Android系统关机的全流程解析

在PowerManager的API文档中，给出了一个关机/重启接口：
public void reboot (String reason)

对于这个接口的描述很简单，就是几句话。
接口的作用就是重启设备，而且，就算重启成功了也没有返回值。
需要包含REBOOT权限，也就是android.permission.REBOOT
唯一参数reason代表需要的特定重启模式，比如recovery，当然也可以为null。

一、上层空间
1.frameworks/base/core/java/android/os/PowerManager.java

/**
 * Reboot the device. Will not return if the reboot is
 * successful. Requires the {@link android.Manifest.permission#REBOOT}
 * permission.
 *
 * @param reason code to pass to the kernel (e.g., "recovery") to
 *        request special boot modes, or null.
 */
public void reboot(String reason)
{
  try {
    mService.reboot(reason);
  } catch (RemoteException e) {
  }
}

mService为IPowerManager Binder接口服务。

/**
 * {@hide}
 */
public PowerManager(IPowerManager service, Handler handler)
{
  mService = service;
  mHandler = handler;
}

2.frameworks/base/core/java/android/os/IPowerManager.aidl

interface IPowerManager
{
...
void reboot(String reason);
...
}

3.frameworks/base/services/java/com/android/server/PowerManagerService.java

/**
 * Reboot the device immediately, passing 'reason' (may be null)
 * to the underlying __reboot system call. Should not return.
 */
public void reboot(String reason)
{
  mContext.enforceCallingOrSelfPermission(android.Manifest.permission.REBOOT, null); 

  if (mHandler == null || !ActivityManagerNative.isSystemReady()) {
    throw new IllegalStateException("Too early to call reboot()");
  }   

  final String finalReason = reason;
  Runnable runnable = new Runnable() {
    public void run() {
      synchronized (this) {
        ShutdownThread.reboot(getUiContext(), finalReason, false);
      }   

    }
  };
  // ShutdownThread must run on a looper capable of displaying the UI.
  mHandler.post(runnable); 

  // PowerManager.reboot() is documented not to return so just wait for the inevitable.
  synchronized (runnable) {
    while (true) {
      try {
        runnable.wait();
      } catch (InterruptedException e) {
      }
    }
  }
}

4.frameworks/base/services/java/com/android/server/pm/ShutdownThread.java

/**
 * Request a clean shutdown, waiting for subsystems to clean up their
 * state etc. Must be called from a Looper thread in which its UI
 * is shown.
 *
 * @param context Context used to display the shutdown progress dialog.
 * @param reason code to pass to the kernel (e.g. "recovery"), or null.
 * @param confirm true if user confirmation is needed before shutting down.
 */
public static void reboot(final Context context, String reason, boolean confirm) {
  mReboot = true;
  mRebootSafeMode = false;
  mRebootReason = reason;
  shutdownInner(context, confirm);
}

这里说明是需要重启，且不是安全模式，重启参数为传递下来的reason，shutdownInner的confirm参数是用来设置是否有确认提示框的，通过reboot接口调用重启是没有的，为false。
重启的实现在run()中，因为ShutdownThread是Thread的扩展，所以run会自动运行。

/**
 * Makes sure we handle the shutdown gracefully.
 * Shuts off power regardless of radio and bluetooth state if the alloted time has passed.
 */
public void run() {
  BroadcastReceiver br = new BroadcastReceiver() {
    @Override public void onReceive(Context context, Intent intent) {
      // We don't allow apps to cancel this, so ignore the result.
      actionDone();
    }
  }; 

  /*
   * Write a system property in case the system_server reboots before we
   * get to the actual hardware restart. If that happens, we'll retry at
   * the beginning of the SystemServer startup.
   */
  {
    String reason = (mReboot ? "1" : "0") + (mRebootReason != null ? mRebootReason : "");
    SystemProperties.set(SHUTDOWN_ACTION_PROPERTY, reason);
  } 

