Make brightness keys work on external displays with a kernel backlight

bin/omarchy-brightness-display

@@ -35,12 +35,38 @@ monitor_is_internal() {
   [[ $monitor =~ ^(eDP|LVDS|DSI)- ]]
 }
 
+# Hyprland monitor names are DRM connector names, so a backlight the kernel registers on an external
+# connector (e.g. a display whose brightness the GPU driver controls) belongs to this monitor. It wins
+# over the Apple and DDC fallbacks, which go through slower side channels. Internal panels keep
+# omarchy-hw-display's heuristic: GPU drivers also register on the eDP connector a backlight that
+# may not be the one driving the panel (gmux on dual-GPU Macs).
+# Only a connected connector counts: another card can expose a connector with the same name.
+connector_backlight() {
+  local subsystem
+
+  if [[ -z $monitor ]] || monitor_is_internal; then
+    return 1
+  fi
+
+  for subsystem in "${OMARCHY_DRM_PATH:-/sys/class/drm}"/card*-"$monitor"/*/subsystem; do
+    if [[ $(<"${subsystem%/*/subsystem}/status") == "connected" && $(readlink "$subsystem") == */class/backlight ]]; then
+      subsystem="${subsystem%/subsystem}"
+      printf '%s\n' "${subsystem##*/}"
+      return 0
+    fi
+  done
+
+  return 1
+}
+
+device="$(connector_backlight)"
+
 use_apple_display() {
-  omarchy-hyprland-monitor-focused-apple "$monitor"
+  [[ -z $device ]] && omarchy-hyprland-monitor-focused-apple "$monitor"
 }
 
 use_ddc_display() {
-  [[ -n $monitor ]] && ! monitor_is_internal
+  [[ -z $device && -n $monitor ]] && ! monitor_is_internal
 }
 
 if (( $# == 0 )); then
@@ -52,7 +78,7 @@ if (( $# == 0 )); then
     exit
   fi
 
-  device="$(omarchy-hw-display)" || exit 1
+  [[ -n $device ]] || device="$(omarchy-hw-display)" || exit 1
   backlight_brightness "$device"
   exit
 fi
@@ -86,8 +112,8 @@ elif use_ddc_display; then
   brightness="$(omarchy-brightness-display-ddc "$monitor" "$step")" || exit 1
   (( no_osd )) || omarchy-osd -i brightness -p "$brightness"
 else
-  # Current device highlighted
-  device="$(omarchy-hw-display)" || exit 1
+  # The focused connector's backlight, else the most likely panel backlight
+  [[ -n $device ]] || device="$(omarchy-hw-display)" || exit 1
 
   # Current brightness percentage
   current=$(backlight_brightness "$device") || exit 1

test/shell.d/brightness-display-test.sh

@@ -10,11 +10,38 @@ trap 'rm -rf "$test_tmp"' EXIT
 mock_bin="$test_tmp/bin"
 call_log="$test_tmp/calls"
 runtime_dir="$test_tmp/runtime"
+drm_dir="$test_tmp/drm"
 mkdir -p "$mock_bin" "$runtime_dir"
 
