[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 at 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 at 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?