[alsa-devel] [RFC] ASoC: soc-pcm: Use different sequence for start/stop trigger

[Mark Brown]

On Tue, Sep 24, 2019 at 02:41:46PM +0300, Peter Ujfalusi wrote:

> It can only fixed by using different sequence within trigger for 'stop' and
> 'start':
> case SNDRV_PCM_TRIGGER_START:
> case SNDRV_PCM_TRIGGER_RESUME:
> case SNDRV_PCM_TRIGGER_PAUSE_RELEASE:
> 	Start DMA first followed by CPU DAI (currently used sequence)
> 
> case SNDRV_PCM_TRIGGER_STOP:
> case SNDRV_PCM_TRIGGER_SUSPEND:
> case SNDRV_PCM_TRIGGER_PAUSE_PUSH:
> 	Stop CPU DAI first followed by DMA

Yeah, this makes sense I think.

> If I think about the issue, I'm not sure why it was not noticed before as the
> behavior makes sense:
> we stop the DMA first then we stop the CPU DAI. If between the DMA stop and DAI
> stop we would need a sample in the DAI (which is still running) then for sure we
> will underrun in the HW (or overrun in case of capture).

There are a bunch of systems where the trigger only actually does
anything with one or the other of the IPs and the startup for the
other is handled by a hardware signal so the ordering doesn't
really matter for them.

> Not sure if anyone else have seen such underrun/overrun when stopping a stream,
> but the fact that I have seen it with both UDMA+PDMA and EDMA on different
> platforms makes me wonder if the issue can be seen on other platforms as well.

I'd guess so, especially with smaller buffers.
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