Android系统的状态包括wake、earlysuspend以及suspend状态，其使用锁和定时器来进行状态的切换。
而在wake状态，屏幕首先是调至设定的亮度，如果没有其他动作，当经过一段时间后屏幕会变暗，再经过一段时间屏幕会关闭，于是屏幕的状态也包括3种：bright、dim、off。
在Android应用框架层中的PowerManagerService.java（framework/base/services/java/com/android/server/）中实现了上述屏幕状态的切换。下面对PowerManagerService.java如何切换屏幕状态进行分析。
在PowerManagerService的初始化函数init中，会进行必要参数的初始化，包括LightsService，BatteryService，Thread等等，然后会使用forceUserActivityLocked点亮屏幕。
void init(Context context, LightsService lights, IActivityManager activity,BatteryService battery) {mLightsService = lights; // LightsService mLightsServicemContext = context;mActivityService = activity;mBatteryStats = BatteryStatsService.getService();mBatteryService = battery;mLcdLight = lights.getLight(LightsService.LIGHT_ID_BACKLIGHT); // LightsService.Light mLcdLightmButtonLight = lights.getLight(LightsService.LIGHT_ID_BUTTONS);mKeyboardLight = lights.getLight(LightsService.LIGHT_ID_KEYBOARD);mAttentionLight = lights.getLight(LightsService.LIGHT_ID_ATTENTION);......synchronized (mLocks) {updateNativePowerStateLocked();forceUserActivityLocked(); // 强制点亮屏幕mInitialized = true;}}
在forceUserActivityLocked中主要是使用userActivity点亮屏幕
private void forceUserActivityLocked() {if (isScreenTurningOffLocked()) {// cancel animation so userActivity will succeedmScreenBrightness.animating = false;}boolean savedActivityAllowed = mUserActivityAllowed;mUserActivityAllowed = true;userActivity(SystemClock.uptimeMillis(), false); // 使用userActivity点亮屏幕mUserActivityAllowed = savedActivityAllowed;}public void userActivity(long time, boolean noChangeLights) {......userActivity(time, -1, noChangeLights, OTHER_EVENT, false);}
在userActivity方法中会收集所有锁的状态（mLocks存储了所有申请的锁），然后通过setPowerState方法来设置系统的状态，最后通过setTimeoutLocked来开启定时器
private void userActivity(long time, long timeoutOverride, boolean noChangeLights,int eventType, boolean force) {......if (!mAutoBrightnessButtonKeyboard) {// Turn on button (and keyboard) backlights on any event, so that they// don't suddenly disappear when the lock screen is unlocked (OTHER_EVENT),// and so capacitive buttons can be found on devices where they lack// identifying surface features.mUserState = (mKeyboardVisible ? ALL_BRIGHT : SCREEN_BUTTON_BRIGHT);} else {// don't clear button/keyboard backlights when the screen is touched.mUserState |= SCREEN_BRIGHT;}mWakeLockState = mLocks.reactivateScreenLocksLocked();setPowerState(mUserState | mWakeLockState, noChangeLights, WindowManagerPolicy.OFF_BECAUSE_OF_USER);setTimeoutLocked(time, timeoutOverride, SCREEN_BRIGHT);......}
setPowerState方法会根据输入的状态调用setScreenStateLocked方法来设置系统状态
private void setPowerState(int newState, boolean noChangeLights, int reason){......boolean oldScreenOn = (mPowerState & SCREEN_ON_BIT) != 0; // 记录系统当前屏幕状态boolean newScreenOn = (newState & SCREEN_ON_BIT) != 0; // 记录新的屏幕状态final boolean stateChanged = mPowerState != newState; // 记录状态是否改变if (oldScreenOn != newScreenOn) { // 新的屏幕状态和当前屏幕状态不一致时if (newScreenOn) { // 新状态是开启屏幕boolean reallyTurnScreenOn = true;if (mPreventScreenOn) {reallyTurnScreenOn = false; // 如果屏幕开启被阻止，则设置reallyTurnScreenOn为false}if (reallyTurnScreenOn) {err = setScreenStateLocked(true); // 使用setScreenStateLocked唤醒系统......