Android电源管理系统调研报告-（4）

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;       }

 

 

打开设备结点后，最终通过 write 将 brigthness 写入到了设备结点。