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

[Pierre-Louis Bossart]

>>> ... Why only 240?  That's the next logical question.
>>> If you have a clarification for it, it may be the rigid reason to
>>> introduce such a hw constraint.
> 
>> According to Brent, the DSP is using 240 period regardless the
>> hw_param. If the period size is 256, DSP will read 256 samples each
>> time but only consume 240 samples until the ring buffer of DSP is
>> full. This behavior makes the samples in the ring buffer of kernel
>> consumed quickly.
> 
>> Not sure whether the explanation is correct. Hi Brent, can you confirm it?
> 
> This seems to be going round and round in circles.  Userspace lets the
> kernel pick the period size, if the period size isn't 240 (or a multiple
> of it?) the DSP doesn't properly pay attention to that apparently due to
> internal hard coding in the DSP firmware which we can't change so the
> constraint logic needs to know about this DSP limitation - it seems like
> none of this is going to change without something new going into the
> mix?  We at least need a new question to ask about the DSP firmware I
> think.

I just tested aplay -Dhw: on a Cyan Chromebook with the Ubuntu kernel 
5.4, and I see no issues with the 240 sample period. Same with 432, 960, 
9600, etc.

I also tried just for fun what happens with 256 samples, and I don't see 
any underflows thrown either, so I am wondering what exactly the problem 
is? Something's not adding up. I would definitively favor multiple of 
1ms periods, since it's the only case that was productized, but there's 
got to me something a side effect of how CRAS programs the hw_params.

root at chrx:~# aplay -Dhw:0,0 --period-size=240 --buffer-size=480 -v 1.wav
Playing WAVE '1.wav' : Signed 16 bit Little Endian, Rate 48000 Hz, Stereo
Hardware PCM card 0 'chtmax98090' device 0 subdevice 0
Its setup is:
   stream       : PLAYBACK
   access       : RW_INTERLEAVED
   format       : S16_LE
   subformat    : STD
   channels     : 2
   rate         : 48000
   exact rate   : 48000 (48000/1)
   msbits       : 16
   buffer_size  : 480
   period_size  : 240
   period_time  : 5000
   tstamp_mode  : NONE
   tstamp_type  : MONOTONIC
   period_step  : 1
   avail_min    : 240
   period_event : 0
   start_threshold  : 480
   stop_threshold   : 480
   silence_threshold: 0
   silence_size : 0
   boundary     : 8646911284551352320
   appl_ptr     : 0
   hw_ptr       : 0

root at chrx:~# aplay -Dhw:0,0 --period-size=256 --buffer-size=512 -v 1.wav
Playing WAVE '1.wav' : Signed 16 bit Little Endian, Rate 48000 Hz, Stereo
Hardware PCM card 0 'chtmax98090' device 0 subdevice 0
Its setup is:
   stream       : PLAYBACK
   access       : RW_INTERLEAVED
   format       : S16_LE
   subformat    : STD
   channels     : 2
   rate         : 48000
   exact rate   : 48000 (48000/1)
   msbits       : 16
   buffer_size  : 512
   period_size  : 256
   period_time  : 5333
   tstamp_mode  : NONE
   tstamp_type  : MONOTONIC
   period_step  : 1
   avail_min    : 256
   period_event : 0
   start_threshold  : 512
   stop_threshold   : 512
   silence_threshold: 0
   silence_size : 0
   boundary     : 4611686018427387904
   appl_ptr     : 0
   hw_ptr       : 0