Single RS232 port Telit and CMUX mode (GSM 27.010 / n_gsm)

[Dan Williams]

On Mon, 2017-07-03 at 13:10 +0100, Tim Small wrote:
> Hello,
> 
> I'm trying to get a (GPRS / 2.5G) Telit GE910-Quad V3 working in CMUX
> mode.  There's only a single RS232 port connection between the modem
> and
> the host computer, so I'd like to use GSM 27.010 ("CMUX") mode.  The
> Telit module does have another physical port, but there isn't a spare
> UART on the host (and I can't use USB), so I'm stuck with the one
> RS232
> connection only.
> 
> So far I've hacked about with things as a proof of concept - an
> external
> binary attaches the line discipline and I've manually setup calls
> using
> pppd and chat scripts on one port, whilst monitoring signal quality
> etc.
> on another multiplexed.
> 
> I've then hacked around with the ModemManager source to force it to
> probe a couple of  /dev/gsmtty, ModemManager then identifies two
> separate Telit modems:
> 
> root at nmtest:~# mmcli -L

There should be some way to link the different ports created with the
gsmtty ldisc to their "physical" serial port.  For each of the gsmtty
devices in sysfs (eg, /sys/class/tty/gsmtty0 or so) can you:

ls -al /sys/class/tty/gsmtty0

and we can go from there.  We might also be able to use udev to figure
it out, though that's often not as useful for virtual devices.  But the
kernel gsmtty code does have specific calls to link the MUX ports to
the parent.

Basically, we just want each of the MUX ports to show as serial ports
of the same base modem.  Then you would get PPP and control muxed over
the same port and MM would be aware of it :)

Dan


> Found 2 modems:
>         /org/freedesktop/ModemManager1/Modem/1 [Telit] GE910-QUAD
>         /org/freedesktop/ModemManager1/Modem/0 [Telit] GE910-QUAD
> 
> /org/freedesktop/ModemManager1/Modem/0
>   -------------------------
>   Status |          state: 'registered'
>          | signal quality: '58' (recent)
>          |          bands: 'egsm, dcs'
>          |    access tech: 'unknown'
>   -------------------------
>   3GPP   |   registration: 'roaming'
>          |  operator code: '23415'
>          |  operator name: 'vodafone UK'
>          |   subscription: 'unknown'
> 
> root at nmtest:~# mmcli -m 1 --simple-status
> 
> /org/freedesktop/ModemManager1/Modem/1
>   -------------------------
>   Status |          state: 'registered'
>          | signal quality: '58' (recent)
>          |          bands: 'egsm, dcs'
>          |    access tech: 'unknown'
>   -------------------------
>   3GPP   |   registration: 'roaming'
>          |  operator code: '23415'
>          |  operator name: 'vodafone UK'
>          |   subscription: 'unknown'
> 
> 
> I can also get nm and mm to set up a ppp connection over one of them.
> 
> I'm not sure what my next steps should be to implement this fully,
> and
> end up with a reasonably clean implementation (and a single modem of
> course).  Any hints appreciated.
> 
> Thanks,
> 
> Tim.
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