Linux Led 子系统

driver/leds/led-class.c
driver/leds/led-core.c
driver/leds/led-triggers.c
include/linux/leds.h

driver/leds/led-gpio.c
driver/leds/wm8350.c
driver/leds/led-xxx.c
driver/leds/trigger/ledtrig-backlight.c
driver/leds/trigger/ledtrig-camera.c
driver/leds/trigger/ledtrig-cpu.c
driver/leds/trigger/ledtrig-default-on.c
driver/leds/trigger/ledtrig-gpio.c
driver/leds/trigger/ledtrig-heartbeat.c
driver/leds/trigger/ledtrig-ide-disk.c
driver/leds/trigger/ledtrig-multi-control.c
driver/leds/trigger/ledtrig-oneshot.c
driver/leds/trigger/ledtrig-timer.c
driver/leds/trigger/ledtrig-transient.c

brightness：设置 LED 亮度，范围 0 ~ max_brightness
max_brightness：最大亮度（255 或其他数字）
trigger：触发方式，如 heartbeat、mmc0、backlight、gpio

delay_off、delay_on：trigger为timer时，LED亮灭的时间，单位ms

enum led_brightness {
 LED_OFF  = 0,    //全暗
 LED_HALF = 127,  //一半亮度
 LED_FULL = 255,  //最大亮度
};

维护 LED 子系统的所有 LED 设备，为 LED 设备提供注册操作函数: 

led_classdev_register()
devm_led_classdev_register()

注销操作函数:
led_classdev_unregister()
devm_led_classdev_unregister();

电源管理的休眠和恢复操作函数: 
led_classdev_suspend()
led_classdev_resume();

用户态操作接口:brightness 、max_brightness

抽象出 LED 操作逻辑，封装成函数导出，供其它文件使用:

led_init_core(): 核心初始化;
led_blink_set(): 设置led闪烁时间:
led_blink_set_oneshot() : 闪烁一次
led_stop_software_blink() : led停止闪烁
led_set_brightness() : 设置led的亮度
led_update_brightness : 更新亮度
led_sysfs_disable : 用户态关闭
led_sysfs enable : 用户态打开
leds_list : leds链表;
leds_list_lock : leds链表锁

维护 LED 子系统的所有触发器，为触发器提供注册操作函数: 

led_trigger_register()
devm_led_trigger_register()
led_trigger_register_simple()

注销操作函数: 

led_trigger_unregister()
led_trigger_unregister_simple()

以及其它触发器相关的操作函数

以 ledtrig-timer.c 为例

入口函数调用 led_trigger_register() 注册触发器，
注册时候传入 led_trigger 结构体，里面有 activate 和 deactivate 成员函数指针，
作用是生成 delay_on 、 delay_off 文件

同时还提供 delay_on 和 delay_off 的用户态操作接口
卸载时，使用 led_trigger_unregister() 注销触发器

以 leds-gpio.c 为例

在通过设备树或者其它途径匹配到设备信息后，将调用 probe() 函数，
然后再根据设备信息设置 led_classdev，
最后调用 devm_led_classdev_register() 注册 LED 设备。

struct led_classdev {
 const char  *name;//名字
 enum led_brightness  brightness;//亮度
 enum led_brightness  max_brightness;//最大亮度
 int    flags;

 /* Lower 16 bits reflect status */
#define LED_SUSPENDED  (1 << 0)
 /* Upper 16 bits reflect control information */
#define LED_CORE_SUSPENDRESUME (1 << 16)
#define LED_BLINK_ONESHOT (1 << 17)
#define LED_BLINK_ONESHOT_STOP (1 << 18)
#define LED_BLINK_INVERT (1 << 19)
#define LED_SYSFS_DISABLE (1 << 20)
#define SET_BRIGHTNESS_ASYNC (1 << 21)
#define SET_BRIGHTNESS_SYNC (1 << 22)
#define LED_DEV_CAP_FLASH (1 << 23)

//设置亮度API
 void  (*brightness_set)(struct led_classdev *led_cdev,enum led_brightness brightness);
 int  (*brightness_set_sync)(struct led_classdev *led_cdev,enum led_brightness brightness);
          
//获取亮度API
 enum led_brightness (*brightness_get)(struct led_classdev *led_cdev);

//闪烁时点亮和熄灭的时间设置
 int  (*blink_set)(struct led_classdev *led_cdev,unsigned long *delay_on,unsigned long *delay_off);

 struct device  *dev;
 const struct attribute_group **groups;

//leds-list的node
 struct list_head  node;
//默认trigger的名字
 const char  *default_trigger;
//闪烁的开关时间
 unsigned long   blink_delay_on, blink_delay_off;
//闪烁的定时器链表
 struct timer_list  blink_timer;
//闪烁的亮度
 int    blink_brightness;
 void   (*flash_resume)(struct led_classdev *led_cdev);

 struct work_struct set_brightness_work;
 int   delayed_set_value;

#ifdef CONFIG_LEDS_TRIGGERS
//trigger的锁
 struct rw_semaphore  trigger_lock;
//led的trigger
 struct led_trigger *trigger;
//trigger的链表
 struct list_head  trig_list;
//trigger的数据
 void   *trigger_data;
 bool   activated;
#endif
 struct mutex  led_access;
};

struct led_trigger {
 /* Trigger Properties */
 const char  *name;
 void  (*activate)(struct led_classdev *led_cdev);
 void  (*deactivate)(struct led_classdev *led_cdev);

 /* LEDs under control by this trigger (for simple triggers) */
 rwlock_t   leddev_list_lock;
 struct list_head  led_cdevs;

 /* Link to next registered trigger */
 struct list_head  next_trig;
};

echo 255 > /sys/class/leds/led1/brightness
cat /sys/class/leds/led1/brightness
cat /sys/class/leds/led1/max_brightness

cat /sys/class/leds/led1/trigger

会看到 trigger_list
[none] mmc0 mmc1 mmc2 timer
其中的 timer 这个 trigger 是 ledtrig-timer.c 中模块初始化的时候注册进去的

echo timer > /sys/class/leds/led1/trigger
这一句会调用
led_trigger_store()->
   led_trigger_set()->
     trigger->activate(led_cdev);
从而调用 ledtrig-timer.c 文件里 的timer_trig_activate()，
在 /sys/class/leds/led1/ 下创建 delay_on、delay_off 两个文件

echo 100 > /sys/class/leds/led1/delay_on
echo 200 > /sys/class/leds/led1/delay_off
这样会闪烁，亮 100ms 灭 200ms

echo 0 > /sys/class/leds/led1/delay_on
或
echo 0 > /sys/class/leds/led1/brightness