Android電源管理系列之PowerManagerService(二)

摘要： WakeLock機制 PowerManager.WakeLock 為了延長電池的使用壽命，Android裝置會在一段時間後使螢幕變暗，然後關閉螢幕顯示，直至停止CPU進入休眠。WakeLock是Android提供的喚醒鎖機制，用來保持CPU執行或避免螢幕變暗/關閉以...

WakeLock機制

PowerManager.WakeLock

為了延長電池的使用壽命，Android裝置會在一段時間後使螢幕變暗，然後關閉螢幕顯示，直至停止CPU進入休眠。WakeLock是Android提供的喚醒鎖機制，用來保持CPU執行或避免螢幕變暗/關閉以及避免鍵盤背光燈熄滅

喚醒鎖的型別：

Flag	CPU	Screen	Keyboard
PARTIAL_WAKE_LOCK	on	off	off
SCREEN_DIM_WAKE_LOCK	on	Dim	off
SCREEN_BRIGHT_WAKE_LOCK	on	Bright	off
FULL_WAKE_LOCK	on	Bright	on

如果是PARTIAL_WAKE_LOCK,無論螢幕的狀態或是按下電源鍵, CPU都將正常工作。如果是其它的喚醒鎖,裝置會在使用者按下電源鈕後停止工作進入休眠狀態。以上四種鎖，除了PARTIAL_WAKE_LOCK，其餘的鎖在API level 17已經被deprecated了。

喚醒鎖的使用方法

程式碼使用：

PowerManager powerManager = (PowerManager) getSystemService(Context.POWER_SERVICE);
PowerManager.WakeLock wl =powerManager.newWakeLock(PowerManager.PARTIAL_WAKE_LOCK, "My Tag");
wl.acquire();　//acquire時儘量申明timeout時間
// ... do work...
wl.release();

許可權申明：

<uses-permission android:name="android.permission.WAKE_LOCK" />
<uses-permission android:name="android.permission.DEVICE_POWER"/>

在應用程式中使用WakeLock時必須申明許可權，acquire請求喚醒鎖時儘量設定timeout時間釋放WakeLock，以避免長時間持有WakeLock導致系統無法休眠。

喚醒鎖的實現：

frameworks/base/core/java/android/os/PowerManager.java

public final class WakeLock {
private int mFlags;
private String mTag;
private final String mPackageName;
private final IBinder mToken;
private int mCount;
private boolean mRefCounted = true;
private boolean mHeld;
private WorkSource mWorkSource;
private String mHistoryTag;
private final String mTraceName;
......
WakeLock(int flags, String tag, String packageName) {
mFlags = flags;
mTag = tag;
mPackageName = packageName;
mToken = new Binder();
mTraceName = "WakeLock (" + mTag + ")";
}
......
}

updatePowerStateLocked

frameworks/base/services/core/java/com/android/server/power/PowerManagerService.java

protected void updatePowerStateLocked() {
// 服務沒有ready,mDirty值沒有設定情況下不做更新操作
if (!mSystemReady || mDirty == 0) {
return;
}
if (!Thread.holdsLock(mLock)) {
Slog.wtf(TAG, "Power manager lock was not held when calling updatePowerStateLocked");
}

try {
// Phase 0: 更新基本狀態
updateIsPoweredLocked(mDirty);
updateStayOnLocked(mDirty);
updateScreenBrightnessBoostLocked(mDirty);

// Phase 1: Update wakefulness.
// Loop because the wake lock and user activity computations are influenced
// by changes in wakefulness.
final long now = SystemClock.uptimeMillis();
int dirtyPhase2 = 0;
for (;;) {
int dirtyPhase1 = mDirty;
dirtyPhase2 |= dirtyPhase1;
mDirty = 0;

updateWakeLockSummaryLocked(dirtyPhase1);
updateUserActivitySummaryLocked(now, dirtyPhase1);
if (!updateWakefulnessLocked(dirtyPhase1)) {
break;
}
}

// Phase 2: Update display power state.
boolean displayBecameReady = updateDisplayPowerStateLocked(dirtyPhase2);

// Phase 3: Update dream state (depends on display ready signal).
updateDreamLocked(dirtyPhase2, displayBecameReady);

// Phase 4: Send notifications, if needed.
finishWakefulnessChangeIfNeededLocked();

// Phase 5: Update suspend blocker.
// Because we might release the last suspend blocker here, we need to make sure
// we finished everything else first!
updateSuspendBlockerLocked();
} finally {
Trace.traceEnd(Trace.TRACE_TAG_POWER);
}
}

首先來看updateIsPoweredLocked(mDirty);

private void updateIsPoweredLocked(int dirty) {
if ((dirty & DIRTY_BATTERY_STATE) != 0) {
final boolean wasPowered = mIsPowered;
final int oldPlugType = mPlugType;
final boolean oldLevelLow = mBatteryLevelLow;
mIsPowered = mBatteryManagerInternal.isPowered(BatteryManager.BATTERY_PLUGGED_ANY);
mPlugType = mBatteryManagerInternal.getPlugType();
mBatteryLevel = mBatteryManagerInternal.getBatteryLevel();
mBatteryLevelLow = mBatteryManagerInternal.getBatteryLevelLow();

if (wasPowered != mIsPowered || oldPlugType != mPlugType) {
mDirty |= DIRTY_IS_POWERED;

// Update wireless dock detection state.
final boolean dockedOnWirelessCharger = mWirelessChargerDetector.update(
mIsPowered, mPlugType, mBatteryLevel);

// Treat plugging and unplugging the devices as a user activity.
// Users find it disconcerting when they plug or unplug the device
// and it shuts off right away.
// Some devices also wake the device when plugged or unplugged because
// they don't have a charging LED.
final long now = SystemClock.uptimeMillis();
if (shouldWakeUpWhenPluggedOrUnpluggedLocked(wasPowered, oldPlugType,
dockedOnWirelessCharger)) {
wakeUpNoUpdateLocked(now, "android.server.power:POWER", Process.SYSTEM_UID,
mContext.getOpPackageName(), Process.SYSTEM_UID);
}
userActivityNoUpdateLocked(
now, PowerManager.USER_ACTIVITY_EVENT_OTHER, 0, Process.SYSTEM_UID);

// Tell the notifier whether wireless charging has started so that
// it can provide feedback to the user.
//Bug293654, zhanghong.wt, modify, 20170914, modify no notificatin ring while plugging USB
if (dockedOnWirelessCharger || (mIsPowered && oldPlugType != mPlugType&&("1".equals(SystemProperties.get("sys.boot_completed"))))) {
mNotifier.onWirelessChargingStarted();
}
}

if (wasPowered != mIsPowered || oldLevelLow != mBatteryLevelLow) {
if (oldLevelLow != mBatteryLevelLow && !mBatteryLevelLow) {
if (DEBUG_SPEW) {
Slog.d(TAG, "updateIsPoweredLocked: resetting low power snooze");
}
mAutoLowPowerModeSnoozing = false;
}
updateLowPowerModeLocked();
}
}
}

最後附上總結圖

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