  /*
   * If we are rebooting into safe mode, write a system property
   * indicating so.
   */
  if (mRebootSafeMode) {
    SystemProperties.set(REBOOT_SAFEMODE_PROPERTY, "1");
  }
  ...
  rebootOrShutdown(mReboot, mRebootReason);
}

在重启前会将重启原因写入sys.shutdown.requested，如果没有则为空，如果是安全模式还会将persist.sys.safemode置1，之后会进行一些关机前的预处理，关闭ActivityManager以及MountService，最终调用rebootOrShutdown进行关机操作。

  /**
   * Do not call this directly. Use {@link #reboot(Context, String, boolean)}
   * or {@link #shutdown(Context, boolean)} instead.
   *
   * @param reboot true to reboot or false to shutdown
   * @param reason reason for reboot
   */
  public static void rebootOrShutdown(boolean reboot, String reason) {
    if (reboot) {
      Log.i(TAG, "Rebooting, reason: " + reason);
      try {
        PowerManagerService.lowLevelReboot(reason);
      } catch (Exception e) {
        Log.e(TAG, "Reboot failed, will attempt shutdown instead", e);
      }
    } else if (SHUTDOWN_VIBRATE_MS > 0) {
      // vibrate before shutting down
      Vibrator vibrator = new SystemVibrator();
      try {
        vibrator.vibrate(SHUTDOWN_VIBRATE_MS);
      } catch (Exception e) {
        // Failure to vibrate shouldn't interrupt shutdown. Just log it.
        Log.w(TAG, "Failed to vibrate during shutdown.", e);
      }   

      // vibrator is asynchronous so we need to wait to avoid shutting down too soon.
      try {
        Thread.sleep(SHUTDOWN_VIBRATE_MS);
      } catch (InterruptedException unused) {
      }
    }   

    // Shutdown power
    Log.i(TAG, "Performing low-level shutdown...");
    PowerManagerService.lowLevelShutdown();
  }
}

如果确认重启，则调用PowerManagerService的lowLevelReboot函数，参数就是传递下来的reason，稍后分析。如果不是重启，即mReboot=false,那就是需要关机了，在shutdown函数中就能够知道。

/**
 * Request a clean shutdown, waiting for subsystems to clean up their
 * state etc. Must be called from a Looper thread in which its UI
 * is shown.
 *
 * @param context Context used to display the shutdown progress dialog.
 * @param confirm true if user confirmation is needed before shutting down.
 */
public static void shutdown(final Context context, boolean confirm) {
  mReboot = false;
  mRebootSafeMode = false;
  shutdownInner(context, confirm);
}

关机的时候需要震动，就是这里了SHUTDOWN_VIBRATE_MS，默认的定义是500ms。但是在代码上看，无论如何，最后都会调用一下lowLevelShutdown函数，也就是关机。逻辑上，这里可能是个问题，但是实际中，如果重启操作能够调用成功的话，整个系统都重启了，后边的代码当然不可能执行到了。
目光转回PowerManagerService
4.frameworks/base/services/java/com/android/server/PowerManagerService.java

/**
 * Low-level function to reboot the device.
 *
 * @param reason code to pass to the kernel (e.g. "recovery"), or null.
 * @throws IOException if reboot fails for some reason (eg, lack of
 *     permission)
 */
public static void lowLevelReboot(String reason) throws IOException {
  nativeReboot(reason);
}  

/**
 * Low-level function turn the device off immediately, without trying
 * to be clean. Most people should use
 * {@link com.android.server.pm.internal.app.ShutdownThread} for a clean shutdown.
 */
public static void lowLevelShutdown() {
  nativeShutdown();
} 

很熟悉的字样native，是JNI调用了：

private static native void nativeShutdown();
private static native void nativeReboot(String reason) throws IOException;

5.frameworks/base/services/jni/com_android_server_PowerManagerService.cpp

static JNINativeMethod gPowerManagerServiceMethods[] = {
  /* name, signature, funcPtr */
  ...
  { "nativeShutdown", "()V",
      (void*) nativeShutdown },
  { "nativeReboot", "(Ljava/lang/String;)V",
      (void*) nativeReboot },
  ...
};

