Modemmanager probes non-modem TTY devices
Submitted by Aleksander Morgado
Assigned to ModemManager bug user
Description
Originally reported at: https://bugzilla.gnome.org/show_bug.cgi?id=688213
The original bugreport in gnome.org has some very useful comments on the issue, which you should read if you're interested in this issue; e.g.: https://bugzilla.gnome.org/show_bug.cgi?id=688213#c23
The current logic to decide whether a TTY is to be probed by ModemManager goes more or less as follows:
- Platform ports (real RS232 ports, if any) need to be explicitly Whitelisted if they should be probed by ModemManager. RS232 devices are no longer very much used by consumer users, but they are still very used in the industry.
- TTY ports which are explicitly Blacklisted in ModemManager will never get probed.
- TTY ports which are Greylisted in ModemManager will never get probed automatically; they will only get probed if explicitly requested to do so via a manual Scan() call.
- Otherwise, the port will get probed.
This strategy is not ideal, as new USB devices that expose TTYs will need to be explicitly blacklisted/greylisted in ModemManager so that they don't get probed.
This issue may be especially relevant with e.g. serial screen-readers for people with disabilities; if MM takes over the TTY, even just for AT probing for some seconds, the main software wanting to use the screen-reader may break. There is of course other kind of devices affected (e.g. devboards with microcontrollers accessed via serial).
Some of the solutions suggested in the original thread are specific to the cdc-acm device (e.g. checking capabilities reported by the driver in the kernel), but if a solution is to be found, I guess it should cover every possible driver exposing TTYs, not just one.