} else {setScreenStateLocked(false); // 使系统睡眠// But continue as if we really did turn the screen on...err = 0;}if (err == 0) {sendNotificationLocked(true, -1);if (stateChanged) {updateLightsLocked(newState, 0);}mPowerState |= SCREEN_ON_BIT;} else { // 如果新状态是关闭屏幕，则使用screenOffFinishedAnimatingLocked方法使系统睡眠if (stateChanged) {updateLightsLocked(newState, 0);}......if (!mScreenBrightness.animating) {err = screenOffFinishedAnimatingLocked(reason); // 该方法也是调用setScreenStateLocked方法睡眠系统} else {err = 0;mLastTouchDown = 0;}}} else if (stateChanged) {// Screen on/off didn't change, but lights may have.updateLightsLocked(newState, 0);}......}
在setScreenStateLocked方法中会使用Power.setScreenState方法调用jni层中的函数，最终会传递至内核层，在内核层中执行相应的睡眠系统或唤醒系统
private int setScreenStateLocked(boolean on) {......int err = Power.setScreenState(on);......}
而setScreenStateLocked方法中，函数updateLightsLocked用来更新lights，后面将进行分析。
以上一小段介绍了userActivity唤醒系统的简单流程，而在PowerManagerService的初始化函数中，不仅会通过userActivity中的setPowerState来唤醒系统，同时也会使用userActivity中的setTimeoutLocked来开启一个定时器，用于切换屏幕的状态
private void setTimeoutLocked(long now, final long originalTimeoutOverride, int nextState) {long timeoutOverride = originalTimeoutOverride;......long when = 0;if (timeoutOverride <= 0) { // 时间设置<=0时，此时系统会使用缺省的定时时间开启计时器switch (nextState){case SCREEN_BRIGHT: // 新状态是BRIGHT，则when加上mKeylightDelaywhen = now + mKeylightDelay;break;case SCREEN_DIM: // 新状态是DIM，则when加上mDimDelayif (mDimDelay >= 0) {when = now + mDimDelay;break;}case SCREEN_OFF: // 新状态是OFF，则when加上mScreenOffDelaysynchronized (mLocks) {when = now + mScreenOffDelay;}break;default:when = now;break;}} else { // 如果定时时间设定，即为timeoutOverrideoverride: {if (timeoutOverride <= mScreenOffDelay) {when = now + timeoutOverride;nextState = SCREEN_OFF;break override;}timeoutOverride -= mScreenOffDelay;if (mDimDelay >= 0) {if (timeoutOverride <= mDimDelay) {when = now + timeoutOverride;nextState = SCREEN_DIM;break override;}timeoutOverride -= mDimDelay;}when = now + timeoutOverride;nextState = SCREEN_BRIGHT;}}mHandler.removeCallbacks(mTimeoutTask); // 移除旧的mTimeoutTask时间mTimeoutTask.nextState = nextState; // 赋值状态mTimeoutTask.remainingTimeoutOverride = timeoutOverride > 0? (originalTimeoutOverride - timeoutOverride): -1;mHandler.postAtTime(mTimeoutTask, when); // 重新启动定时器，在when时间后执行mTimeoutTask任务mNextTimeout = when;}
在PowerManagerService的初始化函数中最终会通过setTimeoutLocked来启动定时器，当定时器到时间后就会执行mTimeoutTask任务，下面可以看看mTimeoutTask任务的定义：
TimeoutTask mTimeoutTask = new TimeoutTask();
mTimeoutTask定义为类型是TimeoutTask的对象，TimeoutTask如下所示：
private class TimeoutTask implements Runnable{int nextState; // access should be synchronized on mLockslong remainingTimeoutOverride;public void run(){synchronized (mLocks) {if (nextState == -1) {return;}mUserState = this.nextState;setPowerState(this.nextState | mWakeLockState); // 调用setPowerState来睡眠或唤醒系统long now = SystemClock.uptimeMillis();switch (this.nextState) // 更新定时器{case SCREEN_BRIGHT: // 如果状态是BRIGHT，则更新定时器，是定时器在到期后执行SCREEN_DIM操作if (mDimDelay >= 0) {setTimeoutLocked(now, remainingTimeoutOverride, SCREEN_DIM);break;}case SCREEN_DIM: // 如果状态是DIM，则更新定时器，是定时器在到期后执行SCREEN_OFF操作setTimeoutLocked(now, remainingTimeoutOverride, SCREEN_OFF);break;}}}}
于是在PowerManagerService中通过定时器来切换屏幕的状态，同时也会调用setPowerState方法来睡眠或唤醒系统，而具体的屏幕亮度是如何实现的呢？
