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
第1篇 ：不写一行代码(一)：实现安卓基于GPIO的LED设备驱动
第2篇 ：不写一行代码(二)：实现安卓基于PWM的LED设备驱动
第3篇：不写一行代码(三)：实现安卓基于i2c bus的Slaver设备驱动
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 #
六、后语
继续周末爆肝……
