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LCD Backlight

LCD backlight usually involves a power supply, PWM controller, and some enable signals. For example on PinePhone, it looks like this:


On my devices PWM controller is configured for 20kHz frequency in DTS (sun50i-a64-pinephone.dts):

backlight: backlight {
        compatible = "pwm-backlight";
        pwms = <&r_pwm 0 50000 PWM_POLARITY_INVERTED>;
        brightness-levels = <0 10 20 30 40 50 60 70 80 90 100>;
        default-brightness-level = <4>;
        enable-gpios = <&pio 7 10 GPIO_ACTIVE_HIGH>; /* PH10 */

Here 50000 means a period of 50000 ns. Frequency thus is 1e9 / 50000 = 20kHz. You can experiment with differnt frequencies by changing this number.

You can control the backlight from userspace via sysfs (to set backlight intensity) or via KMS atomic modesetting API (it's enabled/di­sabled automatically when suspending the display pipeline).

Backlight intensity

Setting the backlight intensity always works via sysfs:

max=`cat /sys/class/backlight/backlight/max_brightness`
echo 0 > /sys/class/backlight/backlight/brightness
echo $max > /sys/class/backlight/backlight/brightness
# and anything in between

Brightness PWM duty cycles for the brightness levels are set in the DTS file. See above. You can add more, or change the scale.

Power saving

Be aware that LCD backlight and LCD panel controller and LCD panel interface are all separate parts of the display pipeline. If you turn off the backlight via sysfs, you're just turning off the light, but rest of the pipeline is still in full gear.

To get maximum power savings you need to shut down the entire pipeline using DPMS via KMS API. If you're using Linux console you can do that via echo 3 > /sys/class/graphics/fb0/blank. This will not work if you're using other KMS based app. (eg. Xorg server) The app needs to tell the kernel to shutdown the pipeline itself.