在setPowerState中使用了updateLightsLocked来更新屏幕的状态。
private void updateLightsLocked(int newState, int forceState) {final int oldState = mPowerState; // 将当前系统状态赋值于oldState......final int realDifference = (newState ^ oldState); // 判断新状态和旧状态的不同之处final int difference = realDifference | forceState;if (difference == 0) {return;}int offMask = 0;int dimMask = 0;int onMask = 0;int preferredBrightness = getPreferredBrightness(); // 获取缺省的亮度值if ((difference & KEYBOARD_BRIGHT_BIT) != 0) { // 如果是键盘亮度不同，则更新offMask和onMaskif ((newState & KEYBOARD_BRIGHT_BIT) == 0) {offMask |= KEYBOARD_BRIGHT_BIT;} else {onMask |= KEYBOARD_BRIGHT_BIT;}}if ((difference & BUTTON_BRIGHT_BIT) != 0) { // 如果是按键亮度不同，则更新offMask和onMaskif ((newState & BUTTON_BRIGHT_BIT) == 0) {offMask |= BUTTON_BRIGHT_BIT;} else {onMask |= BUTTON_BRIGHT_BIT;}}if ((difference & (SCREEN_ON_BIT | SCREEN_BRIGHT_BIT)) != 0) { // 如果是屏幕开启或点亮状态位不同int nominalCurrentValue = -1; // 当前亮度if ((realDifference & (SCREEN_ON_BIT | SCREEN_BRIGHT_BIT)) != 0) {switch (oldState & (SCREEN_BRIGHT_BIT|SCREEN_ON_BIT)) { // 判断旧的状态case SCREEN_BRIGHT_BIT | SCREEN_ON_BIT: // 如果旧状态是开启并点亮屏幕nominalCurrentValue = preferredBrightness; // 则将preferredBrightness赋给nominalCurrentValuebreak;case SCREEN_ON_BIT: // 如果旧状态仅仅是开启屏幕nominalCurrentValue = mScreenDim;break;case 0: // 如果旧状态是关闭屏幕nominalCurrentValue = Power.BRIGHTNESS_OFF;break;case SCREEN_BRIGHT_BIT:default:// not possiblenominalCurrentValue = (int)mScreenBrightness.curValue;break;}}int brightness = preferredBrightness;int steps = ANIM_STEPS;if ((newState & SCREEN_BRIGHT_BIT) == 0) { // 如果新状态不是点亮屏幕，即变暗屏幕或关闭屏幕// 此时会使用动画操作渐渐的使屏幕变暗或关闭，这里计算step// dim or turn off backlight, depending on if the screen is on// the scale is because the brightness ramp isn't linear and this biases// it so the later parts take longer.final float scale = 1.5f;float ratio = (((float)mScreenDim)/preferredBrightness);if (ratio > 1.0f) ratio = 1.0f;if ((newState & SCREEN_ON_BIT) == 0) {if ((oldState & SCREEN_BRIGHT_BIT) != 0) {// was brightsteps = ANIM_STEPS;} else {// was dimsteps = (int)(ANIM_STEPS*ratio*scale);}brightness = Power.BRIGHTNESS_OFF; // 如果新状态是关闭屏幕，则赋值brightness为Power.BRIGHTNESS_OFF} else { // brightness为新状态的亮度if ((oldState & SCREEN_ON_BIT) != 0) {// was brightsteps = (int)(ANIM_STEPS*(1.0f-ratio)*scale);} else {// was dimsteps = (int)(ANIM_STEPS*ratio);}if (mStayOnConditions != 0 && mBatteryService.isPowered(mStayOnConditions)) {// If the "stay on while plugged in" option is// turned on, then the screen will often not// automatically turn off while plugged in. To// still have a sense of when it is inactive, we// will then count going dim as turning off.mScreenOffTime = SystemClock.elapsedRealtime();mAlwaysOnAndDimmed = true;}brightness = mScreenDim; // 如果新状态是变暗屏幕，则赋值brightness为mScreenDim}}if (!mSkippedScreenOn) {mScreenBrightness.setTargetLocked(brightness, steps,INITIAL_SCREEN_BRIGHTNESS, nominalCurrentValue); // 设置屏幕的亮度}}// 以下根据bit位的不同调用setLightBrightness来设置亮度，包括屏幕、键盘和按键if (offMask != 0) {setLightBrightness(offMask, Power.BRIGHTNESS_OFF);}if (dimMask != 0) {int brightness = mScreenBrightnessDim;if ((newState & BATTERY_LOW_BIT) != 0 &&brightness > Power.BRIGHTNESS_LOW_BATTERY) {brightness = Power.BRIGHTNESS_LOW_BATTERY;}setLightBrightness(dimMask, brightness);}if (onMask != 0) {int brightness = getPreferredBrightness();if ((newState & BATTERY_LOW_BIT) != 0 &&brightness > Power.BRIGHTNESS_LOW_BATTERY) {brightness = Power.BRIGHTNESS_LOW_BATTERY;}setLightBrightness(onMask, brightness);}}
