feat(redshift): dim backlight via acpilight/ddcci on day/night cycle

.gitmodules

@@ -646,3 +646,7 @@
 [submodule "home/.local/share/nvim/site/pack/default/opt/snacks.nvim"]
 	path = home/.local/share/nvim/site/pack/default/opt/snacks.nvim
 	url = https://github.com/folke/snacks.nvim.git
+[submodule "deps/acpilight"]
+	path = deps/acpilight
+	url = https://gitlab.com/wavexx/acpilight
+	ignore = untracked

README.md

@@ -92,3 +92,50 @@ Custom uBlock Origin filters ("My filters") are declared in [`home/.mozilla/nati
 Notes:
 - A full browser restart is required to pick up changes to this file.
 - `toOverwrite` fully replaces the "My filters" pane content on every launch, so any filters added manually through the uBlock UI will be reverted on restart — treat this file as the source of truth and edit it instead.
+
+### Backlight control (acpilight)
+
+[`acpilight`](https://gitlab.com/wavexx/acpilight) provides a backward-compatible `xbacklight` replacement that controls brightness via `/sys/class/backlight` instead of X11's RandR `Backlight` property, so it works for laptop panels the RandR property was never wired up for, and it's driven by a redshift hook ([`home/.config/redshift/hooks/backlight`](home/.config/redshift/hooks/backlight)) that dims the display along with the day/night cycle.
+
+Controlling an external/desktop monitor's brightness this way additionally requires the **`ddcci-driver-linux`** kernel module, which exposes DDC/CI-controlled monitors under `/sys/class/backlight/ddcci*` (the same interface as a native panel), so the same hook script works unmodified on both a laptop (native panel) and a desktop (DDC/CI monitor):
+
+- **Arch Linux**: AUR package `ddcci-driver-linux-dkms` (or `-git` for newer kernels)
+- **Slackware**: [`ddcci-driver-linux`](https://slackbuilds.org/repository/15.0/system/ddcci-driver-linux/) on SlackBuilds.org
+
+System settings required:
+
+- **Registering each monitor's DDC/CI device**, since `ddcci-driver-linux` doesn't autodetect monitors on its own -- it needs the main device (address `0x37`) added manually per i2c bus. Add the following snippet to your system's startup (adjust the `card0-*` glob to match which GPU your monitors are on):
+  ```sh
+  /sbin/modprobe ddcci-backlight
+
+  # DisplayPort connectors expose the real DDC/CI-capable channel as a direct
+  # i2c-N subdirectory of the connector itself (the AUX-channel-backed
+  # adapter); their `ddc` symlink instead points to a separate, more limited
+  # legacy adapter that can't complete the full DDC/CI handshake. HDMI
+  # connectors have no such split -- their `ddc` symlink is the only (and
+  # correct) adapter.
+  (
+    for conn in /sys/class/drm/card0-*/; do
+      [ "$(cat "$conn/status" 2>/dev/null)" = "connected" ] || continue
+      direct=$(ls -d "$conn"i2c-* 2>/dev/null | head -1)
+      if [ -n "$direct" ]; then
+        # DP
+        ddc_bus=$(basename "$direct")
+      else
+        # HDMI
+        ddc_link="$conn/ddc"
+        [ -e "$ddc_link" ] || continue
+        ddc_bus=$(basename "$(readlink -f "$ddc_link")")
+      fi
+      bus_num=${ddc_bus#i2c-}
+      [ -e "/sys/bus/ddcci/devices/ddcci$bus_num" ] && continue
+      echo ddcci 0x37 > "/sys/bus/i2c/devices/$ddc_bus/new_device" 2>/dev/null
+    done
+  ) & # writing to new_device is a synchronous sysfs call that blocks until the driver's real I2C handshake with that monitor finishes, and it's slow
+  ```
+- **Installing acpilight's udev rule** so `xbacklight` can write to `/sys/class/backlight` without root:
+  ```sh
+  sudo install -vCDt /etc/udev/rules.d deps/acpilight/90-backlight.rules
+  sudo udevadm trigger -s backlight -c add
+  ```
+  (also requires your user to be in the `video` group)

