2009/11/05 pl bossart bossart.nospam@gmail.com

Hi, I have a very low-*latency* full-duplex application where I would like up- and downlink to be synchronous, i.e. period events happen at the same time for capture and playback. This would minimize the number of wake-ups and enable the platform to go to low power.

What kind of full-duplex application and how low latency your application need ?

I thought snd_pcm_link() would enable this coupling between capture and playback, but it looks like it only buids a list of linked devices and uses this information for state transitions. Said transitions don't look quite atomic by looking at the code. The alsa-lib/test/*latency*.c code even tries to measure the difference between the number of samples read/written. In short the question is whether I can force a synchronous behavior from my application, or if I need to roll up my sleeves and fix the drivers to see this behavior?

http://thread.gmane.org/gmane.linux.alsa.devel/37367/focus=37388

To get low latency , your application have to use small period size (i.e. your system is unlikely go to low power mode )

BTW. do you know how PA calculate "Final latency" ?

D: memblockq.c: memblockq requested: maxlength=4194304, tlength=44100, base=4, prebuf=44100, minreq=3528 maxrewind=0 D: memblockq.c: memblockq sanitized: maxlength=4194304, tlength=44100, base=4, prebuf=44100, minreq=3528 maxrewind=0 I: protocol-native.c: Final latency 460.00 ms = 210.00 ms + 2*20.00 ms + 210.00 ms D: module-alsa-sink.c: latency set to 46.44ms D: module-alsa-sink.c: hwbuf_unused_frames=1 D: module-alsa-sink.c: setting avail_min=4662 D: module-alsa-sink.c: Requesting rewind due to latency change.

Another solution I thought of would be to disable the period events on one of the devices, and handle both devices at once when one period event fires. But I am not sure you can disable all events for an audio device (was on the wish list of Lennart, not sure if it became reality?) Thanks for your feedback.

• Pierre