这两个好哥俩的实现也是在一起的：

static void nativeShutdown(JNIEnv *env, jobject clazz) {
  android_reboot(ANDROID_RB_POWEROFF, 0, 0);
} 

static void nativeReboot(JNIEnv *env, jobject clazz, jstring reason) {
  if (reason == NULL) {
    android_reboot(ANDROID_RB_RESTART, 0, 0);
  } else {
    const char *chars = env->GetStringUTFChars(reason, NULL);
    android_reboot(ANDROID_RB_RESTART2, 0, (char *) chars);
    env->ReleaseStringUTFChars(reason, chars); // In case it fails.
  }
  jniThrowIOException(env, errno);
}

可以看到无论是关机还是重启，都是调用android_reboot来实现的，只是参数不一样而已。

6.system/core/libcutils/android_reboot.c

int android_reboot(int cmd, int flags, char *arg)
{
  int ret = 0;
  int reason = -1; 

#ifdef RECOVERY_PRE_COMMAND
  if (cmd == (int) ANDROID_RB_RESTART2) {
    if (arg && strlen(arg) > 0) {
      char cmd[PATH_MAX];
      sprintf(cmd, RECOVERY_PRE_COMMAND " %s", arg);
      system(cmd);
    }
  }
#endif 

  if (!(flags & ANDROID_RB_FLAG_NO_SYNC))
    sync(); 

  if (!(flags & ANDROID_RB_FLAG_NO_REMOUNT_RO))
    remount_ro(); 

  switch (cmd) {
    case ANDROID_RB_RESTART:
      reason = RB_AUTOBOOT;
      break; 

    case ANDROID_RB_POWEROFF:
      ret = reboot(RB_POWER_OFF);
      return ret; 

    case ANDROID_RB_RESTART2:
      // REBOOT_MAGIC
      break; 

    default:
      return -1;
  } 

#ifdef RECOVERY_PRE_COMMAND_CLEAR_REASON
  reason = RB_AUTOBOOT;
#endif 

  if (reason != -1)
    ret = reboot(reason);
  else
    ret = __reboot(LINUX_REBOOT_MAGIC1, LINUX_REBOOT_MAGIC2,
              LINUX_REBOOT_CMD_RESTART2, arg); 

  return ret;
}

以reboot recovery为例，arg即为recovery,所在在第五步的时候会传入ANDROID_RB_RESTART2。到了android_reboot函数中，会看到这样的定义#ifdef RECOVERY_PRE_COMMAND，即属于重启前会执行的命令，如果定义了就会执行。
下面也是做了一些关机重启前的预处理工作，sync()作用是将缓存中的信息写入磁盘，以免程序异常结束导致文件被损坏，linux系统关机前会做几次这样的动作；而remount_ro()作用是通过调用emergency_remount()强制将文件系统挂载为只读，不再允许任何写入操作，同时会通过检查/proc/mounts的设备状态来确认是否当前的所有写入工作已经完成，这个检查过程是阻塞操作。
接下来才是对参数的解析处理：
1）普通重启 ANDROID_RB_RESTART， reason = RB_AUTOBOOT；
2）关机 ANDROID_RB_POWEROFF， 无需reason，直接调用reboot进行关机；
3）带参数的特殊重启 ANDROID_RB_RESTART2， reason 将为默认值 -1
这里又出现一个#ifdef RECOVERY_PRE_COMMAND_CLEAR_REASON，如果定义了它，则无论上层传下来的参数是什么样的，最终都只是普通重启而已。定义它的方式是在BoardConfig.mk中加入TARGET_RECOVERY_PRE_COMMAND_CLEAR_REASON := true，应该有厂商会喜欢这么做的，毕竟除了普通重启，都可能带给用户一定的风险。
最后会对reason进行一个检测，那么通过上边的分析，其实只有带参数的特殊重启才会为-1,而不等于-1的情况中有普通重启和关机，而关机已经自行解决了……所以，不等于-1的情况到了这里也只有普通重启了。最终这里就是区分普通重启与特殊重启的地方了。这里再插入一个问题，其他的几个cmd都是什么值呢？答案在bionic/libc/include/sys/reboot.h中：

