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Date:   Wed, 12 Aug 2020 16:55:29 +0200
From:   Takashi Iwai <tiwai@...e.de>
To:     Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>
Cc:     Yu-Hsuan Hsu <yuhsuan@...omium.org>,
        Guennadi Liakhovetski <guennadi.liakhovetski@...ux.intel.com>,
        "alsa-devel@...a-project.org" <alsa-devel@...a-project.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Kuninori Morimoto <kuninori.morimoto.gx@...esas.com>,
        Kai Vehmanen <kai.vehmanen@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Rojewski, Cezary" <cezary.rojewski@...el.com>,
        Jie Yang <yang.jie@...ux.intel.com>,
        Takashi Iwai <tiwai@...e.com>,
        Liam Girdwood <liam.r.girdwood@...ux.intel.com>,
        Sam McNally <sammc@...omium.org>,
        Mark Brown <broonie@...nel.org>,
        Ranjani Sridharan <ranjani.sridharan@...ux.intel.com>,
        Daniel Stuart <daniel.stuart14@...il.com>,
        "yuhsuan@...gle.com" <yuhsuan@...gle.com>,
        "Lu, Brent" <brent.lu@...el.com>,
        Damian van Soelen <dj.vsoelen@...il.com>
Subject: Re: [PATCH v3 2/2] ASoC: Intel: Add period size constraint on strago board

On Wed, 12 Aug 2020 16:46:40 +0200,
Pierre-Louis Bossart wrote:
> 
> 
> >>>>>> 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@...x:~# 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@...x:~# 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

What if with --period-size=256 --buffer-size=512 and --test-position?
Can you reproduce the problem in your side?

> 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?

I also wonder what's really missing, too :)

BTW, I took a look back at the thread, and CRAS seems using a very
large buffer, namely:
[   52.434791] sound pcmC1D0p:   PERIOD_SIZE [240:240]
[   52.434802] sound pcmC1D0p:   BUFFER_SIZE [204480:204480]


Takashi

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