#!/bin/bash
#
# Author: Christian Birchinger <cbirchinger@netswarm.net>
#
# Daemon that monitors the DPMS state of a given display port and adjusts the
# CPU scheduling governor. Typically powersave or shedutil during screen
# blanking and performance while the display is on.

PORT=card0-DVI-D-1
INTERVAL=60
ON_GOV=performance
OFF_GOV=schedutil

snore()
{
	local IFS
	[[ -n "${_snore_fd:-}" ]] || { exec {_snore_fd}<> <(:); } 2>/dev/null
	read -r ${1:+-t "$1"} -u $_snore_fd || :
}

dpms_path="/sys/class/drm/${PORT}/dpms"
prevstat=''

if [[ ! -r "$dpms_path" ]]; then
	echo "ERROR: Unable to read ${dpms_path}" >&2
	exit 1
fi

while true; do
	curstat=$(<"${dpms_path}")
	if [[ "$prevstat" != "$curstat" ]]; then
		if [[ "$curstat" == "On" ]]; then
			cpupower frequency-set -g "${ON_GOV}" &> /dev/null
			logger -t "dpms_cpufreq[$$]" -p daemon.info "Switching governor to ${ON_GOV}"
		else
			cpupower frequency-set -g "${OFF_GOV}" &> /dev/null
			logger -t "dpms_cpufreq[$$]" -p daemon.info "Switching governor to ${OFF_GOV}"
		fi
	fi
	prevstat=$curstat
	snore ${INTERVAL}
done