一、前言

在完成了基于GPIO的LED设备驱动的文章后，本文继续介绍基于PWM的LED设备驱动。同样的，站在巨人的肩膀上，不写一行代码……，但工资还是要领……

二、系列文章

1.基于GPIO的LED设备驱动
链接：实现安卓LED设备驱动：不写一行代码

2.基于PWM的LED设备驱动：即本文

三、准备工作

3.1 查找PWM引脚

（1）通过查看Datasheet，结合原理图，姑且选择GPIOZ_6作为实验所用的PWM引脚。

（2）GPIOZ_6在复用为Func4时，可作为PWM_D使用

3.2 原理图：确认引脚位置

（1）GPIOZ_6引脚，通过原理图可知，它位于J31插座左边的第7个PIN，即PIN-13

3.3 PWM Controller

（1）通过查阅板子的DTS设备树文件，查找PWM_D的控制器

（2）如下所示，控制PWM_D的控制器名为:pwm_cd，此控制器同时控制了PWM_C和PWM_D

//文件：common\arch\arm\boot\dts\amlogic\mesontl1.dtsi

			pwm_cd: pwm@1a000 {
				compatible = "amlogic,tl1-ee-pwm";
				reg = <0x1a000 0x20>;
				#pwm-cells = <3>;
				clocks = <&xtal>,
						<&xtal>,
						<&xtal>,
						<&xtal>;
				clock-names = "clkin0",
						"clkin1",
						"clkin2",
						"clkin3";
				status = "disabled";
			};

四、查阅PWM bindings

想要站在大神的肩膀上，自然要查看大神遗留的Leds-pwm说明文档，以了解什么样的设备才能与大神的leds-pwm驱动绑定

文件位置：Documentation/devicetree/bindings/leds/leds-pwmtxt

LED connected to PWM

Required properties:
- compatible : should be "pwm-leds".

Each LED is represented as a sub-node of the pwm-leds device.  Each
node's name represents the name of the corresponding LED.

LED sub-node properties:
- pwms : PWM property to point to the PWM device (phandle)/port (id) and to
  specify the period time to be used: <&phandle id period_ns>;
- pwm-names : (optional) Name to be used by the PWM subsystem for the PWM device
  For the pwms and pwm-names property please refer to:
  Documentation/devicetree/bindings/pwm/pwm.txt
- max-brightness : Maximum brightness possible for the LED
- active-low : (optional) For PWMs where the LED is wired to supply
  rather than ground.
- label :  (optional)
  see Documentation/devicetree/bindings/leds/common.txt
- linux,default-trigger :  (optional)
  see Documentation/devicetree/bindings/leds/common.txt

Example:

twl_pwm: pwm {
	/* provides two PWMs (id 0, 1 for PWM1 and PWM2) */
	compatible = "ti,twl6030-pwm";
	#pwm-cells = <2>;
};

twl_pwmled: pwmled {
	/* provides one PWM (id 0 for Charing indicator LED) */
	compatible = "ti,twl6030-pwmled";
	#pwm-cells = <2>;
};

pwmleds {
	compatible = "pwm-leds";
	kpad {
		label = "omap4::keypad";
		pwms = <&twl_pwm 0 7812500>;
		max-brightness = <127>; 
	};

	charging {
		label = "omap4:green:chrg";
		pwms = <&twl_pwmled 0 7812500>;
		max-brightness = <255>;
	};
};

五、编写设备树节点

5.1 实现节点：pwm-leds

	pwmleds {
		status = "okay";
		compatible = "pwm-leds";
			pwm_cd-led {
				//向&pwm_cd控制器传送了3个参数，分别是：1.index=6   2.Period=30541  3.极性=0
				pwms = <&pwm_cd 3 30541 0>;  //GPIOZ-6的index=3，这个具体要查阅datasheet和驱动源码
				max-brightness = <255>; //设置Levels，最大256级调整
			};
	};

5.2 测试命令

（1）进入pwm_cd-led设备目录
:/sys/class/leds/pwm_cd-led # ls
brightness device max_brightness power subsystem trigger uevent
:/sys/class/leds/pwm_cd-led # cat brightness
0
:/sys/class/leds/pwm_cd-led # cat max_brightness
255

（2）尝试不同LEV的亮度值，值越大则占空比越大（越亮），反正则越小（越暗）
:/sys/class/leds/pwm_cd-led # echo 200 >  brightness
:/sys/class/leds/pwm_cd-led # echo 100 >  brightness
:/sys/class/leds/pwm_cd-led # echo 255 >  brightness
:/sys/class/leds/pwm_cd-led # echo 1 >  brightness
:/sys/class/leds/pwm_cd-led # echo 20 >  brightness
:/sys/class/leds/pwm_cd-led # echo 80 >  brightness
:/sys/class/leds/pwm_cd-led # echo 200 >  brightness
:/sys/class/leds/pwm_cd-led # echo 250 >  brightness
:/sys/class/leds/pwm_cd-led #