... 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@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@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