#define RB_AUTOBOOT   LINUX_REBOOT_CMD_RESTART
#define RB_HALT_SYSTEM LINUX_REBOOT_CMD_HALT
#define RB_ENABLE_CAD  LINUX_REBOOT_CMD_CAD_ON
#define RB_DISABLE_CAD LINUX_REBOOT_CMD_CAD_OFF
#define RB_POWER_OFF  LINUX_REBOOT_CMD_POWER_OFF

而，LINUX_REBOOT_XXXX之类的在bionic/libc/kernel/common/linux/reboot.h中：

#define LINUX_REBOOT_MAGIC1 0xfee1dead
#define LINUX_REBOOT_MAGIC2 672274793
/* WARNING: DO NOT EDIT, AUTO-GENERATED CODE - SEE TOP FOR INSTRUCTIONS */
#define LINUX_REBOOT_MAGIC2A 85072278
#define LINUX_REBOOT_MAGIC2B 369367448
#define LINUX_REBOOT_MAGIC2C 537993216
#define LINUX_REBOOT_CMD_RESTART 0x01234567
/* WARNING: DO NOT EDIT, AUTO-GENERATED CODE - SEE TOP FOR INSTRUCTIONS */
#define LINUX_REBOOT_CMD_HALT 0xCDEF0123
#define LINUX_REBOOT_CMD_CAD_ON 0x89ABCDEF
#define LINUX_REBOOT_CMD_CAD_OFF 0x00000000
#define LINUX_REBOOT_CMD_POWER_OFF 0x4321FEDC
/* WARNING: DO NOT EDIT, AUTO-GENERATED CODE - SEE TOP FOR INSTRUCTIONS */
#define LINUX_REBOOT_CMD_RESTART2 0xA1B2C3D4
#define LINUX_REBOOT_CMD_SW_SUSPEND 0xD000FCE2
#define LINUX_REBOOT_CMD_KEXEC 0x45584543

至于为什么他们是这样奇怪的值这个问题，我只能说他们是magic number，魔法嘛，本来就是正常人不能够理解的，所以~~~放过他们吧，只要知道他们没有是-1的就OK啦。
先来看reboot函数，按照往常的经验，reboot最终一定会调用到__reboot的。

7.bionic/libc/unistd/reboot.c

int reboot (int mode)
{
  return __reboot( LINUX_REBOOT_MAGIC1, LINUX_REBOOT_MAGIC2, mode, NULL );
}

Bingo！果然是这样，如此说来reboot(reason) -> reboot(RB_AUTOBOOT) -> __reboot( LINUX_REBOOT_MAGIC1, LINUX_REBOOT_MAGIC2, LINUX_REBOOT_CMD_RESTART, NULL )，要是直接这样写多好~~~免得绕这一层了。

二、KERNEL域
8.__reboot通过syscall来到内核
这里用一些篇幅简要介绍syscall，以后遇到类似的东西更好追踪一些。
第七步中的__reboot在arm架构的实现是这样的（bionic/libc/arch-arm/syscalls/__reboot.S）

ENTRY(__reboot)
  .save  {r4, r7}
  stmfd  sp!, {r4, r7}
  ldr   r7, =__NR_reboot
  swi   #0
  ldmfd  sp!, {r4, r7}
  movs  r0, r0
  bxpl  lr
  b    __set_syscall_errno
END(__reboot)

可以看出来，这里将__reboot的实现映射到了__NR_reboot, 而在bionic/libc/sys/linux-syscalls.h能够找到：

#define __NR_reboot            (__NR_SYSCALL_BASE + 88)

