[LAU] Why doesn't Jack work in duplex mode?

[François-Xavier Thomas]

Hello all,

I posted this on Reddit a while ago, but I hope you folks might have some
more luck than people on r/linuxquestions.

I'm trying to find out why Jack2 works with playback-only as well as
capture-only, but not both. If anybody has any clue as to why that is or
how to start debugging it, I'd be grateful!

* `jackd -d alsa -D -d hw:1,0`: Duplex mode, doesn't work.
* `jackd -d alsa -C -d hw:1,0`: Capture-only mode, works.
* `jackd -d alsa -P -d hw:1,0`: Playback-only mode, works.

I'm using the default settings (1024 frames/period with 2 periods/buffer @
48kHz sampling rate). The sound card is a `bytcrrt5640` (`hw:0` is another
card connected to HDMI outputs).

Pulseaudio is using 48000 frames/period with 2 periods/buffer @ 48kHz for
comparison, and appears to work fine on the exact same soundcard (I used
`pacmd list-sinks` and `/proc/asound/card1/**/hw_params` to get that info).

By "doesn't work", I mean : when trying to run `jackd` in duplex mode, a
slight "pop" sound can be heard from the speakers, then the dmesg logs fill
with the following line:

    intel_sst_acpi 808622A8:00: sst: Busy wait failed, cant send this msg

`jackd` then freezes, and nothing short of `kill -9` can make it go away
(which incidentally makes the speakers emit another small "pop").

The dmesg line is the same mentioned in https://lkml.org/lkml/2018/2/18/117,
but I did not find a lot of info on it.

Cheers,
François-Xavier
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