For upside-down or flipped installations, add:
--rotation 180
Example:
rpicam-hello -t 0 \
--fullscreen \
--qt-preview 0 \
--framerate 30 \
--viewfinder-mode 1920:1080:12:P \
--viewfinder-width 1280 \
--viewfinder-height 720 \
--denoise off \
--buffer-count 2 \
--rotation 180