其被指定了一个固定的偏移量，在被调用的时候就是通过这个偏移量去内核中寻找对应的入口的，由此可见，内核中一定有着相同的定义，否则将不能成功调用。内核中对syscall偏移量的定义在内核源码中的arch/arm/include/asm/unistd.h，相关信息完全一致。
已经找到了内核中的对应映射，那么下一步就要去找寻真正的实现函数了，在include/asm-generic/unistd.h中可以找到内核对__NR_reboot的syscall函数映射，即

/* kernel/sys.c */
#define __NR_setpriority 140
__SYSCALL(__NR_setpriority, sys_setpriority)
#define __NR_getpriority 141
__SYSCALL(__NR_getpriority, sys_getpriority)
#define __NR_reboot 142
__SYSCALL(__NR_reboot, sys_reboot)

同时，能够发现如此温馨的一幕，内核已经指引我们下一步该去哪里寻找sys_reboot，即kernel/sys.c。

9.kernel/sys.c
在进入这个文件前，我们先去include/linux/syscalls.h中查看一下sys_reboot的定义：

asmlinkage long sys_reboot(int magic1, int magic2, unsigned int cmd,
        void __user *arg);

与__reboot的调用参数一致。
进入sys.c文件后，并没有找到名为sys_reboot的函数，而通过仔细查找，发现一个很有趣的函数，其定义为SYSCALL_DEFINE4(reboot, int, magic1, int, magic2, unsigned int, cmd, void __user *, arg)，对比__reboot的参数，能够符合。究竟是不是这个函数？
同样在include/linux/syscalls.h文件中，能够找到这样几个定义：

#define SYSCALL_DEFINE1(name, ...) SYSCALL_DEFINEx(1, _##name, __VA_ARGS__)
#define SYSCALL_DEFINE2(name, ...) SYSCALL_DEFINEx(2, _##name, __VA_ARGS__)
#define SYSCALL_DEFINE3(name, ...) SYSCALL_DEFINEx(3, _##name, __VA_ARGS__)
#define SYSCALL_DEFINE4(name, ...) SYSCALL_DEFINEx(4, _##name, __VA_ARGS__)
#define SYSCALL_DEFINE5(name, ...) SYSCALL_DEFINEx(5, _##name, __VA_ARGS__)
#define SYSCALL_DEFINE6(name, ...) SYSCALL_DEFINEx(6, _##name, __VA_ARGS__)
... 

#define SYSCALL_DEFINEx(x, sname, ...)       \
  __SYSCALL_DEFINEx(x, sname, __VA_ARGS__)
... 

#define __SYSCALL_DEFINEx(x, name, ...)         \
  asmlinkage long sys##name(__SC_DECL##x(__VA_ARGS__))

整合后等价于：

#define SYSCALL_DEFINE4(name, ...) \
  asmlinkage long sys##_name(__SC_DECL##4(__VA_ARGS__))

这样就不难看出，SYSCALL_DEFINE4(reboot, int, magic1, int, magic2, unsigned int, cmd, void __user *, arg)就是sys_reboot,也就是上层调用的__reboot的最终实现。函数实现如下：

/*
 * Reboot system call: for obvious reasons only root may call it,
 * and even root needs to set up some magic numbers in the registers
 * so that some mistake won't make this reboot the whole machine.
 * You can also set the meaning of the ctrl-alt-del-key here.
 *
 * reboot doesn't sync: do that yourself before calling this.
 */
SYSCALL_DEFINE4(reboot, int, magic1, int, magic2, unsigned int, cmd,
    void __user *, arg)
{
  char buffer[256];
  int ret = 0; 

  /* We only trust the superuser with rebooting the system. */
  if (!capable(CAP_SYS_BOOT))
    return -EPERM; 

  /* For safety, we require "magic" arguments. */
  if (magic1 != LINUX_REBOOT_MAGIC1 ||
    (magic2 != LINUX_REBOOT_MAGIC2 &&
          magic2 != LINUX_REBOOT_MAGIC2A &&
      magic2 != LINUX_REBOOT_MAGIC2B &&
          magic2 != LINUX_REBOOT_MAGIC2C))
    return -EINVAL; 

