ModemManager and modem low power with USB/QMI

[Filip Kubicz]

Hi!
I'm using Quectel EC21EUX modem, connected with USB, and QMI to control it
via ModemManager.

I have a question about going to sleep with a TCP connection kept so the
device can wake up.

I have configured the modem to go to low power mode whenever there is no
traffic (AT+QSCLK=1, AT+QURCCFG="urcport","usbat", setting DTR and AP_READY
pins).

With this configuration, LTE modem should wake up when there is an incoming
data packet.
The easiest way for me put modem to sleep was to stop ModemManager. Now, if
I disable ModemManager, and then try to send a network packet from backend
to the device, the modem never wakes up. My guess is that when I stop
ModemManager, the TCP connection is lost, and this is why modem cannot wake
up as it doesn't get the message from backend.
My preferred way to sleep modem is to disable USB_VBUS.

On the ModemManager side, what should I do to maintain the TCP connection
when putting the modem to sleep, so it can wake up when getting a message
over network?

Kind regards,
Filip
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