Improved support for ALSA devices for buffer = 2 x period

[siraaris]

In a recent DIY thread investigation of running CDSP with rate adjustments via Loopback identified a potential issue in CDSP supporting playback devices where the buffer size is limited to 2x period.

A note was mentioned that implementing another thread as in Coreaudio and Wasapi in ALSA would help this case.

I'm creating this issue as placeholder.

Incidentally using the "alsa_optional_params" code to set the capture period to the same as the Loopback play period seems to help reduce xruns.

Ready to assist when/if this issue gets traction!

[siraaris]

A data point. Running with the following configuration:

devices:
multithreaded: true
samplerate: 96000
chunksize: 1920
queuelimit: 4

silence_threshold: -80
silence_timeout: 0

volume_ramp_time: 100.0

enable_rate_adjust: true

adjust_period: 2

capture:
type: Alsa
device: "hw:CARD=Loopback,DEV=1"
format: S32LE
channels: 2
period: 1920
buffersize: 7680

playback:
type: Alsa
device: "plughw:CARD=HDSPMx2673a5,DEV=0"
format: S32LE
channels: 32
period: 2048
buffersize: 4096

xruns are minimal (1 observed in 8 hours!).

1 Loopback/pcm0p/sub0 Loopback/pcm1c/sub0 HDSPMx2673a5/pcm0p/sub0
2 0 0 1
3 1920 1920 2048
4 3840 7680 4096

[siraaris]

Whilst the above does not report xruns, there are clearly audible periodic clicks.

[HEnquist]

Closing in favor of #376