+# DP-3 has a kernel backlight registered on its connector, as external displays that support it do.
+# DP-1's connector only has a non-backlight child. eDP-1's connector has a GPU backlight, as i915
+# registers, which must not override omarchy-hw-display's pick for the internal panel. card1 has a
+# disconnected DP-3 of its own with a backlight, which must not be mistaken for the focused DP-3.
+mkdir -p "$drm_dir/card2-DP-3/apple-DP-3-bl" "$drm_dir/card2-DP-1/power" "$drm_dir/card0-eDP-1/intel_backlight" \
+  "$drm_dir/card1-DP-3/other-DP-3-bl"
+ln -s ../../../../class/backlight "$drm_dir/card2-DP-3/apple-DP-3-bl/subsystem"
+ln -s ../../../../class/backlight "$drm_dir/card0-eDP-1/intel_backlight/subsystem"
+ln -s ../../../../bus/platform "$drm_dir/card2-DP-1/power/subsystem"
+ln -s ../../../../class/backlight "$drm_dir/card1-DP-3/other-DP-3-bl/subsystem"
+for connector in card2-DP-3 card2-DP-1 card0-eDP-1; do
+  printf 'connected\n' >"$drm_dir/$connector/status"
+done
+printf 'disconnected\n' >"$drm_dir/card1-DP-3/status"
+
 cat >"$mock_bin/omarchy-hyprland-monitor-focused-apple" <<'SH'
 #!/bin/bash
-exit 1
+printf 'omarchy-hyprland-monitor-focused-apple %s\n' "$*" >>"$CALL_LOG"
+[[ ${APPLE_DISPLAY:-0} == "1" ]]
+SH
+
+cat >"$mock_bin/omarchy-brightness-display-apple" <<'SH'
+#!/bin/bash
+printf 'omarchy-brightness-display-apple %s\n' "$*" >>"$CALL_LOG"
+SH
+
+cat >"$mock_bin/omarchy-osd" <<'SH'
+#!/bin/bash
+printf 'omarchy-osd %s\n' "$*" >>"$CALL_LOG"
 SH
 
 cat >"$mock_bin/omarchy-hyprland-monitor-focused" <<'SH'
@@ -54,7 +81,7 @@ SH
 chmod +x "$mock_bin"/*
 
 run_brightness() {
-  CALL_LOG="$call_log" XDG_RUNTIME_DIR="$runtime_dir" PATH="$mock_bin:$ROOT/bin:$PATH" \
+  CALL_LOG="$call_log" XDG_RUNTIME_DIR="$runtime_dir" OMARCHY_DRM_PATH="$drm_dir" PATH="$mock_bin:$ROOT/bin:$PATH" \
     "$ROOT/bin/omarchy-brightness-display" "$@"
 }
 
@@ -129,6 +156,36 @@ grep -F 'ddcutil --bus 7 --skip-ddc-checks --noverify setvcp 10 5' "$call_log" >
   fail "external low brightness writes the one-percent target"
 pass "external low brightness uses a one-percent step"
 
+: >"$call_log"
+brightness=$(APPLE_DISPLAY=1 run_brightness --monitor DP-3)
+[[ $brightness == "40" ]] || fail "connector backlight reports brightness" "actual: $brightness"
+grep -F 'brightnessctl -d apple-DP-3-bl -m' "$call_log" >/dev/null || \
+  fail "connector backlight is queried through brightnessctl"
+pass "connector backlight reports brightness"
+
+: >"$call_log"
+APPLE_DISPLAY=1 run_brightness --monitor DP-3 +5%
+grep -Fx 'brightnessctl -d apple-DP-3-bl set 45%' "$call_log" >/dev/null || \
+  fail "connector backlight steps through brightnessctl" "$(cat "$call_log")"
+grep -Fx 'omarchy-osd -i brightness -p 40' "$call_log" >/dev/null || \
+  fail "connector backlight shows the OSD"
+if grep -E 'omarchy-brightness-display-apple|omarchy-hyprland-monitor-focused-apple|ddcutil' "$call_log"; then
+  fail "connector backlight skips the Apple and DDC backends"
+fi
+pass "connector backlight wins over the Apple and DDC backends"
+
+: >"$call_log"
+FOCUSED_MONITOR=DP-3 run_brightness --no-osd 1%-
+grep -Fx 'brightnessctl -d apple-DP-3-bl set 1%-' "$call_log" >/dev/null || \
+  fail "focused monitor's connector backlight takes precise steps" "$(cat "$call_log")"
+pass "focused monitor's connector backlight takes precise steps"
+
+: >"$call_log"
+APPLE_DISPLAY=1 run_brightness --no-osd --monitor DP-1 +5%
+grep -Fx 'omarchy-brightness-display-apple --no-osd +5%' "$call_log" >/dev/null || \
+  fail "Apple display without a connector backlight keeps asdcontrol" "$(cat "$call_log")"
+pass "Apple display without a connector backlight keeps asdcontrol"
+
 cat >"$mock_bin/hyprctl" <<'SH'
 #!/bin/bash
 printf '%s\n' '[