MOTU M2 stopped working after upgrade to 5.5

[Michael Gebetsroither]

Hi,

I'm using a MOTU M2 as an audio interface for my headset and after upgrading to kernel 5.5 (from 5.4)
the interface stopped working in full duplex mode, means it does not work anymore for me with a headset at all.
(The headset is a sennheiser hmdc27)

If i go back to kernel 5.4 everything works (also without any cracks in the audio output).

It's a bit unfortunate that a working audio interface is suddenly disabled by software.
Is there any way this can be made working again?

I suspect c249177944b650816069f6c49b769baaa94339dc to be the reason for this
"ALSA: usb-audio: add implicit fb quirk for MOTU M Series"

With kernel 5.5 i also get numerous warnings in kernel log like:

| Apr 25 20:23:16 localhost kernel: [ 2631.701391] usb 5-2.3.2: Unable to change format on ep #81: already in use
| Apr 25 20:23:16 localhost kernel: [ 2631.701694] usb 5-2.3.2: Unable to change format on ep #81: already in use
| Apr 25 20:23:16 localhost kernel: [ 2631.701951] usb 5-2.3.2: Unable to change format on ep #81: already in use
| Apr 25 20:23:16 localhost kernel: [ 2631.705521] usb 5-2.3.2: Unable to change format on ep #81: already in use
| ...

For audio output i've used pulseaudio with the following settings in daemon.conf:
| resample-method = src-sinc-best-quality
| default-sample-format = s24le
| default-sample-rate = 96000
| alternate-sample-rate = 48000

And for jack to not produce cracks i've used:
| sample rate: 96khz
| Frames/periode: 256
| Periodes/buffer: 25

greets,
gebi