The script in the /etc/hotplug.d/usb/10-modem is not being executed when system boots.
I have a modem device powered up from the power button press. Kernel log shows the system finds USB controller 1d6b/2/601, and invokes the script above.
But further log shows USB modem is being detected and its ttyUSBx device files are created, but the hotplug script is not invoked.
If I turn modem device off and then on (manipulating gpios), then yes, hotplug script is executed as expected.
I have found this dated back 2012, and can't believe there's no better solution other than power cycle the modem (= losing time on its initialization and connecting).
Edit: I found logical cause of the issue, but still no idea how to solve it.
On bootup the script is actually called, but it is only called with "bind" action, while on modem power cycle there's also "add" action before the "bind".
Edit 2: the page states for "coldplug" I muse use bind (and thus unbind) instead of add. This is not a big problem for me as long at the script is being executed once for every instance of the device.