GPS support using Quectel EC21

[Aleksander Morgado]

Hey,

> 
> I am trying to use ModemManager with the Quectel EC21 modem to get GPS data.
> 
> By default this modem uses QMI, but I see the following message when
> starting ModemManager:
> 
> [1504793139.991929] [../../git/src/mm-broadband-modem-qmi.c:11204]
> qmi_port_allocate_client_ready(): Couldn't allocate client for service
> 'pds': Couldn't create client for service 'pds': Service 'pds' not
> supported by the device
> 
> From that, I interpret that the modem firmware (I am using
> EC21EFAR02A05M4G) is not implementing the PDS QMI service, which is
> used for GPS support. I have asked Quectel about new firmware versions
> for this modem, just in case there is support for PDS service, but I
> am still waiting for their answer.
> 
> In the meanwhile, I decided to test without QMI, using the option
> driver to probe the USB serial devices. I removed the QMI driver and I
> manually probed the USB serial ports with the option driver:
> 
> echo "2c7c 0121" > /sys/bus/usb-serial/drivers/option1/new_id
> option 1-1.3:1.0: GSM modem (1-port) converter detected
> usb 1-1.3: GSM modem (1-port) converter now attached to ttyUSB0
> option 1-1.3:1.1: GSM modem (1-port) converter detected
> usb 1-1.3: GSM modem (1-port) converter now attached to ttyUSB1
> option 1-1.3:1.2: GSM modem (1-port) converter detected
> usb 1-1.3: GSM modem (1-port) converter now attached to ttyUSB2
> option 1-1.3:1.3: GSM modem (1-port) converter detected
> usb 1-1.3: GSM modem (1-port) converter now attached to ttyUSB3
> 
> In this mode, I have verified that the GPS seems to work. I use the
> 'AT+QGPS=1' at command to enable GPS and then I can see NMEA traces in
> the ttyUSB1 port:
> 
> root at ccimx6ulsbc:~# mmcli -m 0 --command="AT+QGPS=1"
> response: ''
> root at ccimx6ulsbc:~# cat /dev/ttyUSB1
> $GPVTG,,T,,M,,N,,K,N*2C
> $GPGSA,A,1,,,,,,,,,,,,,,,*1E
> $GPGGA,,,,,,0,,,,,,,,*66
> $GPRMC,,V,,,,,,,,,,N*53
> $GPVTG,,T,,M,,N,,K,N*2C
> $GPGSA,A,1,,,,,,,,,,,,,,,*1E
> $GPGGA,,,,,,0,,,,,,,,*66
> $GPRMC,,V,,,,,,,,,,N*53
> $GPVTG,,T,,M,,N,,K,N*2C
> $GPGSA,A,1,,,,,,,,,,,,,,,*1E
> $GPGGA,,,,,,0,,,,,,,,*66
> $GPRMC,,V,,,,,,,,,,N*53
> $GPVTG,,T,,M,,N,,K,N*2C
> $GPGSA,A,1,,,,,,,,,,,,,,,*1E
> $GPGGA,,,,,,0,,,,,,,,*66
> 
> The problem is trying to make this work with ModemManager. For some
> reason (I don't quite understand why) seems like ModemManager doesn't
> recognize the ttyUSB1 port as the GPS port. I am not sure if AT+QGPS
> is a standard AT command or is something specific from Quectell
> modems/Qualcomm chipsets.
> 
> Is this expected to work, am I missing something?
> 

This is expected; AT+QGPS isn't standard and no plugins currently use it. The way to go would be to setup GPS location gathering in the same way as done in the Cinterion QMI plugin, where we have custom AT commands to enable/disable GPS and then we read NMEA traces from a TTY. ModemManager needs to know which tty will have the output of NMEA traces, and to do that we currently require some previous knowledge of the device (i.e. udev tags to specify which device interface handles the NMEA output). Again, the Cinterion plugin is a perfect example of how to subclass the QMI implementation to provide an AT/TTY based GPS location fallback.


-- 
Aleksander
https://aleksander.es