Re: [PATCH v3 2/2] ASoC: Intel: Add period size constraint on strago board

[Pierre-Louis Bossart]

> > > > > > After doing some experiments, I think I can identify the problem more precisely.
> > > > > > 1. aplay can not reproduce this issue because it writes samples
> > > > > > immediately when there are some space in the buffer. However, you can
> > > > > > add --test-position to see how the delay grows with period size 256.
> > > > > > > aplay -Dhw:1,0 --period-size=256 --buffer-size=480 /dev/zero -d 1 -f dat --test-position
> > > > > > Playing raw data '/dev/zero' : Signed 16 bit Little Endian, Rate 48000
> > > > > > Hz, Stereo
> > > > > > Suspicious buffer position (1 total): avail = 0, delay = 2064, buffer = 512
> > > > > > Suspicious buffer position (2 total): avail = 0, delay = 2064, buffer = 512
> > > > > > Suspicious buffer position (3 total): avail = 0, delay = 2096, buffer = 512
> > > > > > ...
> > > > >
> > > > > Isn't this about the alignment of the buffer size against the period
> > > > > size, not the period size itself?  i.e. in the example above, the
> > > > > buffer size isn't a multiple of period size, and DSP can't handle if
> > > > > the position overlaps the buffer size in a half way.
> > > > >
> > > > > If that's the problem (and it's an oft-seen restriction), the right
> > > > > constraint is
> > > > >    snd_pcm_hw_constraint_integer(runtime, SNDRV_PCM_HW_PARAM_PERIODS);
> > > > >
> > > > >
> > > > > Takashi
> > > > Oh sorry for my typo. The issue happens no matter what buffer size is
> > > > set. Actually, even if I want to set 480, it will change to 512
> > > > automatically.
> > > > Suspicious buffer position (1 total): avail = 0, delay = 2064, buffer
> > > > = 512 <-this one is the buffer size
> > >
> > > OK, then it means that the buffer size alignment is already in place.
> > >
> > > And this large delay won't happen if you use period size 240?
> > >
> > >
> > > Takashi
> > Yes! If I set the period size to 240, it will not print "Suspicious
> > buffer position ..."
>
> So it sounds like DSP handles the delay report incorrectly.
> Then it comes to another question: the driver supports both SOF and
> SST.  Is there the behavior difference between both DSPs wrt this
> delay issue?

I still don't get what the issue is. The two following cases work fine 
with the SST/Atom driver:

root@chrx:~# aplay -Dhw:0,0 --period-size=240 --buffer-size=480 
/dev/zero -d 2 -f dat --test-position
Playing raw data '/dev/zero' : Signed 16 bit Little Endian, Rate 48000 
Hz, Stereo
root@chrx:~# aplay -Dhw:0,0 --period-size=960 --buffer-size=4800 
/dev/zero -d 2 -f dat --test-position
Playing raw data '/dev/zero' : Signed 16 bit Little Endian, Rate 48000 
Hz, Stereo

The existing code has this:

	/* Make sure, that the period size is always even */
	snd_pcm_hw_constraint_step(substream->runtime, 0,
			   SNDRV_PCM_HW_PARAM_PERIODS, 2);

	return snd_pcm_hw_constraint_integer(runtime,
			 SNDRV_PCM_HW_PARAM_PERIODS);

and with the addition of period size being a multiple of 1ms all 
requirements should be met?