Android电源管理系统调研报告-（4）

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二、Brightness的调节

现在通过一个具体的电源管理实例来了解从andriod上层到内核驱动层的整个调用流程。

如果你使用过android操作系统，无论是模拟器还是开发板亦或手机，对里面setting这个服务一定很熟悉吧。其中有一项是用于调节显示屏亮度的：

setting/sound & display settings/brightness。

这个功能是怎么实现的呢。通过分析，我们可以清晰看到整个调用的流程，上层是如何一步一步到达驱动层，把LCD屏幕亮度改变的。

1、android层的调用流程。1）、BrightnessPreference.java

/packages/apps/Settings/src/com/android/settings/

// Backlight range is from 0 - 255. Need to make sure that user

// doesn't set the backlight to 0 and get stuck

private static final int MINIMUM_BACKLIGHT = android.os.Power.BRIGHTNESS_DIM + 10;

private static final int MAXIMUM_BACKLIGHT = android.os.Power.BRIGHTNESS_ON;

当系统检测到调节亮度的调节栏改变时，会重新设置屏幕的亮度：

public void (CompoundButton buttonView, boolean isChecked) {

setMode(isChecked ? Settings.System.SCREEN_BRIGHTNESS_MODE_AUTOMATIC

: Settings.System.SCREEN_BRIGHTNESS_MODE_MANUAL);

if (!isChecked) {

setBrightness(mSeekBar.getProgress() + MINIMUM_BACKLIGHT);

}

private void setBrightness(int brightness) {

try {

IPowerManager power = IPowerManager.Stub.asInterface(

ServiceManager.getService("power"));

if (power != null) {

power.setBacklightBrightness(brightness);

}

} catch (RemoteException doe) {

}

首先，onCheckedChanged会检查当前亮度调节的模式，如果是AUTOMATIC模式，则isChecked为1（MANUAL为0）。当isChecked为0时，即当前模式为手动调节模式，则会调用setBrightness调节亮度。

可以看到，setBrightness先会通过调用ServiceManager.getService("power")获得power manager service这个服务的对象power,获取成功后会调用power.setBacklightBrightness(brightness);我们进入PowerManagerService.java看这个方法的具体内容。

2）、PowerManagerService.java

/frameworks/base/services/java/com/android/server/

手动调用的范围是MINI到MAX。而setBrightness传进来的参数为 MINI + MINIMUM_BACKLIGHT到 MAX + MINIMUM_BACKLIGHT，以确保brightness最小为MINIMUM_BACKLIGHT。在setBrightness方法的一开始又对brightness进行了修正，从而确保应用程序不能将屏幕全部熄灭。

// Don't let applications turn the screen all the way off

brightness = Math.max(brightness, Power.BRIGHTNESS_DIM)

setBacklightBrightness这个方法会调节lcd backlight、keyboard light、和buttons light这三个设备的亮度，但都是通过调用setLightBrightness_UNCHECKED实现的。

mHardware.setLightBrightness_UNCHECKED(HardwareService.LIGHT_ID_BACKLIGHT, brightness, HardwareService.BRIGHTNESS_MODE_USER);

mHardware.setLightBrightness_UNCHECKED(HardwareService.LIGHT_ID_KEYBOARD, (mKeyboardVisible ? brightness : 0), HardwareService.BRIGHTNESS_MODE_USER);

mHardware.setLightBrightness_UNCHECKED(HardwareService.LIGHT_ID_BUTTONS, brightness, HardwareService.BRIGHTNESS_MODE_USER);

让我们先看一下第一个设备亮度的调节，注意参数LIGHT_ID_BACKLIGHT，它将决定选择操作哪一个设备。

3）、HardwareService.java

/frameworks/base/services/java/com/android/server/

void setLightBrightness_UNCHECKED(int light, int brightness, int brightnessMode) {

int b = brightness & 0x000000ff;

b = 0xff000000 | (b << 16) | (b << 8) | b;

setLight_native(mNativePointer, light, b, LIGHT_FLASH_NONE, 0, 0, brightnessMode);

}

setLightBrightness_UNCHECKED调用了本地方法setLight_native，同时将brightness做了相就的处理，处理后的数据作为colorARGB传入本地方法。

4）、com_android_server_HardwareService.java

/frameworks/base/services/jni/

setLight_native函数中定义了一个类型为struct light_state_t的结构体，并对这个结构体的成员进行赋值，最后对一个函数指针进行了赋值：

devices->lights[light]->set_light(devices->lights[light],&state);

先来看看这个结构体：

struct light_state_t {

/**

* The color of the LED in ARGB.

* Do your best here.

* - If your light can only do red or green, if they ask for blue,

* you should do green.

* - If you can only do a brightness ramp, then use this formula:

* unsigned char brightness = ((77*((color>>16)&0x00ff))

* + (150*((color>>8)&0x00ff)) + (29*(color&0x00ff))) >> 8;

* - If you can only do on or off, 0 is off, anything else is on.

* The high byte should be ignored. Callers will set it to 0xff (which

* would correspond to 255 alpha).

unsigned int color;

int flashMode;

int flashOnMS;

int flashOffMS;

/**

* Policy used by the framework to manage the light's brightness.

* Currently the values are BRIGHTNESS_MODE_USER and BRIGHTNESS_MODE_SENSOR.

int brightnessMode;

};

再来看这个函数指针指向的函数。在Lights.c这个文件中的open_lights这个函数中，对传入参数的不同对set_light进行了赋值：

在这里，name传入的是LIGHT_ID_BACKLIGHT

if (0 == strcmp(LIGHT_ID_BACKLIGHT, name)) {

set_light = set_light_backlight;

}

set_light_backlight的函数原型如下：

static int

set_light_backlight(struct light_device_t* dev,

struct light_state_t const* state)

再对比一下函数指针的赋值：

devices->lights[light]->set_light(devices->lights[light],&state);

可见，参数devices->lights[light]，&state分别传入了dev和state。

让我们来看一下set_light_backlight的内容：

5）、Lights.c

/hardware/msm7k/liblights/

这个文件已经不陌生了吧，上文刚刚提到呵。

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;

}

(1)、在这里对brightness进行了特殊的赋值：

rgb_to_brightness(struct light_state_t const* state)

{

int color = state->color & 0x00ffffff;

return ((77*((color>>16)&0x00ff))

+ (150*((color>>8)&0x00ff)) + (29*(color&0x00ff))) >> 8;

}

之所以这样赋值在struct light_state_t的注释中有解释。

(2)、write_int的参数LCD_FILE

char const*const LCD_FILE

= "/sys/class/leds/lcd-backlight/brightness";

这个路径是kernel中sysfs生成的设备节点，用户可以直接对这个设备节点进行操作。

sysfs ~~~~~

When a driver is registered, a sysfs directoryis created in its bus's directory. In this directory, the driver can export aninterface to userspace to control operation of the driver on a global basis;e.g. toggling debugging output in the driver.

A future feature of this directory will be a'devices' directory. This directory will contain symlinks to the directories ofdevices it supports.

(3)、

err = write_int(LCD_FILE, brightness);

write_int(char const* path, int value)

{

int fd;

static int already_warned = 0;

fd = open(path, O_RDWR);

if (fd >= 0) {

char buffer[20];

int bytes = sprintf(buffer, "%d/n", value);

int amt = write(fd, buffer, bytes);

close(fd);

return amt == -1 ? -errno : 0;

} else {

if (already_warned == 0) {

LOGE("write_int failed to open %s/n", path);

already_warned = 1;

}

return -errno;

}