How can I make gpsd restrict to a certain serial port?

Question

After reinstalling Raspbian Buster I'm always running into an issue with my gpsd 3.17: Despite the fact that I start it with the corresponding device file /dev/ttyS0 as a parameter (because the GPS receiver hangs on it), it seems to access other serial devices as well, including /dev/ttyUSB0, which stands for a GNS FM9 TMC receiver without any GPS functionality, thereby causing a deadlock in my app. Here is the status of the gpsd I get from systemctl:

pi@autoradio:/import/valen/autoradio $ sudo systemctl status gpsd
● gpsd.service - GPS (Global Positioning System) Daemon
   Loaded: loaded (/lib/systemd/system/gpsd.service; enabled; vendor preset: enabled)
   Active: active (running) since Thu 2021-09-16 23:14:08 CEST; 24h ago
  Process: 412 ExecStart=/usr/sbin/gpsd $GPSD_OPTIONS $DEVICES (code=exited, status=0/SUCCESS)
 Main PID: 463 (gpsd)
    Tasks: 1 (limit: 2059)
   CGroup: /system.slice/gpsd.service
           └─463 /usr/sbin/gpsd /dev/ttyS0

Sep 16 23:14:08 autoradio systemd[1]: Starting GPS (Global Positioning System) Daemon...
Sep 16 23:14:08 autoradio systemd[1]: Started GPS (Global Positioning System) Daemon.

So it should be restricted to the Pi's on-board UART named /dev/ttyS0. Before being accessed for the first time, gpsd seems not to open any of the serial devices:

pi@autoradio:/import/valen/autoradio $ sudo lsof /dev/ttyUSB0

pi@autoradio:/import/valen/autoradio $ sudo lsof /dev/ttyS0

pi@autoradio:/import/valen/autoradio $

But: Once I start my app (named autoradio), the daemon suddenly opens both serial devices:

pi@autoradio:/import/valen/autoradio $ sudo lsof /dev/ttyS0
COMMAND PID USER   FD   TYPE DEVICE SIZE/OFF NODE NAME
gpsd    463 gpsd    8u   CHR   4,64      0t0  133 /dev/ttyS0

pi@autoradio:/import/valen/autoradio $ sudo lsof /dev/ttyUSB0
COMMAND   PID USER   FD   TYPE DEVICE SIZE/OFF NODE NAME
gpsd      463 gpsd    9u   CHR  188,0      0t0  433 /dev/ttyUSB0
autoradio 937   pi    7u   CHR  188,0      0t0  433 /dev/ttyUSB0

And this is what I do not want, as I'm accessing /dev/ttyUSB0 directly from my app. The code connecting to gpsd looks very simple (irrelevant parts omitted):

tmc::tmc ()
: gps_handler ("localhost", DEFAULT_GPSD_PORT)
{
}

void tmc::Initialize ()
{
gps_data_t* response;

response = this -> gps_handler.stream (WATCH_ENABLE | WATCH_JSON);
}

So: How do I prevent gpsd from accessing any device other than /dev/ttyUSB0, preferably already when starting it up? What's really strange: Why does this issue arise after reinstalling Raspbian? Thank you.

UPDATE: I've looked into my /etc/default/gpsd config file to find this (comments removed):

START_DAEMON="true"
USBAUTO="true"
DEVICES="/dev/ttyS0"
GPSD_OPTIONS=""

Setting the -n option did not work BTW.

Answer 1

OK, I've already had a slight suspicion that the following line in the config file was the culprit, and now I've found out that this was really the case:

USBAUTO="true"

Set this to false to prevent the daemon to look for GPS receivers hanging on USB ports, and the problem is solved.