How to pin USB serial devices to a specific probe order

Hey there folks,

At the dayjob we have some 16-port USB serial console expanders, and we recently also plugged in the USB console for a Netgate firewall, which lacks a traditional DE9 port, but instead has a USB micro port that bridges to its own internal Prolific PL2032 port and presents a usb-to-serial adapter there.  (They also default to 115200, grr).

Here’s the problem: after a reboot, the probe order changed, and the netgate box became ttyU0.

We need them to stay consistently ordered, i.e. such that the device with a given USB ID will always turn out to be ttyU1, and the 16-port pod that has 16 serials will always be ttyU1-ttyU16 (or vice versa).

Is there something one can put in devfs.rules to enforce this, or something that once can do to force a given device to be probed earlier or later in the process?

I imagine there’s a similar issue for USB ethernet devices and the like.

-Dan