create a udev rule file: sudo nano /etc/udev/rules.d/99-ftdi.rules
add the following line: ATTRS{idVendor}=="0403", ATTRS{idProduct}=="6015", MODE="0666"
exit nano
enter this command in a shell: sudo udevadm control --reload-rules && sudo udevadm trigger
see also trabucayre/openFPGALoader#245