GSM Multiplexing on the OpenMoko FreeRunner
We want Qtopia (or other similar system) to handle GSM calls/SMS messages and we want concurrently
pppd(8) to place GSM/GPRS data calls. The problem is that there is only one GSM modem.
We need a software multiplexer:
gsm0710muxd is our choice. Install it with
opkg install gsm0710muxd.
Be sure that any program accessing the modem (
gsmd) is terminated. Be sure the modem is properly restetted, I suggest this script:
echo "0" > /sys/bus/platform/devices/neo1973-pm-gsm.0/power_on sleep 1 echo "1" > /sys/bus/platform/devices/neo1973-pm-gsm.0/power_on sleep 1 stty -F /dev/ttySAC0 115200 crtscts min 1 ignbrk ignpar -ocrnl -onlcr -iexten -echoctl -echoke echo -e '\r' > /dev/ttySAC0
This is an example script (
gsm_mux_alloc) allocating a virtual channel:
#!/bin/sh # Ask the GSM modem muxer to allocate a virtual channel. tag=$1 test -z "$tag" && echo "Usage: $0 <tag>" && exit 1 test -L /dev/$tag && rm /dev/$tag test -e /dev/$tag && exit 1 set -e pts="$(dbus-send --system --print-reply --type=method_call \ --dest=org.pyneo.muxer /org/pyneo/Muxer \ org.freesmartphone.GSM.MUX.AllocChannel string:$tag \ | grep string | cut -f2 -d'"')" test -c "$pts" ln -s $pts /dev/$tag
We use the D-Bus system to communicate with gsm0710muxd, the sense of the command is:
- Communicate through the system message bus, not the user's one.
- Require to print a reply.
- Message is of type method call (ask for an answer), not a one-way signal type.
- The destination is org.pyneo.muxer: this is the name choosen by gsm0710muxd.
- The object to communicate to (inside org.pyneo.muxer) has path
- The message is a channel allocation request.
- We pass an unique identifier along with the request.
The GSM muxer D-Bus interface is documented on this freesmartphone.org page. The full answer (without
awk filters) is:
method return sender=:1.17 -> dest=:1.22 reply_serial=2 string "/dev/pts/3"
Run the script as
gsm_mux_alloc minicom and you will get a symbolic link
/dev/minicom pointing to
/dev/pts/3 (if 3 is the allocated pseudo tty).
The device can now be used, say by
minicom(1) or by
pppd(8). When the program closes the device, it will be released by the muxer an cannot be longer used. This is the syslog:
gsm0710muxd.c:693:c_alloc_channel(): Connecting /dev/pts/3 to virtual channel 2 for minicom on /dev/ttySAC0 gsm0710muxd.c:569:pseudo_device_read(): Write to a channel which wasn't acked to be open. gsm0710muxd.c:1410:extract_frames(): Logical channel 2 opened gsm0710muxd.c:609:pseudo_device_read(): Logical channel 2 for (null) closed
The multiplexed GSM modem
You can use the multiplexed modem at 115200 8N1. Wich flow chontrol?
After 10 seconds of inactivity the virtual serial line goes to sleep. Send a carriage return to awake it and get the OK response from the modem. If you send an AT command while the line is sleeping, it will be ignored.
You cannot run more than one program with the allocated muxed device. Once the first program closes the device, it will be released. This means - for example - that you cannot initialize the serial line with
stty(1) before running