zalsa plugin not working with Loopback ALSA device

[farshield]

No sound when using zalsa_in plugin with a Loopback ALSA card.

Steps to reproduce:

/usr/bin/jackd -dalsa -dhw:PCH -r48000 -p256 -n2 -i2 -o2

sudo modprobe snd-aloop pcm_substreams=2
jack_load zalsa_in -i "-d hw:Loopback,1,0 -L -r 48000"

jack_connect zita-a2j:capture_1 system:playback_1
jack_connect zita-a2j:capture_2 system:playback_2

aplay -D hw:Loopback,0,0 -f S16_LE -r 48000 -c 2 < /dev/urandom

Starting aplay doesn't produce any sound. Using zita-a2j (v0.8.4) instead of zalsa_in plugin, produces the following messages:

zita-a2j -v -d hw:Loopback,1,0 -r 48000 -L
playback : not enabled
capture  :
  nchan  : 2
  fsamp  : 48000
  fsize  : 256
  nfrag  : 2
  format : S16_LE
Starting synchronisation.
   0.876   0.999698     0
   0.305   0.999683   411
  -0.274   0.999853   409
  -0.192   0.999890   408
   0.024   0.999841   407
Starting synchronisation.     <-- aplay was started at this point
Starting synchronisation. 
(repeats for ever)

Environment:

• Fedora 39
• jackd v1.9.22
• jack-tools-server v4.1
• snd_aloop v6.9.8

[farshield]

Using loopback devices with Zalsa/Zita won't work because:

"ALSA's 'software' devices do not behave in the same way as the hw:. ones although ideally they should.
Zita-a2j/j2a is for use with real hardware devices only, as stated in the man pages."
Author of zita-a2jbridge

Instead, it's better to just use the ALSA PCM plugin for Jack: https://jackaudio.org/faq/routing_alsa.html