在updateLightsLocked方法中使用了mScreenBrightness.setTargetLocked方法来设置屏幕的亮度，而mScreenBrightness是类型为BrightnessState的对象，以下是其定义：
private final BrightnessState mScreenBrightness = new BrightnessState(SCREEN_BRIGHT_BIT);
此类是通过逐渐减少step值的亮度来是的屏幕到达最终的亮度，下面分析setTargetLocked方法
void setTargetLocked(int target, int stepsToTarget, int initialValue,int nominalCurrentValue) {if (!initialized) { // 如果正在进行且目标亮度相同则返回initialized = true;curValue = (float)initialValue;} else if (targetValue == target) {return;}targetValue = target;delta = (targetValue -(nominalCurrentValue >= 0 ? nominalCurrentValue : curValue))/ stepsToTarget; // 计算delta值animating = true;mScreenOffHandler.removeCallbacks(this); // 移除旧的事件mScreenOffHandler.post(this); // 开始新的事件}
其中mScreenOffHandler移除或开始的事件即为mScreenBrightness本身，此时会执行BrightnessState类中的run方法。
public void run() {synchronized (mLocks) {final boolean turningOn = animating && (int)curValue == Power.BRIGHTNESS_OFF; // 是否是打开屏幕final boolean turningOff = animating && targetValue == Power.BRIGHTNESS_OFF; // 是否是关闭屏幕// Check for the electron beam for fully on/off transitions.// Otherwise, allow it to fade the brightness as normal.final boolean electrifying =((mElectronBeamAnimationOff && turningOff) ||(mElectronBeamAnimationOn && turningOn));if (!electrifying && (mAnimateScreenLights || !turningOff)) { long now = SystemClock.uptimeMillis();boolean more = mScreenBrightness.stepLocked(); // 使用stepLocked逐渐调至目标亮度if (more) {mScreenOffHandler.postAtTime(this, now+(1000/60));}} else {// It's pretty scary to hold mLocks for this long, and we should// redesign this, but it works for now.if (turningOff) {if (electrifying) {nativeStartSurfaceFlingerOffAnimation(mScreenOffReason == WindowManagerPolicy.OFF_BECAUSE_OF_PROX_SENSOR? 0 : mAnimationSetting);}mScreenBrightness.jumpToTargetLocked(); // 直接变成目标亮度} else if (turningOn) {if (electrifying) {int delay=mContext.getResources().getInteger(com.android.internal.R.integer.config_screenOnAnimation);if(delay>0) {startElectronBeamDelayed(new Runnable() {@Overridepublic void run() {startElectronBeamOnAnimation();synchronized(mElectronBeamOnHandler) {mElectronBeamOnHandler.notifyAll();}}},delay);} else {startElectronBeamOnAnimation();}} else {mScreenBrightness.jumpToTargetLocked(); // 直接变成目标亮度}}}}}
在run方法中，不论是逐渐调制目标亮度stepLocked，或者直接变成目标亮度jumpToTargetLocked，都会调用到setLightBrightness方法用于改变亮度，下面来分析setLightBrightness方法。
private void setLightBrightness(int mask, int value) {int brightnessMode = (mAutoBrightessEnabled? LightsService.BRIGHTNESS_MODE_SENSOR: LightsService.BRIGHTNESS_MODE_USER);if ((mask & SCREEN_BRIGHT_BIT) != 0) { // 如果是屏幕亮度，则使用mLcdLight.setBrightnessmLcdLight.setBrightness(value, brightnessMode);mLastLcdValue = value;}if ((mask & BUTTON_BRIGHT_BIT) != 0) { // 如果是按键亮度，则使用mButtonLight.setBrightness// Use sensor-determined brightness values when the button (or keyboard)// light is on, since users may want to specify a custom brightness setting// that disables the button (or keyboard) backlight entirely in low-ambient// light situations.mButtonLight.setBrightness(mLightSensorButtonBrightness >= 0 && value > 0 ?mLightSensorButtonBrightness : value);}if ((mask & KEYBOARD_BRIGHT_BIT) != 0) { // 如果是键盘亮度，则使用mKeyboardLight.setBrightnessmKeyboardLight.setBrightness(mLightSensorKeyboardBrightness >= 0 && value > 0 ?mLightSensorKeyboardBrightness : value);}}
