nixos_flake_config/home/fuji/tablet-mode.sh

#!/usr/bin/env bash
# tablet-mode — run commands as the machine is folded into, and out of, a tablet.
#
# sway has `bindswitch tablet:on`, but att_wm has none and river's init is a
# startup script rather than an IPC surface, so there is nothing to bind there.
# Reading SW_TABLET_MODE off evdev is the one mechanism that works in both
# sessions, which is why it is the only one used here — sway's bindswitch is
# left gating rotation and nothing else.
#
# evtest does the reading: without --grab it only listens, so the compositor
# still gets the switch, and its opening dump reports the *current* state. That
# last part is the reason it is evtest and not `libinput debug-events` — a
# session that starts already folded has no transition to react to.

set -uo pipefail

DEVICE="${TABLET_MODE_DEVICE:-}"
ON_CMD=""
OFF_CMD=""
VERBOSE=0

usage() {
	cat <<-EOF
		usage: tablet-mode [-d DEVICE] [--on CMD] [--off CMD] [-v]

		  -d, --device DEVICE   evdev node to watch (default: found by capability)
		      --on CMD          run CMD when the machine enters tablet mode
		      --off CMD         run CMD when it leaves
		  -v, --verbose         log every line evtest prints

		Both commands run through \`bash -c\` with \$TABLET_MODE set to 1 or 0.
	EOF
}

while [ $# -gt 0 ]; do
	case "$1" in
	-d | --device)
		DEVICE="${2:?--device needs a value}"
		shift 2
		;;
	--on)
		ON_CMD="${2:?--on needs a value}"
		shift 2
		;;
	--off)
		OFF_CMD="${2:?--off needs a value}"
		shift 2
		;;
	-v | --verbose)
		VERBOSE=1
		shift
		;;
	-h | --help)
		usage
		exit 0
		;;
	*)
		printf 'tablet-mode: unknown argument: %s\n' "$1" >&2
		usage >&2
		exit 1
		;;
	esac
done

command -v evtest >/dev/null || {
	printf 'tablet-mode: evtest not found in PATH\n' >&2
	exit 1
}

log() { printf '[tablet-mode] %s\n' "$*"; }
vlog() {
	[ "$VERBOSE" = 1 ] && log "$@"
	return 0
}

# Find the switch by capability rather than by name or by-path: on fuji it is an
# intel-hid device that is registered after boot, so neither its event number
# nor its presence at our start time can be counted on. Not finding it is a
# non-zero exit, which is what makes systemd try us again a few seconds later.
#
# In /proc/bus/input/devices each device ends with its capability bitmaps, and
# SW_TABLET_MODE is bit 1 of the switch one. `H:` precedes `B:` in every record,
# so the handler list is always already in hand by the time the bitmap arrives.
find_device() {
	local line handlers="" bitmap handler
	while IFS= read -r line; do
		case "$line" in
		"H: Handlers="*)
			handlers="${line#H: Handlers=}"
			;;
		"B: SW="*)
			# A bitmap is hex words, most significant first; bit 1
			# lives in the last of them.
			bitmap="${line#B: SW=}"
			bitmap="${bitmap##* }"
			[ -n "$bitmap" ] || continue
			(((0x$bitmap & 2) == 0)) && continue
			for handler in $handlers; do
				case "$handler" in
				event*)
					printf '/dev/input/%s' "$handler"
					return 0
					;;
				esac
			done
			;;
		esac
	done </proc/bus/input/devices
	return 1
}

if [ -z "$DEVICE" ]; then
	DEVICE="$(find_device)" || {
		printf 'tablet-mode: no input device reports SW_TABLET_MODE\n' >&2
		exit 1
	}
fi
[ -r "$DEVICE" ] || {
	printf 'tablet-mode: cannot read %s\n' "$DEVICE" >&2
	exit 1
}
log "watching ${DEVICE}"

state="" # last seen switch state, "" until evtest reports the first one

apply() {
	local cmd
	case "$1" in
	1) cmd="$ON_CMD" ;;
	0) cmd="$OFF_CMD" ;;
	*) return 0 ;;
	esac
	[ -n "$cmd" ] || return 0
	TABLET_MODE="$1" bash -c "$cmd" || log "hook failed: ${cmd}"
}

exec 3< <(stdbuf -oL evtest "$DEVICE" 2>&1)
evtest_pid=$!
# Only kill the child here — an `exit` in an EXIT trap would overwrite the
# status below, and systemd needs to see a failure to restart us.
trap 'kill "$evtest_pid" 2>/dev/null' EXIT
trap 'exit 0' INT TERM

while IFS= read -r line <&3; do
	case "$line" in
	# A transition:
	#   Event: time ..., type 5 (EV_SW), code 1 (SW_TABLET_MODE), value 1
	*"(SW_TABLET_MODE), value "*)
		new="${line##*, value }"
		;;
	# The opening dump, which is where the state at startup comes from:
	#   Event code 1 (SW_TABLET_MODE) state 0
	*"(SW_TABLET_MODE) state "*)
		new="${line##* state }"
		;;
	*)
		vlog "$line"
		continue
		;;
	esac

	[ "$new" = "$state" ] && continue
	state="$new"
	[ "$state" = 1 ] && log "tablet mode on" || log "tablet mode off"
	apply "$state"
done

log "evtest exited"
exit 1