
> We could assume that the sDMA FIFO is kept full and report that as a 'delay'
> or do not account this information.
> 
> For now I think the cyclic mode should not set the prefetch. If I recall right
> the cyclic mode is only used by audio at the moment.
> 
> > I'm merely pointing out here that we need their feedback here before
> > deciding if there's anything further that needs to happen.
> 
> Thanks Russell, I'll take a look at the implication of the prefetch for audio.

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