在setLightBrightness方法中会根据mask的值来相应的改变屏幕、键盘和按键的亮度，屏幕的亮度主要使用mLcdLight.setBrightness方法，其中mLcdLight是类型为LightsService.Light的对象，在PowerManagerService的初始化函数中进行了赋值：
mLcdLight = lights.getLight(LightsService.LIGHT_ID_BACKLIGHT);
LightsService.Light类型定义在framework/base/services/java/com/android/server/LightsService.java中，下面来分析其setBrightness方法：
public void setBrightness(int brightness, int brightnessMode) {synchronized (this) {int color = brightness & 0x000000ff;color = 0xff000000 | (color << 16) | (color << 8) | color;setLightLocked(color, LIGHT_FLASH_NONE, 0, 0, brightnessMode);}}
在setBrightness方法中，主要是将亮度值扩展成32bit的color值来提供给setLightLocked处理。
private void setLightLocked(int color, int mode, int onMS, int offMS, int brightnessMode) {if (color != mColor || mode != mMode || onMS != mOnMS || offMS != mOffMS) {mColor = color;mMode = mode;mOnMS = onMS;mOffMS = offMS;setLight_native(mNativePointer, mId, color, mode, onMS, offMS, brightnessMode);}}
setLightLocked会调用setLight_native来将亮度值传递至jni层，setLight_native在文件framework/base/services/jni/com_android_server_LightsService.cpp中实现。
static void setLight_native(JNIEnv *env, jobject clazz, int ptr,int light, int colorARGB, int flashMode, int onMS, int offMS, int brightnessMode){Devices* devices = (Devices*)ptr;light_state_t state;if (light < 0 || light >= LIGHT_COUNT || devices->lights[light] == NULL) {return ;}memset(&state, 0, sizeof(light_state_t));state.color = colorARGB;state.flashMode = flashMode;state.flashOnMS = onMS;state.flashOffMS = offMS;state.brightnessMode = brightnessMode;devices->lights[light]->set_light(devices->lights[light], &state);}
而setLight_native方法使用devices->lights[light]->set_light来设定亮度值。在com_android_server_LightsService.cpp的初始化函数中，会对devices进行初始化，如下所示：
static jint init_native(JNIEnv *env, jobject clazz){int err;hw_module_t* module;Devices* devices;devices = (Devices*)malloc(sizeof(Devices));err = hw_get_module(LIGHTS_HARDWARE_MODULE_ID, (hw_module_t const**)&module);if (err == 0) {devices->lights[LIGHT_INDEX_BACKLIGHT]= get_device(module, LIGHT_ID_BACKLIGHT);devices->lights[LIGHT_INDEX_KEYBOARD]= get_device(module, LIGHT_ID_KEYBOARD);devices->lights[LIGHT_INDEX_BUTTONS]= get_device(module, LIGHT_ID_BUTTONS);devices->lights[LIGHT_INDEX_BATTERY]= get_device(module, LIGHT_ID_BATTERY);devices->lights[LIGHT_INDEX_NOTIFICATIONS]= get_device(module, LIGHT_ID_NOTIFICATIONS);devices->lights[LIGHT_INDEX_ATTENTION]= get_device(module, LIGHT_ID_ATTENTION);devices->lights[LIGHT_INDEX_BLUETOOTH]= get_device(module, LIGHT_ID_BLUETOOTH);devices->lights[LIGHT_INDEX_WIFI]= get_device(module, LIGHT_ID_WIFI);} else {memset(devices, 0, sizeof(Devices));}return (jint)devices;}static light_device_t* get_device(hw_module_t* module, char const* name){int err;hw_device_t* device;err = module->methods->open(module, name, &device);if (err == 0) {return (light_device_t*)device;} else {return NULL;}}
