前言
目前大部分嵌入式设备还是采用PWM调光的方式，当然我的mangopi也是采用这种方式进行背光调节的。linux是自带背光控制驱动程序的，要使用背光控制接口首先得调通PWM得驱动，可以参考上一篇博客PWM驱动适配。
既然都有了PWM驱动了，其实就可以控制屏幕背光了，还去调专用的背光控制接口也仅仅是为了简化控制方法吧。
设备树修改
PWM驱动调试成功以后，还需要在设备树根节点中添加backlight节点，如下所示：
backlight: backlight {
compatible = "pwm-backlight"; //适配内核pwm-backlight驱动
pwms = <&pwm 1 500000 0>; //引用PWM1 也就是PWM驱动
pwm-names = "backlight";
brightness-levels = <0 4 8 16 32 64 128 255>;
default-brightness-level = <7>;
status = "okay";
};
其中关于节点中的各属性含义可参考内核绑定文档Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt。引用如下：
equired properties:
- compatible: "pwm-backlight"
- pwms: OF device-tree PWM specification (see PWM binding[0])
- power-supply: regulator for supply voltage
Optional properties:
- pwm-names: a list of names for the PWM devices specified in the
"pwms" property (see PWM binding[0])
- enable-gpios: contains a single GPIO specifier for the GPIO which enables
and disables the backlight (see GPIO binding[1])
- post-pwm-on-delay-ms: Delay in ms between setting an initial (non-zero) PWM
and enabling the backlight using GPIO.
- pwm-off-delay-ms: Delay in ms between disabling the backlight using GPIO
and setting PWM value to 0.
- brightness-levels: Array of distinct brightness levels. Typically these
are in the range from 0 to 255, but any range starting at
0 will do. The actual brightness level (PWM duty cycle)
will be interpolated from these values. 0 means a 0% duty
cycle (darkest/off), while the last value in the array
represents a 100% duty cycle (brightest).
- default-brightness-level: The default brightness level (index into the
array defined by the "brightness-levels" property).
- num-interpolated-steps: Number of interpolated steps between each value
of brightness-levels table. This way a high
resolution pwm duty cycle can be used without
having to list out every possible value in the
brightness-level array.
其中brightness-levels意思就是：
这是一个描述亮度等级的数组，一般是从0到255，0代表亮度为0，255代表亮度为最亮。中间的亮度通过插值进行确定。
在这里brightness-levels = <0 4 8 16 32 64 128 255>，一共分为了8个等级(从0到7)。
default-brightness-level：默认的亮度等级，这里设置为最高亮度等级7。
内核配置
在menuconfig中打开以下选项以使能PWM背光控制：
结果
启动系统后，在目录/sys/class/backlight/backlight中可以看到和背光相关的目录：
root@mangopi:/sys/class/backlight/backlight# ls
actual_brightness brightness max_brightness subsystem uevent
bl_power device power type
通过修改brightness即可修改屏幕亮度：
echo 7 > brightness
echo 6 > brightness