android 静默重启 QUIESCENT REBOOT

背景

项目中需要处理Android的原生开机动画，一定条件下还需要做到静默重启（android系统启动进入到桌面前，屏幕保持完全没有亮度的状态）。因为项目使用的rom是MTK平台支持，一开始并不知道Android 的QUIESCENT_REBOOT模式，所以自己想办法实现了此功能，详细见博客：基于Q的Android开机动画。因为后续Linux启用了SELinux增强了权限限制，原有的方案因为权限限制了文件的执行和读写，需要进行比较大的变动。后来，学习了下Android 的QUIESCENT_REBOOT模式，并把它用到了项目中解决了问题。这里介绍下QUIESCENT_REBOOT模式原理。

Android Quiescent的原理和流程

一、powerManager.reboot(PowerManager.REBOOT_QUIESCENT) 来静默重启（本地测试可以通过命令行触发svc power reboot quiescent）

1、代码：frameworks/base/core/java/android/os/PowerManager.java

public void reboot(String reason) {try {// mService是PowerManagerServicemService.reboot(false, reason, true);} catch (RemoteException e) {throw e.rethrowFromSystemServer();}
}

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

// True if the lights should stay off until an explicit user action.
// 这个标记记录是否静默重启
private static boolean sQuiescent;@Override // Binder call
public void reboot(boolean confirm, String reason, boolean wait) {mContext.enforceCallingOrSelfPermission(android.Manifest.permission.REBOOT, null);......final long ident = Binder.clearCallingIdentity();try {shutdownOrRebootInternal(HALT_MODE_REBOOT, confirm, reason, wait);} finally {Binder.restoreCallingIdentity(ident);}
}private void shutdownOrRebootInternal(final @HaltMode int haltMode, final boolean confirm,final String reason, boolean wait) {// lowLevelReboot会去解析reason并存储到SystemPropertiesif (mHandler == null || !mSystemReady) {if (RescueParty.isAttemptingFactoryReset()) {// If we're stuck in a really low-level reboot loop, and a// rescue party is trying to prompt the user for a factory data// reset, we must GET TO DA CHOPPA!PowerManagerService.lowLevelReboot(reason);} else {throw new IllegalStateException("Too early to call shutdown() or reboot()");}}// 调用ShutdownThread重启Runnable runnable = new Runnable() {@Overridepublic void run() {synchronized (this) {if (haltMode == HALT_MODE_REBOOT_SAFE_MODE) {ShutdownThread.rebootSafeMode(getUiContext(), confirm);} else if (haltMode == HALT_MODE_REBOOT) {ShutdownThread.reboot(getUiContext(), reason, confirm);} else {ShutdownThread.shutdown(getUiContext(), reason, confirm);}}}};......
}public static void lowLevelReboot(String reason) {if (reason == null) {reason = "";}// If the reason is "quiescent", it means that the boot process should proceed// without turning on the screen/lights.// The "quiescent" property is sticky, meaning that any number// of subsequent reboots should honor the property until it is reset.if (reason.equals(PowerManager.REBOOT_QUIESCENT)) {sQuiescent = true;reason = "";} else if (reason.endsWith("," + PowerManager.REBOOT_QUIESCENT)) {sQuiescent = true;reason = reason.substring(0,reason.length() - PowerManager.REBOOT_QUIESCENT.length() - 1);}if (sQuiescent) {// Pass the optional "quiescent" argument to the bootloader to let it know// that it should not turn the screen/lights on.reason = reason + ",quiescent";}// 这里会存储到系统参数SystemProperties.set("sys.powerctl", "reboot," + reason);try {Thread.sleep(20 * 1000L);} catch (InterruptedException e) {Thread.currentThread().interrupt();}Slog.wtf(TAG, "Unexpected return from lowLevelReboot!");
}

sQuiescent记录是否静默重启，lowLevelReboot会去解析reason并设置sQuiescent为true，同时把reason存储到SystemProperties，最后调用ShutdownThread带着reason传参。

3、代码frameworks/base/services/core/java/com/android/server/power/ShutdownThread.java
这个类中主要是根据一些条件做了一些单独处理，例如recovery等可能还会展示弹窗，这里有兴趣的可以根据代码路径自行观看，静默重启这个类中没有用到。

4、前边说到了SystemProperties.set(“sys.powerctl”, “reboot,” + reason)，这里很重要。静默重启会把这个值存储到内核特定的启动参数，这个启动参数在下次重启时会被系统拿到。这里内部就涉及SystemProperties原理和源代码了，不特意分析，知道大概用做啥。

5、同时，内核还会解析reason，存储另一个SystemProperties；大概的流程是lk 中读到 rtc 的 Quiescent 标志位，则不显示开机logo，并且在 cmdline 中添加 androidboot.quiescent=1；lk -> kernel -> init，init 会解析 cmdline，并把其中的 androidboot.quiescent=1 解析出来，并设置成 ro.boot.quiescent=1，这样后续 Android 所有地方都能知道此次是 quiescent 开机。

6、bootanimation (frameworks/base/cmds/bootanimation) 中，读到 ro.boot.quiescent=1，则不显示开机动画
代码：frameworks/base/cmds/bootanimation/BootAnimationUtil.cpp

    bool bootAnimationDisabled() {45    char value[PROPERTY_VALUE_MAX];
46    property_get("debug.sf.nobootanimation", value, "0");
47    if (atoi(value) > 0) {48        return true;
49    }
50
51    property_get("ro.boot.quiescent", value, "0");
52    return atoi(value) > 0;
53}

代码：frameworks/base/cmds/bootanimation/bootanimation_main.cpp

36int main()
37{38    setpriority(PRIO_PROCESS, 0, ANDROID_PRIORITY_DISPLAY);
39    // 如果noBootAnimation，不展示开机动画
40    bool noBootAnimation = bootAnimationDisabled();
41    ALOGI_IF(noBootAnimation,  "boot animation disabled");
42    if (!noBootAnimation) {43
44        sp<ProcessState> proc(ProcessState::self());
45        ProcessState::self()->startThreadPool();
46
47        // create the boot animation object (may take up to 200ms for 2MB zip)
48        sp<BootAnimation> boot = new BootAnimation(audioplay::createAnimationCallbacks());
49
50        waitForSurfaceFlinger();
51
52        boot->run("BootAnimation", PRIORITY_DISPLAY);
53
54        ALOGV("Boot animation set up. Joining pool.");
55
56        IPCThreadState::self()->joinThreadPool();
57    }
58    return 0;
59}

7、这里还会涉及开机log的隐藏，在kernal层根据存储在内核的启动参数作为判断，隐藏logo或者亮度。这里大概属于猜测，具体有兴趣的可以自己深入研究。

解决项目中问题

项目中除了要隐藏开机动画，还需要把亮度调为最低。
1、需要把开机默认设置调度的代码给屏蔽掉
代码：frameworks/base/services/core/java/com/android/server/display/DisplayManagerService.java

    @VisibleForTestingDisplayManagerService(Context context, Injector injector) {super(context);......PowerManager pm = mContext.getSystemService(PowerManager.class);// 注释掉的为源代码，这里会取一个默认160的亮度并设置//mGlobalDisplayBrightness = pm.getDefaultScreenBrightnessSetting();// 这里根据静默的参数，增加判断if (SystemProperties.getInt("ro.boot.quiescent", 0) == 0) {mGlobalDisplayBrightness = pm.getDefaultScreenBrightnessSetting();}......}

2、因为除了这个地方会设置默认亮度，系统起来后还会被android层重新设置亮度，就恢复到正常亮度了。