Help with UART multiplexing

[nabelekt-vl]

This is related to my question at https://www.linux.org/threads/help-with-modemmanager-and-modem-on-uart-port.55625/#post-260222, but is different.

I have a SARA-R52M10 cellular+GNSS modem connected to Raspberry Pi CM4 UART3. The RPi is running Ubuntu 24.04. I have a ppp interface working fine with the standard UART connection. But, because the ppp interface hogs the connection, and I need to be able to send commands and get information back while maintaining an Internet connection, I am trying to multiplex the UART. The modem supposedly supports this.

This is what I've gotten to for setup:

# Start fresh
pkill ldattach; sleep 0.5
modprobe -r n_gsm; sleep 0.5

# Load the kernel module
modprobe n_gsm; sleep 0.5

# Configure interface settings for serial device
stty -F /dev/ttyAMA3 115200 raw -echo -crtscts -ixon -ixoff; sleep 0.5

# Use example command from the documentation to enable CMUX on the modem
echo -ne 'AT+CMUX=0,0,,1500,50,3,90\r' > /dev/ttyAMA3; sleep 0.5

# Explicit baud rate
ldattach -s 115200 21 /dev/ttyAMA3 & sleep 1

# Check if ldattach remains running
pgrep ldattach

# Test comms
screen /dev/gsmtty1 115200

After the ldattach command, I see 63 /dev/gsmtty* devices, but as soon as I try to start a screen session with any of them (which works fine beforehand with /dev/ttyAMA3), I just get [screen is terminating].
If I try a cat command on the /dev/ file, I get cat: /dev/gsmtty2: Level 2 halted.

Am I setting up CMUX correctly?
Do you have any ideas on this?
Are you able to reproduce?

Thank you!

 

[nabelekt-vl]

I was able to get this working using https://github.com/ya-jeks/gsmmux/. I did:

apt update
apt install -y build-essential automake autoconf libtool
wget https://github.com/ya-jeks/gsmmux/archive/refs/heads/master.zip
unzip master.zip
cd gsmmux-master
make
make install
/usr/sbin/gsmMuxd -p /dev/ttyAMA3 -b 115200 -s /dev/modem-mux -w /dev/ptmx /dev/ptmx

This gives two separate devices at /dev/modem-mux0 and /dev/modem-mux1 that can be used simultaneously.