该方法使用hw_get_module来获取动态库模块，其中LIGHTS_HARDWARE_MODULE_ID在/hardware/libhardware/include/hardware/lights.h中声明：
#define LIGHTS_HARDWARE_MODULE_ID "lights"
所以在硬件抽象层，如果要编写lights的模块供上层使用，需要将自身命名为lights的模块，下面以hardware/msm7k/liblights/lights.c为例，在其中就声明了“lights”模块
const struct hw_module_t HAL_MODULE_INFO_SYM = {.tag = HARDWARE_MODULE_TAG,.version_major = 1,.version_minor = 0,.id = LIGHTS_HARDWARE_MODULE_ID,.name = "QCT MSM7K lights Module",.author = "Google, Inc.",.methods = &lights_module_methods,};
然后在com_android_server_LightsService.cpp的初始化函数中使用get_device来打开相应的设备，其调用了module->methods->open的方法，而在hardware/msm7k/liblights/lights.c也定义了回调函数：
static struct hw_module_methods_t lights_module_methods = {.open = open_lights,};static int open_lights(const struct hw_module_t* module, char const* name,struct hw_device_t** device){int (*set_light)(struct light_device_t* dev,struct light_state_t const* state);if (0 == strcmp(LIGHT_ID_BACKLIGHT, name)) {set_light = set_light_backlight;}else if (0 == strcmp(LIGHT_ID_KEYBOARD, name)) {set_light = set_light_keyboard;}else if (0 == strcmp(LIGHT_ID_BUTTONS, name)) {set_light = set_light_buttons;}else if (0 == strcmp(LIGHT_ID_BATTERY, name)) {set_light = set_light_battery;}else if (0 == strcmp(LIGHT_ID_NOTIFICATIONS, name)) {set_light = set_light_notifications;}else if (0 == strcmp(LIGHT_ID_ATTENTION, name)) {set_light = set_light_attention;}else {return -EINVAL;}pthread_once(&g_init, init_globals);struct light_device_t *dev = malloc(sizeof(struct light_device_t));memset(dev, 0, sizeof(*dev));dev->common.tag = HARDWARE_DEVICE_TAG;dev->common.version = 0;dev->common.module = (struct hw_module_t*)module;dev->common.close = (int (*)(struct hw_device_t*))close_lights;dev->set_light = set_light;*device = (struct hw_device_t*)dev;return 0;}
在open_lights方法中也映射了set_light方法，于是在com_android_server_LightsService.cpp的devices->lights[light]->set_light最终会调用hardware/msm7k/liblights/lights.c中set_light所映射的方法，set_light_backlight就是设置屏幕亮度的方法。
static int set_light_backlight(struct light_device_t* dev,struct light_state_t const* state){int err = 0;int brightness = rgb_to_brightness(state);pthread_mutex_lock(&g_lock);g_backlight = brightness;err = write_int(LCD_FILE, brightness);if (g_haveTrackballLight) {handle_trackball_light_locked(dev);}pthread_mutex_unlock(&g_lock);return err;}
其中char const*const LCD_FILE = "/sys/class/leds/lcd-backlight/brightness";
该方法会使用write_int函数将亮度写入至路径为LCD_FILE的文件中，即传递至内核层。
用户空间
//
内核空间
在内核空间中的kernel/drivers/leds/led-class.c模块初始化函数中，使用了class_create创建了“leds”类目录，同时在这个模块中给出了led_classdev_register用于注册led设备，除此之外，还给出了这个class下的相关属性：
// 模块初始化函数static int __init leds_init(void){leds_class = class_create(THIS_MODULE, "leds");if (IS_ERR(leds_class))return PTR_ERR(leds_class);leds_class->suspend = led_suspend;leds_class->resume = led_resume;leds_class->dev_attrs = led_class_attrs;return 0;}// led设备注册函数int led_classdev_register(struct device *parent, struct led_classdev *led_cdev){led_cdev->dev = device_create(leds_class, parent, 0, led_cdev,"%s", led_cdev->name);if (IS_ERR(led_cdev->dev))return PTR_ERR(led_cdev->dev);#ifdef