Re: [PATCH v2] ASoC: fsl: sai: Fix underrrun for playback stream start

From: Daniel Baluta
Date: Mon Mar 11 2019 - 03:24:14 EST

On Fri, Mar 8, 2019 at 10:59 PM Nicolin Chen <nicoleotsuka@xxxxxxxxx> wrote:
> On Fri, Mar 08, 2019 at 05:39:30PM +0000, Daniel Baluta wrote:
> > @@ -542,6 +544,11 @@ static int fsl_sai_trigger(struct snd_pcm_substream *substream, int cmd,
> > + for (i = 0; tx && i < channels; i++)
> > + regmap_write(sai->regmap, FSL_SAI_TDR, 0x0);
> Though it could reduce underrrun for sure, I don't feel it's a
> thorough fix, especially when dealing with a duplex case where
> RX is the first stream while the driver enables both TE and RE
> at the same time. So the transmitter might have started before
> TX DMA request gets enabled.

I see your point here. This is trickier than I expected.
> We actually have something similar in the ESAI driver as that
> one is well documented. But SAI is apparently more tricky at
> the trigger() function. And the paragraph you pasted from the
> RM doesn't explicitly sound like we should do this.
> Btw, Instead of writing dummy data, have you tried polling the
> WFP pointer of the TFR0 register to make sure data is copied
> by the DMA?

This is a good suggestion. Will give it a try.

> > + if (tx)
> > + udelay(10);
> Any justification for the delay? Missing a line of comments? I
> guess it's to address the 3-bit clocks timing. However, putting
> it before enabling DMA request doesn't make much sense to me.

Indeed. Will come back with a better patch in v3.