home/.config/redshift/hooks/backlight

@@ -0,0 +1,57 @@
+#!/bin/sh
+# redshift hook: dim the backlight along with the day/night cycle, via acpilight's
+# xbacklight (see deps/acpilight, README). Portable across machines: acpilight
+# enumerates both native ACPI panels (/sys/class/backlight/<panel>) and, once the
+# ddcci-driver-linux kernel module is loaded, DDC/CI-controlled external monitors
+# (/sys/class/backlight/ddcci*) under the same interface.
+#
+# Called by redshift as: backlight period-changed <old-period> <new-period>
+set -u
+
+brightness_day=70
+brightness_transition=50
+brightness_night=30
+# redshift sets period to "none" (and fires this same period-changed hook)
+# when disabled via SIGUSR1/the redshift-gtk tray toggle, alongside resetting
+# color settings to neutral -- restore full brightness to match.
+brightness_default=100
+
+have() { command -v "$1" >/dev/null 2>&1; }
+
+have xbacklight || exit 0
+
+case "${3:-}" in
+	daytime) brightness=$brightness_day ;;
+	transition) brightness=$brightness_transition ;;
+	night) brightness=$brightness_night ;;
+	none) brightness=$brightness_default ;;
+	*) exit 0 ;;
+esac
+
+# Only look at actual display backlights, never acpilight's /sys/class/leds
+# fallback (keyboard/capslock/etc. LEDs) -- on a machine with no backlight
+# device at all, xbacklight's "first controller found" default would otherwise
+# silently target one of those instead of a screen and fail or do the wrong
+# thing. No backlight device present (e.g. ddcci-driver-linux not loaded
+# yet): nothing to do.
+backlight_ctrls=$(ls /sys/class/backlight 2>/dev/null) || exit 0
+[ -n "$backlight_ctrls" ] || exit 0
+
+# Set every ddcci-exposed external monitor (a desktop may have several,
+# independently registered per i2c bus -- see README); on machines with no
+# ddcci device at all (e.g. a laptop's native panel only), fall through to
+# xbacklight's own default controller, which is safe here since we already
+# confirmed at least one /sys/class/backlight entry exists. Matched via
+# `case`, not `grep ddcci*`, so it never risks shell pathname expansion
+# against files in the current directory.
+found_ddcci=0
+for name in $backlight_ctrls; do
+	case "$name" in
+		ddcci*)
+			xbacklight -ctrl "$name" -set "$brightness"
+			found_ddcci=1
+			;;
+	esac
+done
+
+[ "$found_ddcci" -eq 1 ] || xbacklight -set "$brightness"

home/.config/redshift/redshift.conf

@@ -0,0 +1,3 @@
+[redshift]
+; slightly warm daytime color temperature (default neutral is 6500K)
+temp-day=5500

home/.local/bin/xbacklight

@@ -0,0 +1 @@
+../../../deps/acpilight/xbacklight

mk/build.mk

@@ -45,6 +45,13 @@ $(xwinwrap_target): $(xwinwrap_head_file)
 	$(MAKE) -C $(XWINWRAP_ROOT)
 xwinwrap: $(xwinwrap_target)
 
+.PHONY: acpilight
+acpilight_target = $(ACPILIGHT_ROOT)/xbacklight
+$(eval $(call git_submodule,acpilight,$(ACPILIGHT_ROOT)))
+# no build step - just ensure submodule
+$(acpilight_target): $(acpilight_head_file)
+acpilight: $(acpilight_target)
+
 i3status-config-template := home/.config/i3status/config.template
 i3status-config          := home/.config/i3status/config
 amd-cpu-temperature-path := /sys/class/hwmon/hwmon2/temp1_input
@@ -292,6 +299,7 @@ logrotate: $(logrotate_target)
 TARGETS = \
 	mpv-mpris \
 	xwinwrap \
+	acpilight \
 	ccls \
 	fzf \
 	fzy \

mk/variables.mk

@@ -21,6 +21,7 @@ FONTS = home/.local/share/fonts
 
 MPV_MPRIS_ROOT = $(DEPS)/mpv-mpris
 XWINWRAP_ROOT = $(DEPS)/xwinwrap
+ACPILIGHT_ROOT = $(DEPS)/acpilight
 CCLS_ROOT = $(DEPS)/ccls
 ATUIN_ROOT = $(DEPS)/atuin
 QMK_CLI_ROOT = $(DEPS)/qmk_cli