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

[Lu, Brent]

> 
> Sorry for the late reply. CRAS does not set the period size when using it.
> The default period size is 256, which consumes the samples quickly(about 49627
> fps when the rate is 48000 fps) at the beginning of the playback.
> Since CRAS write samples with the fixed frequency, it triggers underruns
> immidiately.
> 
> 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. Need Brent to confirm it.)
> 
> Unfortunately, we can not change the behavior of DSP. After some experiments,
> we found that the issue can be fixed if we set the period size to 240. With the
> same frequency as the DSP, the samples are consumed stably. Because everyone
> can trigger this issue when using the driver without setting the period size, we
> think it is a general issue that should be fixed in the kernel.

I check the code and just realized CRAS does nothing but request maximum buffer
size. As I know the application needs to decide the buffer time and period time so
ALSA could generate a hw_param structure with proper period size instead of using
fixed constraint in machine driver because driver has no idea about the latency you
want.

You can use snd_pcm_hw_params_set_buffer_time_near() and
snd_pcm_hw_params_set_period_time_near() to get a proper configuration of
buffer and period parameters according to the latency requirement. In the CRAS
code, there is a UCM variable to support this: DmaPeriodMicrosecs. I tested it on
Celes and it looks quite promising. It seems to me that adding constraint in machine
driver is not necessary.

SectionDevice."Speaker".0 {
	Value {
		PlaybackPCM "hw:chtrt5650,0"
		DmaPeriodMicrosecs "5000"
...

[   52.434761] sound pcmC1D0p: hw_param
[   52.434767] sound pcmC1D0p:   ACCESS 0x1
[   52.434770] sound pcmC1D0p:   FORMAT 0x4
[   52.434772] sound pcmC1D0p:   SUBFORMAT 0x1
[   52.434776] sound pcmC1D0p:   SAMPLE_BITS [16:16]
[   52.434779] sound pcmC1D0p:   FRAME_BITS [32:32]
[   52.434782] sound pcmC1D0p:   CHANNELS [2:2]
[   52.434785] sound pcmC1D0p:   RATE [48000:48000]
[   52.434788] sound pcmC1D0p:   PERIOD_TIME [5000:5000]
[   52.434791] sound pcmC1D0p:   PERIOD_SIZE [240:240]
[   52.434794] sound pcmC1D0p:   PERIOD_BYTES [960:960]
[   52.434797] sound pcmC1D0p:   PERIODS [852:852]
[   52.434799] sound pcmC1D0p:   BUFFER_TIME [4260000:4260000]
[   52.434802] sound pcmC1D0p:   BUFFER_SIZE [204480:204480]
[   52.434805] sound pcmC1D0p:   BUFFER_BYTES [817920:817920]
[   52.434808] sound pcmC1D0p:   TICK_TIME [0:0]

Regards,
Brent

> 
> Thanks,
> Yu-Hsuan