背景

开发板上没有led灯，且没用其他引出的GPIO，所以就借rgb屏幕的引脚来点个灯（不过分吧@狗头），实现控制继电器。

（一）寻找合适的GPIO

看了好久的原理图，怕与其他功能冲突，所以就使用了 PD13 作为本次实验的引脚。
该引脚除了作为rgb屏幕的功能外，好像没其他作用了，好，就它了。

（二）跳线

焊接飞线接到继电器上。

（三）修改设备树

参考开发文档
Documentation/devicetree/bindings/leds/leds-gpio.txt

LEDs connected to GPIO lines

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

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

LED sub-node properties:
- gpios :  Should specify the LED's GPIO, see "gpios property" in
  Documentation/devicetree/bindings/gpio/gpio.txt.  Active low LEDs should be
  indicated using flags in the GPIO specifier.
- function :  (optional)
  see Documentation/devicetree/bindings/leds/common.txt
- color :  (optional)
  see Documentation/devicetree/bindings/leds/common.txt
- label :  (optional)
  see Documentation/devicetree/bindings/leds/common.txt (deprecated)
- linux,default-trigger :  (optional)
  see Documentation/devicetree/bindings/leds/common.txt
- default-state:  (optional) The initial state of the LED.
  see Documentation/devicetree/bindings/leds/common.txt
- retain-state-suspended: (optional) The suspend state can be retained.Such
  as charge-led gpio.
- retain-state-shutdown: (optional) Retain the state of the LED on shutdown.
  Useful in BMC systems, for example when the BMC is rebooted while the host
  remains up.
- panic-indicator : (optional)
  see Documentation/devicetree/bindings/leds/common.txt

Examples:

#include 
#include 

leds {
        compatible = "gpio-leds";
        led0 {
                gpios = <&mcu_pio 0 GPIO_ACTIVE_LOW>;
                linux,default-trigger = "disk-activity";
                function = LED_FUNCTION_DISK;
        };

        led1 {
                gpios = <&mcu_pio 1 GPIO_ACTIVE_HIGH>;
                /* Keep LED on if BIOS detected hardware fault */
                default-state = "keep";
                function = LED_FUNCTION_FAULT;
        };
};

run-control {
        compatible = "gpio-leds";
        led0 {
                gpios = <&mpc8572 6 GPIO_ACTIVE_HIGH>;
                color = <LED_COLOR_ID_RED>;
                default-state = "off";
        };
        led1 {
                gpios = <&mpc8572 7 GPIO_ACTIVE_HIGH>;
                color = <LED_COLOR_ID_GREEN>;
                default-state = "on";
        };
};

leds {
        compatible = "gpio-leds";

        led0 {
                gpios = <&gpio1 2 GPIO_ACTIVE_HIGH>;
                linux,default-trigger = "max8903-charger-charging";
                retain-state-suspended;
                function = LED_FUNCTION_CHARGE;
        };
};
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简单来说就是指定 compatible = “gpio-leds” 就好了
在根节点下添加：

dtsleds { 
 	compatible = "gpio-leds";
 	led0 { 
	 	label = "red";
	  	gpios = <&pio PD 13 GPIO_ACTIVE_HIGH>; 
	  	default-state = "off"; 
	}; 
};
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实例：

（四）使能内核的LED驱动

路径参考：

 Symbol: LEDS_GPIO [=y]                                                                                       │
  │ Type  : tristate                                                                                             │
  │ Prompt: LED Support for GPIO connected LEDs                                                                  │
  │   Location:                                                                                                  │
  │     -> Device Drivers                                                                                        │
  │       -> LED Support (NEW_LEDS [=y])                                                                         │
  │   Defined at drivers/leds/Kconfig:307                                                                        │
  │   Depends on: NEW_LEDS [=y] && LEDS_CLASS [=y] && (GPIOLIB [=y] || COMPILE_TEST [=n])                        │
  │   Selected by [n]:                                                                                           │
  │   - PCENGINES_APU2 [=n] && X86 && X86_PLATFORM_DEVICES [=n] && INPUT [=y] && INPUT_KEYBOARD [=y] && \        │
  │ GPIOLIB [=y] && LEDS_CLASS [=y]                
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然后编译烧写系统。

（五）测试验证

1、寻找设备树，在 /proc/device-tree 下

2、点亮
在 /sys/class/leds/red 下可以找到相关的属性
下面使 brightness 属性为 1 .即可点亮

echo 1 > /sys/class/leds/red/brightness
1

继电器接通！！

关闭的话，使用以下命令即可

echo 0 > /sys/class/leds/red/brightness
1

至此测试完毕