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（framework/base/services/java/com/android/server/）中实现了上述屏幕状态的切换。下面对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 mLightsService
mContext = context;
mActivityService = activity;
mBatteryStats = ();
mBatteryService = battery;
mLcdLight = (LightsService.LIGHT_ID_BACKLIGHT); // mLcdLight
mButtonLight = (LightsService.LIGHT_ID_BUTTONS);
mKeyboardLight = (LightsService.LIGHT_ID_KEYBOARD);
mAttentionLight = (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 succeed
= false;
}
boolean savedActivityAllowed = mUserActivityAllowed;
mUserActivityAllowed = true;
userActivity((), 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 = ();
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 (!) {
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 = (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mKeylightDelay
when = now + mKeylightDelay;
break;
case SCREEN_DIM: // 新状态是DIM，则when加上mDimDelay
if (mDimDelay >= 0) {
when = now + mDimDelay;
break;
}
case SCREEN_OFF: // 新状态是OFF，则when加上mScreenOffDelay
synchronized (mLocks) {
when = now + mScreenOffDelay;
}
break;
default:
when = now;
break;
}
} else { // 如果定时时间设定，即为timeoutOverride
override: {
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;
}
}
(mTimeoutTask); // 移除旧的mTimeoutTask时间
= nextState; // 赋值状态
= timeoutOverride > 0
? (originalTimeoutOverride - timeoutOverride)
: -1;
(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 mLocks
long remainingTimeoutOverride;
public void run()
{
synchronized (mLocks) {
if (nextState == -1) {
return;
}
mUserState = ;
setPowerState( | mWakeLockState); // 调用setPowerState来睡眠或唤醒系统
long now = ();
switch () // 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onMask
if ((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onMask
if ((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nominalCurrentValue
break;
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 possible
nominalCurrentValue = (int);
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 bright
steps = ANIM_STEPS;
} else {
// was dim
steps = (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 bright
steps = (int)(ANIM_STEPS*(1.0f-ratio)*scale);
} else {
// was dim
steps = (int)(ANIM_STEPS*ratio);
}
if (mStayOnConditions != 0 && (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 = ();
mAlwaysOnAndDimmed = true;
}
brightness = mScreenDim; // 如果新状态是变暗屏幕，则赋值brightness为mScreenDim
}
}
if (!mSkippedScreenOn) {
(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;
(this); // 移除旧的事件
(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 = ();
boolean more = (); // 使用stepLocked逐渐调至目标亮度
if (more) {
(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);
}
(); // 直接变成目标亮度
} else if (turningOn) {
if (electrifying) {
int delay=().getInteger(.config_screenOnAnimation);
if(delay>0) {
startElectronBeamDelayed(new Runnable() {
@Override
public void run() {
startElectronBeamOnAnimation();
synchronized(mElectronBeamOnHandler) {
();
}
}
},delay);
} else {
startElectronBeamOnAnimation();
}
} else {
(); // 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
(value, brightnessMode);
mLastLcdValue = value;
}
if ((mask & BUTTON_BRIGHT_BIT) != 0) { // 如果是按键亮度，则使用
// 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.
(mLightSensorButtonBrightness >= 0 && value > 0 ?
mLightSensorButtonBrightness : value);
}
if ((mask & KEYBOARD_BRIGHT_BIT) != 0) { // 如果是键盘亮度，则使用
(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方法，其中mLcdLight是类型为的对象，在PowerManagerService的初始化函数中进行了赋值：
mLcdLight = (LightsService.LIGHT_ID_BACKLIGHT);
类型定义在framework/base/services/java/com/android/server/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));
= colorARGB;
= flashMode;
= onMS;
= offMS;
= 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/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/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/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-> = HARDWARE_DEVICE_TAG;
dev-> = 0;
dev-> = (struct hw_module_t*)module;
dev-> = (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/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/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_TRIGGERS
init_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_TRIGGERS
led_trigger_set_default(led_cdev);
#endif
printk(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,
};//
模块为例子（有regulator）
在kernel/drivers/leds/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/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_led
ssize_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/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/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);
else
drvdata->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,
};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_led
ssize_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/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 into
driver 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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