  /* Instead of trying to make the power_off code look like
   * halt when pm_power_off is not set do it the easy way.
   */
  if ((cmd == LINUX_REBOOT_CMD_POWER_OFF) && !pm_power_off)
    cmd = LINUX_REBOOT_CMD_HALT; 

  mutex_lock(&reboot_mutex);
  switch (cmd) {
  case LINUX_REBOOT_CMD_RESTART:
    kernel_restart(NULL);
    break; 

  case LINUX_REBOOT_CMD_CAD_ON:
    C_A_D = 1;
    break; 

  case LINUX_REBOOT_CMD_CAD_OFF:
    C_A_D = 0;
    break; 

  case LINUX_REBOOT_CMD_HALT:
    kernel_halt();
    do_exit(0);
    panic("cannot halt"); 

  case LINUX_REBOOT_CMD_POWER_OFF:
    kernel_power_off();
    do_exit(0);
    break; 

  case LINUX_REBOOT_CMD_RESTART2:
    if (strncpy_from_user(&buffer[0], arg, sizeof(buffer) - 1) < 0) {
      ret = -EFAULT;
      break;
    }
    buffer[sizeof(buffer) - 1] = '\0'; 

    kernel_restart(buffer);
    break; 

#ifdef CONFIG_KEXEC
  case LINUX_REBOOT_CMD_KEXEC:
    ret = kernel_kexec();
    break;
#endif 

#ifdef CONFIG_HIBERNATION
  case LINUX_REBOOT_CMD_SW_SUSPEND:
    ret = hibernate();
    break;
#endif 

  default:
    ret = -EINVAL;
    break;
  }
  mutex_unlock(&reboot_mutex);
  return ret;
}

在此函数中，首先会检测权限问题，只有超级用户才可以执行重启系统的操作：

/* We only trust the superuser with rebooting the system. */
if (!capable(CAP_SYS_BOOT))
  return -EPERM;

否则将返回权限错误。对应的权限列表在include/linux/capability.h中，重启操作为22.
随后对magic number进行了校验：

/* For safety, we require "magic" arguments. */
if (magic1 != LINUX_REBOOT_MAGIC1 ||
  (magic2 != LINUX_REBOOT_MAGIC2 &&
        magic2 != LINUX_REBOOT_MAGIC2A &&
    magic2 != LINUX_REBOOT_MAGIC2B &&
        magic2 != LINUX_REBOOT_MAGIC2C))
  return -EINVAL;

如果数据传输过程中没有发生错误的话，这里也当然不会有问题，所以只是一个安全性校验，基本不会发生错误。
之后有一个很有趣的检查，如果用户要求关机，而pm_power_off为空的话，就把用户的关机命令转换为挂起：

/* Instead of trying to make the power_off code look like
 * halt when pm_power_off is not set do it the easy way.
 */
if ((cmd == LINUX_REBOOT_CMD_POWER_OFF) && !pm_power_off)
  cmd = LINUX_REBOOT_CMD_HALT;

在arch/arm/kernel/process.c中可以找到它的定义：

/*
 * Function pointers to optional machine specific functions
 */
void (*pm_power_off)(void);
EXPORT_SYMBOL(pm_power_off);

好的，只是一个函数指针，而且做了全局操作，整个kernel都可以调用它。以高通msm7x30为例，在arch/arm/mach-msm/pm2.c中对这个函数指针进行了赋值：

pm_power_off = msm_pm_power_off;

msm_pm_power_off的具体实现就不再跟踪了，各家的都不一样，跟下去没有太大意义。现在只要知道，我分析的这个kernel是给了这个函数指针赋值的，所以不为空，关机命令将正常执行。
接下来就是这个函数的正题了，对用户命令进行解析操作，同时这个过程是用reboot_mutex互斥锁来进行保护的，以保证同一时间只可能有一个解析过程，避免冲突。
下边贴出所有关机重启相关的命令定义：