CONFIG_LEDS_TRIGGERSinit_rwsem(&led_cdev->trigger_lock);#endif/* add to the list of leds */down_write(&leds_list_lock);list_add_tail(&led_cdev->node, &leds_list);up_write(&leds_list_lock);if (!led_cdev->max_brightness)led_cdev->max_brightness = LED_FULL;led_update_brightness(led_cdev);init_timer(&led_cdev->blink_timer);led_cdev->blink_timer.function = led_timer_function;led_cdev->blink_timer.data = (unsigned long)led_cdev;#ifdef CONFIG_LEDS_TRIGGERSled_trigger_set_default(led_cdev);#endifprintk(KERN_DEBUG "Registered led device: %s\n",led_cdev->name);return 0;}EXPORT_SYMBOL_GPL(led_classdev_register);// led属性static struct device_attribute led_class_attrs[] = {__ATTR(brightness, 0644, led_brightness_show, led_brightness_store), __ATTR(max_brightness, 0444, led_max_brightness_show, NULL),#ifdef CONFIG_LEDS_TRIGGERS__ATTR(trigger, 0644, led_trigger_show, led_trigger_store),#endif__ATTR_NULL,};
//
leds-lm3530.c模块为例子（有regulator）
在kernel/drivers/leds/leds-lm3530.c模块中，声明了名称为“lcd-backlight”的led设备，
并使用了led_classdev_register将其注册进入led class中。
drvdata->mode = pdata->mode;drvdata->client = client;drvdata->pdata = pdata;drvdata->brightness = LED_OFF;drvdata->enable = false;drvdata->led_dev.name = LM3530_LED_DEV; // #define LM3530_LED_DEV "lcd-backlight"drvdata->led_dev.brightness_set = lm3530_brightness_set;i2c_set_clientdata(client, drvdata);drvdata->regulator = regulator_get(&client->dev, "vin");err = led_classdev_register(&client->dev, &drvdata->led_dev);
于是就会产生/sys/class/leds/lcd-backlight/brightness的目录，所以上层使用write_int(LCD_FILE, brightness)向内核写入亮度值会调用kernel/drivers/leds/led-class.c模块中的led_brightness_store方法。
static ssize_t led_brightness_store(struct device *dev,struct device_attribute *attr, const char *buf, size_t size){struct led_classdev *led_cdev = dev_get_drvdata(dev); // 此处的led_cdev就是kernel/drivers/staging/msm/msm_fb.c中注册的backlight_ledssize_t ret = -EINVAL;char *after;unsigned long state = simple_strtoul(buf, &after, 10);size_t count = after - buf;if (isspace(*after))count++;if (count == size) {ret = count;if (state == LED_OFF)led_trigger_remove(led_cdev);led_set_brightness(led_cdev, state); // 设置屏幕亮度}return ret;}
该方法调用led_set_brightness设置屏幕亮度，其在kernel/drivers/leds/leds.h中
static inline void led_set_brightness(struct led_classdev *led_cdev,enum led_brightness value){if (value > led_cdev->max_brightness)value = led_cdev->max_brightness;led_cdev->brightness = value;if (!(led_cdev->flags & LED_SUSPENDED))led_cdev->brightness_set(led_cdev, value); // 此处的led_cdev就是kernel/drivers/staging/msm/msm_fb.c中注册的backlight_led}
于是就调用了kernel/drivers/leds/leds-lm3530.c模块中的回调函数brightness_set，即lm3530_brightness_set函数
static void lm3530_brightness_set(struct led_classdev *led_cdev,enum led_brightness brt_val){int err;struct lm3530_data *drvdata = container_of(led_cdev, struct lm3530_data, led_dev);switch (drvdata->mode) {case LM3530_BL_MODE_MANUAL:if (!drvdata->enable) {err = lm3530_init_registers(drvdata); // 如果没有使能则会先使能，在这个方法中会调用regulator_enable来开启regulator输出if (err) {dev_err(&drvdata->client->dev, "Register Init failed: %d\n", err);break;}}/* set the brightness in brightness control register*/err = i2c_smbus_write_byte_data(drvdata->client,LM3530_BRT_CTRL_REG, brt_val / 2); // 设置亮度if (err)dev_err(&drvdata->client->dev, "Unable to set brightness: %d\n", err);elsedrvdata->brightness = brt_val / 2;if (brt_val == 0) {err = regulator_disable(drvdata->regulator); // 如果设置亮光为0，则会调用regulator_disable来关闭regulator输出if (err)dev_err(&drvdata->client->dev, "Disable regulator failed\n");drvdata->enable = false;}break;case LM3530_BL_MODE_ALS:break;case LM3530_BL_MODE_PWM:break;default:break;}}
