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

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Date:   Wed, 12 Aug 2020 09:46:40 -0500
From:   Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>
To:     Takashi Iwai <tiwai@...e.de>, Yu-Hsuan Hsu <yuhsuan@...omium.org>
Cc:     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


>>>>>> 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

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?