My Motu UltraLite AVB is getting a very reliable AVB audio stream with the latest Drumfix avb4linux ALSA driver (
https://github.com/Drumfix/avb4linux) on Arch Linux (core repo kernel 5.11.11.arch1-1). If I go to my Motu UltraLite AVB's routing config page and set one channel to directly loopback the AVB input to the AVB output, and then run the precise time measurement tool jack_delay with that capture port and input port, then I get the following roundtrip latency measurements (which do include the inherent jack buffering latency) at 44.1 kHz for the following buffer_size settings form a 6-frame buffer setting at minimum to a 1024-frame buffer setting at maximum:
6-frame buffer: 44.000 frames 0.998 ms
7-frame buffer: 47.000 frames 1.066 ms
8-frame buffer: 50.000 frames 1.134 ms
9-frame buffer: 53.000 frames 1.202 ms
10-frame buffer: 56.000 frames 1.270 ms
11-frame buffer: 59.000 frames 1.338 ms
12-frame buffer: 62.000 frames 1.406 ms
13-frame buffer: 65.000 frames 1.474 ms
14-frame buffer: 68.000 frames 1.542 ms
15-frame buffer: 71.000 frames 1.610 ms
16-frame buffer: 74.000 frames 1.678 ms
17-frame buffer: 77.000 frames 1.746 ms
18-frame buffer: 80.000 frames 1.814 ms
24-frame buffer: 98.000 frames 2.222 ms
30-frame buffer: 116.000 frames 2.630 ms
32-frame buffer: 122.000 frames 2.766 ms
36-frame buffer: 134.000 frames 3.039 ms
42-frame buffer: 152.000 frames 3.447 ms
48-frame buffer: 170.000 frames 3.855 ms
54-frame buffer: 188.000 frames 4.263 ms
60-frame buffer: 206.000 frames 4.671 ms
64-frame buffer: 218.000 frames 4.943 ms
128-frame buffer: 410.000 frames 9.297 ms
256-frame buffer: 794.000 frames 18.005 ms
512-frame buffer: 1562.000 frames 35.420 ms
1024-frame buffer: 3098.000 frames 70.249 ms
(Note: there is an additional .950 ms latency to go from my UltraLite's physical audio DAC through my headphones into a mic and back into the UItraLite's ADC.)
I get these same latency measurements even if I use the Motu AVB Switch in between my computer's i210 network card and my UltraLite or if directly connect my computer's i210 network card to the UltraLite.
Turns out there is a nice simple relationship between latency in frames & buffer_size:
latency in frames = 26 + 3*buffer_size
(and can calculate the latency in milliseconds by dividing frames by 44.1)
Even on reboot, the latency is deterministic according to that equation. For 48 kHz, I get latency in frames = 28 + 3*buffer_size
I've measured a sine wave sent from my computer to audacity and looped back through the motu and I'm able to get a clean signal back, for at least an hour. (Though I did have a couple xruns in that hour, but that was simply a fault of my cpu failing to keep up). I get quite a few xruns with 6 samples, but at 12 samples it is very reliable with only the occasional xrun on my AMD Ryzen 5 1600 Six-Core Processor computer, though of course your mileage may vary on your computer.
(note: however on the other hand if I try to use my UltraLite AVB over USB, the audio connection is very unreliable and gets lots of glitches)