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PX4 startup on VOXL2

PX4 is auto-started via the systemd service using the file etc\systemd\system\voxl-px4.service This service file relies on the /usr/bin/voxl-px4 convenience script to start PX4 in daemon mode. The voxl-px4 script is a simple bash script.


The voxl-px4 convenience script performs a few simple tasks:

  • Checks for the existence of the configuration file in /etc/modalai/voxl-px4.conf. If the configuration file exists it will be sourced to set the values of various important environment variables that control the startup process
  • Checks for the existence of the signature file that is required to run any custom software on the sensors DSP. If it does not exist it is created.
  • Parse any command line options. These options can be used to overwrite any environment variable values set by the configuration file (if one existed).
  • Check for existence of a parameters file in /data/px4/param. If one does not exist it will create one with the parameters listed in /etc/modalai/voxl-px4-set-default-parameters.config
  • Finally it will start the px4 and pass it the option to run the startup script located at /usr/bin/voxl-px4-start. The voxl-px4-start startup script is also a simple bash script.
  • As the voxl-px4 convenience script runs it will print out useful information

Creating the configuration file

The voxl-configure-px4 script will create the configuration file at /etc/modalai/voxl-px4.conf.

Auto start and daemon mode

By default PX4 will be auto-started and will run as a system daemon. For development purposes it is often more appropriate to start PX4 manually from the command line. To disable the auto-start function use systemctl disable voxl-px4 and then reboot the system. Once the system has restarted you can manually start PX4 by running voxl-px4. Be aware that the configuration file may be set up to run PX4 in daemon mode so even though it was started manually you will not be able to interact with the PX4 shell. In order to not start in daemon mode the configuration script can be updated.

If voxl-px4 was auto-started and you want to see all of the normal console messages you can use this command: journalctl -b -u voxl-px4

voxl-px4-start startup script

The startup script /usr/bin/voxl-px4-start is the most important part of PX4 startup process. This script is responsible for starting all drivers and modules on both the applications processor and the DSP. It is important to note that the qshell driver is needed to interact with the PX4 code running on the DSP. All drivers and modules started on the DSP must be done via qshell.

Interacting with PX4 after startup

If PX4 has been started manually and is not in daemon mode then you can interact directly with the PX4 shell. If it has been auto-started or is in daemon mode then you cannot directly interact with the PX4 shell. However, many of the commands are still available from an adb shell or ssh session terminal prompt. Simply prefix the desired command with px4- and it will interact with the running PX4 process. A very common use case is to run px4-listener from a different shell to see the current output of a desired topic. For example px4-listener sensor_gps will give you the current value of the sensor_gps topic if it is being currently published by the GPS device driver.