Menno van der Graaf menno.vandergraaf at
Mon Feb 19 18:15:41 UTC 2024

Hey all,

I'm running into trouble with ModemManager, there seems to be no way to disable probing for certain devices.
I have found countless threads discussing the lack of this feature, but none of them refer to my specific troubles.

We have some CDC devices that implement ACM and ether.
We want to communicate on the ttyACM of these devices, but our data keeps getting corrupted.
After many hours we finally found out ModemManager is probing the devices, so we added the udev rules:

/etc/udev/rules.d/78-custom-mm-blacklist-internal-modem.rules :

ACTION!="add|change|move", GOTO="custom_mm_blacklist_internal_modem_end"
ATTRS{idVendor}=="[redacted1]" ATTRS{idProduct}=="[redacted2]", ENV{ID_MM_DEVICE_IGNORE}="1"
ATTRS{idVendor}=="[redacted1]" ATTRS{idProduct}=="[redacted3]", ENV{ID_MM_DEVICE_IGNORE}="1"

but looking at the logs I see:

ModemManager[3751]: <debug> [filter] (tty/ttyACM0): port filtered: device is blocklisted
ModemManager[3751]: <debug> [filter] (net/enxd[redacted4]) port allowed: net device
ModemManager[3751]: <debug> [filter] (net/enxd[redacted4]) port allowed: net device
ModemManager[3751]: <debug> [filter] (tty/ttyACM0): port allowed: device also exports a net interface

So if ModemManager successfully found the device is blocklisted, why does it continue to probe the port?
What is the recommended mechanism to disable the probing of these devices?

I would prefer to exclude these specific devices, and not have to resort to heavy handed solutions that alter the global rules.

Best Regards,

Menno van der Graaf
Software Developer

Snom Technology GmbH
Phone: +49 30 39833 1708
Email: menno.vandergraaf at<mailto:menno.vandergraaf at>

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