The default powerline frequency for the Logitech C270 webcam is 60Hz, which causes flickering. It can be changed manually via v4l2-ctl or cameractrls, but the change isn’t permanent. To persist the change, we need to create a udev rule. Put the following lines in /etc/udev/rules.d/99-logitech-default-powerline.rules:
# 046d:0825 Logitech, Inc. Webcam C270
SUBSYSTEM=="video4linux", KERNEL=="video[0-9]*", ATTR{index}=="0", ATTRS{idVendor}=="046d", ATTRS{idProduct}=="0825", RUN+="/usr/bin/v4l2-ctl -d $devnode --set-ctrl=power_line_frequency=1"
power_line_frequency value 1 corresponds to 50Hz, 2 to 60Hz and 0 disables the anti-flickering algorithm.