lm3530_brightness_set方法在打开屏幕时会使用regulator_enable开启电源管理芯片的regulator输出，在关闭屏幕时会调用regulator_disable关闭电源管理芯片的regulator输出，在调整屏幕亮度时使用i2c_smbus_write_byte_data向寄存器中写入数值来调整亮度。
///
msm_fb.c模块为例子（无regulator）
在kernel/drivers/staging/msm/msm_fb.c模块中，声明了名称为“lcd-backlight”的led设备，并使用了led_classdev_register将其注册进入led class中。
static struct led_classdev backlight_led = {.name = "lcd-backlight",.brightness = MAX_BACKLIGHT_BRIGHTNESS,.brightness_set= msm_fb_set_bl_brightness,};
于是就会产生/sys/class/leds/lcd-backlight/brightness的目录，所以上层使用write_int(LCD_FILE, brightness)向内核写入亮度值会调用kernel/drivers/leds/led-class.c模块中的led_brightness_store方法。
static ssize_t led_brightness_store(struct device *dev,struct device_attribute *attr, const char *buf, size_t size){struct led_classdev *led_cdev = dev_get_drvdata(dev); // 此处的led_cdev就是kernel/drivers/staging/msm/msm_fb.c中注册的backlight_ledssize_t ret = -EINVAL;char *after;unsigned long state = simple_strtoul(buf, &after, 10);size_t count = after - buf;if (isspace(*after))count++;if (count == size) {ret = count;if (state == LED_OFF)led_trigger_remove(led_cdev);led_set_brightness(led_cdev, state); // 设置屏幕亮度}return ret;}
该方法调用led_set_brightness设置屏幕亮度，其在kernel/drivers/leds/leds.h中
static inline void led_set_brightness(struct led_classdev *led_cdev,enum led_brightness value){if (value > led_cdev->max_brightness)value = led_cdev->max_brightness;led_cdev->brightness = value;if (!(led_cdev->flags & LED_SUSPENDED))led_cdev->brightness_set(led_cdev, value); // 此处的led_cdev就是kernel/drivers/staging/msm/msm_fb.c中注册的backlight_led}
于是就调用了kernel/drivers/staging/msm/msm_fb.c模块中的回调函数brightness_set，即msm_fb_set_bl_brightness函数
static void msm_fb_set_bl_brightness(struct led_classdev *led_cdev,enum led_brightness value){struct msm_fb_data_type *mfd = dev_get_drvdata(led_cdev->dev->parent);int bl_lvl;if (value > MAX_BACKLIGHT_BRIGHTNESS)value = MAX_BACKLIGHT_BRIGHTNESS;/* This maps android backlight level 0 to 255 intodriver backlight level 0 to bl_max with rounding */bl_lvl = (2 * value * mfd->panel_info.bl_max + MAX_BACKLIGHT_BRIGHTNESS)/(2 * MAX_BACKLIGHT_BRIGHTNESS);if (!bl_lvl && value)bl_lvl = 1;msm_fb_set_backlight(mfd, bl_lvl, 1);}
在msm_fb_set_bl_brightness函数中将亮度从0-255映射成0-bl_max，然后使用msm_fb_set_backlight设置屏幕亮度
void msm_fb_set_backlight(struct msm_fb_data_type *mfd, __u32 bkl_lvl, u32 save){struct msm_fb_panel_data *pdata;pdata = (struct msm_fb_panel_data *)mfd->pdev->dev.platform_data;if ((pdata) && (pdata->set_backlight)) {down(&mfd->sem);if ((bkl_lvl != mfd->bl_level) || (!save)) {u32 old_lvl;old_lvl = mfd->bl_level;mfd->bl_level = bkl_lvl;pdata->set_backlight(mfd);if (!save)mfd->bl_level = old_lvl;}up(&mfd->sem);}}
在msm_fb_set_backlight方法中主要调用了各个具体设备的set_backlight回调函数，实现了lcd屏幕的亮度调节。
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