Ever since I got Linux 3.15 booting on the Chromebook, the one thing that has bothered me the most was that the backlight brightness was always at 100%. This caused eye strain, especially at night.

Little did I know that a working /sys/class/backlight/ entry was two simple kernel patches away…

In menuconfig:

[*] Backlight & LCD device support  --->
<*>   Lowlevel LCD controls
<*>     Platform LCD controls
<*>   Lowlevel Backlight controls
<*>     Generic PWM based Backlight Driver
<*>     Generic GPIO based Backlight Driver

Next, apply a combination of a subset of the following patches:

ARM: dts: enable pwm backlight for exynos5250-snow (62ffa706d89925427096f419b70d0416aca255c8)

ARM: dts: Add pwmX_out pinctrl nodes to exynos5250 (235a1976d868d3e992223e4a7475ac7fa01e35a6)

Here is the full patch (11-chromebook-backlight.patch on GitHub):

diff --git arch/arm/boot/dts/exynos5250-pinctrl.dtsi arch/arm/boot/dts/exynos5250-pinctrl.dtsi
index 9a49e68..b0b46cd 100644
--- arch/arm/boot/dts/exynos5250-pinctrl.dtsi
+++ arch/arm/boot/dts/exynos5250-pinctrl.dtsi
@@ -351,6 +351,13 @@
            samsung,pin-drv = <0>;
        };
 
+        pwm0_out: pwm0-out {
+            samsung,pins = "gpb2-0";
+            samsung,pin-function = <2>;
+            samsung,pin-pud = <0>;
+            samsung,pin-drv = <0>;
+        };
+
        i2c7_bus: i2c7-bus {
            samsung,pins = "gpb2-2", "gpb2-3";
            samsung,pin-function = <3>;
diff --git arch/arm/boot/dts/exynos5250-snow.dts arch/arm/boot/dts/exynos5250-snow.dts
index 87e37f4..d9a550e 100644
--- arch/arm/boot/dts/exynos5250-snow.dts
+++ arch/arm/boot/dts/exynos5250-snow.dts
@@ -215,4 +215,13 @@
            clock-frequency = <24000000>;
        };
    };
+
+    backlight {
+        compatible = "pwm-backlight";
+        pwms = <&pwm 0 1000000 0>;
+        brightness-levels = <0 100 500 1000 1500 2000 2500 2800>;
+        default-brightness-level = <7>;
+        pinctrl-0 = <&pwm0_out>;
+        pinctrl-names = "default";
+    };
 };

And that’s it :) Recompile, reboot, and you should see something similar to:

$ ls /sys/class/backlight/backlight.8
actual_brightness  brightness  max_brightness  subsystem  uevent
bl_power           device      power           type

Change brightness by echoing a value from 0 to 7 into /sys/class/backlight/backlight.8/brightness.