/*
 * Commands accepted by the _reboot() system call.
 *
 * RESTART   Restart system using default command and mode.
 * HALT    Stop OS and give system control to ROM monitor, if any.
 * CAD_ON   Ctrl-Alt-Del sequence causes RESTART command.
 * CAD_OFF   Ctrl-Alt-Del sequence sends SIGINT to init task.
 * POWER_OFF  Stop OS and remove all power from system, if possible.
 * RESTART2  Restart system using given command string.
 * SW_SUSPEND Suspend system using software suspend if compiled in.
 * KEXEC    Restart system using a previously loaded Linux kernel
 */    

#define LINUX_REBOOT_CMD_RESTART  0x01234567
#define LINUX_REBOOT_CMD_HALT    0xCDEF0123
#define LINUX_REBOOT_CMD_CAD_ON   0x89ABCDEF
#define LINUX_REBOOT_CMD_CAD_OFF  0x00000000
#define LINUX_REBOOT_CMD_POWER_OFF 0x4321FEDC
#define LINUX_REBOOT_CMD_RESTART2  0xA1B2C3D4
#define LINUX_REBOOT_CMD_SW_SUSPEND 0xD000FCE2
#define LINUX_REBOOT_CMD_KEXEC   0x45584543

注释中的说明很详细了，比较陌生的就是关于CAD，其实就是用来想用Ctrl+Alt+Del操作的；然后SW_SYSPEND是软件休眠；KEXEC就太高端了，属于内核的一个补丁，用来利用老内核重启，详细资料：http://www.ibm.com/developerworks/cn/linux/l-kexec/?ca=dwcn-newsletter-linux
以上这些只有前六个命令被Android系统所使用，为什么这么说，可以去看bionic/libc/include/sys/reboot.h，上边已经贴出了。LINUX_REBOOT_CMD_HALT虽有定义，但是也没有发现Android系统中哪里有调用，有高手找到的话，希望能够告知一下。最终的最终，能够用到的就只有三个：

• RESTART
• POWER_OFF
• RESTART2

10.最终实现
重启调用的是kernel_restart，区别是参数是不是空，关机则调用kernel_power_off()，先看关机：

/**
 * kernel_power_off - power_off the system
 *
 * Shutdown everything and perform a clean system power_off.
 */
void kernel_power_off(void)
{
  kernel_shutdown_prepare(SYSTEM_POWER_OFF);
  if (pm_power_off_prepare)
    pm_power_off_prepare();
  disable_nonboot_cpus();
  syscore_shutdown();
  printk(KERN_EMERG "Power down.\n");
  kmsg_dump(KMSG_DUMP_POWEROFF);
  machine_power_off();
}
EXPORT_SYMBOL_GPL(kernel_power_off);

最了一系列准备工作，最终调用machine_power_off()：

void machine_power_off(void)
{
  machine_shutdown();
  if (pm_power_off)
    pm_power_off();
}

之前找寻的pm_power_off在这里就有用处了，是关机的最后一步操作。关机完成，之后看下重启操作：

/**
 * kernel_restart - reboot the system
 * @cmd: pointer to buffer containing command to execute for restart
 *   or %NULL
 *
 * Shutdown everything and perform a clean reboot.
 * This is not safe to call in interrupt context.
 */
void kernel_restart(char *cmd)
{
  kernel_restart_prepare(cmd);
  if (!cmd)
    printk(KERN_EMERG "Restarting system.\n");
  else
    printk(KERN_EMERG "Restarting system with command '%s'.\n", cmd);
  kmsg_dump(KMSG_DUMP_RESTART);
  machine_restart(cmd);
}
EXPORT_SYMBOL_GPL(kernel_restart);

同样的套路，也是会进行一些准备工作，之后调用machine_restart(cmd)， 如果是普通重启，那么中个cmd就为NULL，如果是特殊重启，那么这个cmd就是一层一层传递下来得那个arg了。

void machine_restart(char *cmd)
{
  machine_shutdown();
  arm_pm_restart(reboot_mode, cmd);
}
...
void (*arm_pm_restart)(char str, const char *cmd) = arm_machine_restart;
EXPORT_SYMBOL_GPL(arm_pm_restart);

