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

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