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LM940 No at ports after reset

Hello,

I have several Telit LM940s that I'm using with Modem Manager. On start all modems have AT ports listed using mmmcli -m <modem number>:

           |             ports: ttyUSB17 (unknown), ttyUSB18 (at), ttyUSB19 (at), 
           |                    ttyUSB15 (unknown), cdc-wdm1 (qmi), ce3p0 (net), ttyUSB16 (unknown)

After resetting the modem using mmmcli -m <modem number> -r the AT ports no longer appear:

           |             ports: ttyUSB17 (unknown), ce3p0 (net), ttyUSB15 (unknown), 
           |                    ttyUSB16 (unknown), cdc-wdm1 (qmi)

The AT ports can still be used manually using screen to send AT commands since the USB port is still visible in /dev.

The only way I have found to force Modem Manager to pick up the AT ports after restart is using systemctl restart ModemManager.

I found some details while debugging:

ModemManager[20208]: <debug> [1599749545.042400] (ttyUSB8) opening serial port...
ModemManager[20208]: <debug> [1599749545.047634] (ttyUSB8): setting up baudrate: 57600
ModemManager[20208]: <debug> [1599749545.047684] (ttyUSB8): no flow control explicitly requested for device
ModemManager[20208]: <debug> [1599749545.047711] (ttyUSB8): port attributes not fully set
ModemManager[20208]: <debug> [1599749545.047755] (ttyUSB8) device open count is 1 (open)
ModemManager[20208]: <debug> [1599749545.047796] (ttyUSB8): port attributes not fully set
ModemManager[20208]: <debug> [1599749545.047947] (ttyUSB9) opening serial port...
ModemManager[20208]: <debug> [1599749545.048703] (ttyUSB9): setting up baudrate: 57600
ModemManager[20208]: <debug> [1599749545.048748] (ttyUSB9): no flow control explicitly requested for device
ModemManager[20208]: <debug> [1599749545.048774] (ttyUSB9): port attributes not fully set
ModemManager[20208]: <debug> [1599749545.048814] (ttyUSB9) device open count is 1 (open)
ModemManager[20208]: <debug> [1599749545.048854] (ttyUSB9): port attributes not fully set

#SKIPED SOME LINES

ModemManager[20208]: <debug> [1599749545.066220] [plugin manager] task 11: extra probing time elapsed
ModemManager[20208]: <debug> [1599749545.066284] [plugin Manager] task 11: still 3 running probes (3 active): ttyUSB8, ttyUSB9, cdc-wdm3
ModemManager[20208]: <debug> [1599749545.148610] (ttyUSB8): --> 'AT<CR>'
ModemManager[20208]: <debug> [1599749545.149947] (ttyUSB9): --> 'AT<CR>'
ModemManager[20208]: <debug> [1599749545.998579] [plugin manager] task 11: min probing time elapsed
ModemManager[20208]: <debug> [1599749545.998647] [plugin Manager] task 11: still 3 running probes (3 active): ttyUSB8, ttyUSB9, cdc-wdm3
ModemManager[20208]: transaction 0x2 aborted, but message is not abortable

#SKIPED SOME LINES

ModemManager[20208]: <debug> [1599749551.122316] Parsing AT got: 'Sending command failed: 'Resource temporarily unavailable''
ModemManager[20208]: <debug> [1599749551.122404] [plugin manager] task 11,ttyUSB8: ignoring port unsupported by physical modem's plugin
ModemManager[20208]: <debug> [1599749551.122436] [plugin manager] task 11,ttyUSB8: finished in '7.616042' seconds
ModemManager[20208]: <debug> [1599749551.122472] [plugin manager] task 11,ttyUSB8: assuming port can be handled by the 'Telit' plugin
ModemManager[20208]: <debug> [1599749551.122505] [plugin Manager] task 11: still 2 running probes (2 active): ttyUSB9, cdc-wdm3
ModemManager[20208]: <debug> [1599749551.122575] (ttyUSB8) device open count is 0 (close)
ModemManager[20208]: <debug> [1599749551.122608] (ttyUSB8) closing serial port...
ModemManager[20208]: <debug> [1599749551.123382] (ttyUSB8) serial port closed
ModemManager[20208]: <debug> [1599749551.123475] (ttyUSB8) forced to close port
ModemManager[20208]: <debug> [1599749551.123537] Parsing AT got: 'Sending command failed: 'Resource temporarily unavailable''
ModemManager[20208]: <debug> [1599749551.123577] [plugin manager] task 11,ttyUSB9: ignoring port unsupported by physical modem's plugin
ModemManager[20208]: <debug> [1599749551.123596] [plugin manager] task 11,ttyUSB9: finished in '7.618339' seconds
ModemManager[20208]: <debug> [1599749551.123641] [plugin manager] task 11,ttyUSB9: assuming port can be handled by the 'Telit' plugin
ModemManager[20208]: <debug> [1599749551.123673] [plugin Manager] task 11: still 1 running probes (1 active): cdc-wdm3
ModemManager[20208]: <debug> [1599749551.123733] (ttyUSB9) device open count is 0 (close)
ModemManager[20208]: <debug> [1599749551.123754] (ttyUSB9) closing serial port...
ModemManager[20208]: <debug> [1599749551.124411] (ttyUSB9) serial port closed
ModemManager[20208]: <debug> [1599749551.124458] (ttyUSB9) forced to close port
ModemManager[20208]: transaction 0x8 aborted, but message is not abortable
ModemManager[20208]: [/dev/cdc-wdm3] sent message...

Different debugging session:

ModemManager[15826]: <warn>  [1599828289.201003] Could not grab port (tty/ttyUSB8): 'Cannot add port 'tty/ttyUSB8', unhandled serial type'
ModemManager[15826]: <warn>  [1599828289.201030] Could not grab port (tty/ttyUSB9): 'Cannot add port 'tty/ttyUSB9', unhandled serial type'

Please let me know if you require any further information.

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