而还记得刚才的pm2.c吗？在那里同样对arm_pm_restart进行了指针赋值：

arm_pm_restart = msm_pm_restart;

赋值的函数为msm_pm_init， 其调用为

late_initcall_sync(msm_pm_init);

late_initcall_sync的启动优先级是最低的，为7。module_init其实是6的优先级，数字越大优先级越低。所以，这样推断的话，最终arm_pm_restart这个函数指针会指向msm_pm_restart。关于msm_pm_restart的具体实现也不细看了，跟前边说的一样，都是各家不一样，就几行代码：

static void msm_pm_restart(char str, const char *cmd)
{
  msm_rpcrouter_close();
  msm_proc_comm(PCOM_RESET_CHIP, &restart_reason, 0); 

  for (;;)
    ;
}

但是细心的朋友可能会发现这里有一个restart_reason，这个并不是传递下来的参数。事实上，这个值已经在之前kernel_restart_prepare(cmd)的时候就已经设置好了。

void kernel_restart_prepare(char *cmd)
{
  blocking_notifier_call_chain(&reboot_notifier_list, SYS_RESTART, cmd);
  system_state = SYSTEM_RESTART;
  usermodehelper_disable();
  device_shutdown();
  syscore_shutdown();
}

就是blocking_notifier机制，这个操作在之前的shutdown关机操作中也有，且是同一个list，都是reboot_notifier_list。也很容易理解，就是将注册在reboot_notifier_list上的函数传入相关参数后执行，作为了解，看一下具体是怎么使用的：(arch/arm/mach-msm/pm2.c)

static int msm_reboot_call
  (struct notifier_block *this, unsigned long code, void *_cmd)
{
  if ((code == SYS_RESTART) && _cmd) {
    char *cmd = _cmd;
    if (!strcmp(cmd, "bootloader")) {
      restart_reason = 0x77665500;
    } else if (!strcmp(cmd, "recovery")) {
      restart_reason = 0x77665502;
    } else if (!strcmp(cmd, "eraseflash")) {
      restart_reason = 0x776655EF;
    } else if (!strncmp(cmd, "oem-", 4)) {
      unsigned code = simple_strtoul(cmd + 4, 0, 16) & 0xff;
      restart_reason = 0x6f656d00 | code;
    } else {
      restart_reason = 0x77665501;
    }
  }
  return NOTIFY_DONE;
}       

static struct notifier_block msm_reboot_notifier = {
  .notifier_call = msm_reboot_call,
}; 

... 

static int __init msm_pm_init(void)
{
...
  register_reboot_notifier(&msm_reboot_notifier);
...
}

OK，万事大吉，在kernel_restart_prepare的时候msm_reboot_call会被首先调用，这个函数的作用就是根据用户命令给restart_reason赋值，从而在之后调用msm_pm_restart的时候使用。这里我们发现在reboot的时候可以带的参数不仅有recovery,bootloader，还有eraseflash和oem-???，字面上看应该是用来擦除ROM和解锁之类的操作了。

三、关机怎么用？
本文的分析是由Android给出的reboot接口开始的，但是分析来分析去，回头想一想会发现，Android给出的接口reboot就真的只能重启而已，不能进行关机操作，可以在跟踪这个流程的过程中会发现，确实是有存在关机的相关接口的。那么关机该怎么用呢？
frameworks/base/services/java/com/android/serverBatteryService.java

private final void shutdownIfNoPower() {
// shut down gracefully if our battery is critically low and we are not powered.
// wait until the system has booted before attempting to display the shutdown dialog.
if (mBatteryLevel == 0 && !isPowered() && ActivityManagerNative.isSystemReady()) {
  Intent intent = new Intent(Intent.ACTION_REQUEST_SHUTDOWN);
  intent.putExtra(Intent.EXTRA_KEY_CONFIRM, false);
  intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
  mContext.startActivity(intent);
}