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 Android教程網 >> Android技術 >> 關於Android編程 >> Android系統關機的全流程解析

Android系統關機的全流程解析

編輯:關於Android編程

在PowerManager的API文檔中,給出了一個關機/重啟接口:
public void reboot (String reason)

對於這個接口的描述很簡單,就是幾句話。
接口的作用就是重啟設備,而且,就算重啟成功了也沒有返回值。
需要包含REBOOT權限,也就是android.permission.REBOOT
唯一參數reason代表需要的特定重啟模式,比如recovery,當然也可以為null。

一、上層空間
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為IPowerManager Binder接口服務。

/** 
 * {@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); 
} 

這裡說明是需要重啟,且不是安全模式,重啟參數為傳遞下來的reason,shutdownInner的confirm參數是用來設置是否有確認提示框的,通過reboot接口調用重啟是沒有的,為false。
重啟的實現在run()中,因為ShutdownThread是Thread的擴展,所以run會自動運行。

/** 
 * 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); 
}  

在重啟前會將重啟原因寫入sys.shutdown.requested,如果沒有則為空,如果是安全模式還會將persist.sys.safemode置1,之後會進行一些關機前的預處理,關閉ActivityManager以及MountService,最終調用rebootOrShutdown進行關機操作。

 

  /** 
   * 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(); 
  } 
} 

如果確認重啟,則調用PowerManagerService的lowLevelReboot函數,參數就是傳遞下來的reason,稍後分析。如果不是重啟,即mReboot=false,那就是需要關機了,在shutdown函數中就能夠知道。

/** 
 * 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); 
} 

關機的時候需要震動,就是這裡了SHUTDOWN_VIBRATE_MS,默認的定義是500ms。但是在代碼上看,無論如何,最後都會調用一下lowLevelShutdown函數,也就是關機。邏輯上,這裡可能是個問題,但是實際中,如果重啟操作能夠調用成功的話,整個系統都重啟了,後邊的代碼當然不可能執行到了。
目光轉回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(); 
} 

 

很熟悉的字樣native,是JNI調用了:

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 }, 
  ... 
}; 

這兩個好哥倆的實現也是在一起的:

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); 
} 

可以看到無論是關機還是重啟,都是調用android_reboot來實現的,只是參數不一樣而已。

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為例,arg即為recovery,所在在第五步的時候會傳入ANDROID_RB_RESTART2。到了android_reboot函數中,會看到這樣的定義#ifdef RECOVERY_PRE_COMMAND,即屬於重啟前會執行的命令,如果定義了就會執行。
下面也是做了一些關機重啟前的預處理工作,sync()作用是將緩存中的信息寫入磁盤,以免程序異常結束導致文件被損壞,linux系統關機前會做幾次這樣的動作;而remount_ro()作用是通過調用emergency_remount()強制將文件系統掛載為只讀,不再允許任何寫入操作,同時會通過檢查/proc/mounts的設備狀態來確認是否當前的所有寫入工作已經完成,這個檢查過程是阻塞操作。
接下來才是對參數的解析處理:
1)普通重啟 ANDROID_RB_RESTART, reason = RB_AUTOBOOT;
2)關機 ANDROID_RB_POWEROFF, 無需reason,直接調用reboot進行關機;
3)帶參數的特殊重啟 ANDROID_RB_RESTART2, reason 將為默認值 -1
這裡又出現一個#ifdef RECOVERY_PRE_COMMAND_CLEAR_REASON,如果定義了它,則無論上層傳下來的參數是什麼樣的,最終都只是普通重啟而已。定義它的方式是在BoardConfig.mk中加入TARGET_RECOVERY_PRE_COMMAND_CLEAR_REASON := true,應該有廠商會喜歡這麼做的,畢竟除了普通重啟,都可能帶給用戶一定的風險。
最後會對reason進行一個檢測,那麼通過上邊的分析,其實只有帶參數的特殊重啟才會為-1,而不等於-1的情況中有普通重啟和關機,而關機已經自行解決了……所以,不等於-1的情況到了這裡也只有普通重啟了。最終這裡就是區分普通重啟與特殊重啟的地方了。這裡再插入一個問題,其他的幾個cmd都是什麼值呢?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之類的在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 

至於為什麼他們是這樣奇怪的值這個問題,我只能說他們是magic number,魔法嘛,本來就是正常人不能夠理解的,所以~~~放過他們吧,只要知道他們沒有是-1的就OK啦。
先來看reboot函數,按照往常的經驗,reboot最終一定會調用到__reboot的。

7.bionic/libc/unistd/reboot.c

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

Bingo!果然是這樣,如此說來reboot(reason) -> reboot(RB_AUTOBOOT) -> __reboot( LINUX_REBOOT_MAGIC1, LINUX_REBOOT_MAGIC2, LINUX_REBOOT_CMD_RESTART, NULL ),要是直接這樣寫多好~~~免得繞這一層了。

二、KERNEL域
8.__reboot通過syscall來到內核
這裡用一些篇幅簡要介紹syscall,以後遇到類似的東西更好追蹤一些。
第七步中的__reboot在arm架構的實現是這樣的(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) 

可以看出來,這裡將__reboot的實現映射到了__NR_reboot, 而在bionic/libc/sys/linux-syscalls.h能夠找到:

#define __NR_reboot            (__NR_SYSCALL_BASE + 88) 

其被指定了一個固定的偏移量,在被調用的時候就是通過這個偏移量去內核中尋找對應的入口的,由此可見,內核中一定有著相同的定義,否則將不能成功調用。內核中對syscall偏移量的定義在內核源碼中的arch/arm/include/asm/unistd.h,相關信息完全一致。
已經找到了內核中的對應映射,那麼下一步就要去找尋真正的實現函數了,在include/asm-generic/unistd.h中可以找到內核對__NR_reboot的syscall函數映射,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) 

同時,能夠發現如此溫馨的一幕,內核已經指引我們下一步該去哪裡尋找sys_reboot,即kernel/sys.c。

9.kernel/sys.c
在進入這個文件前,我們先去include/linux/syscalls.h中查看一下sys_reboot的定義:

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

與__reboot的調用參數一致。
進入sys.c文件後,並沒有找到名為sys_reboot的函數,而通過仔細查找,發現一個很有趣的函數,其定義為SYSCALL_DEFINE4(reboot, int, magic1, int, magic2, unsigned int, cmd, void __user *, arg),對比__reboot的參數,能夠符合。究竟是不是這個函數?
同樣在include/linux/syscalls.h文件中,能夠找到這樣幾個定義:

#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__)) 

整合後等價於:

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

這樣就不難看出,SYSCALL_DEFINE4(reboot, int, magic1, int, magic2, unsigned int, cmd, void __user *, arg)就是sys_reboot,也就是上層調用的__reboot的最終實現。函數實現如下:

/* 
 * 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; 
} 

在此函數中,首先會檢測權限問題,只有超級用戶才可以執行重啟系統的操作:

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

否則將返回權限錯誤。對應的權限列表在include/linux/capability.h中,重啟操作為22.
隨後對magic number進行了校驗:

/* 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; 

如果數據傳輸過程中沒有發生錯誤的話,這裡也當然不會有問題,所以只是一個安全性校驗,基本不會發生錯誤。
之後有一個很有趣的檢查,如果用戶要求關機,而pm_power_off為空的話,就把用戶的關機命令轉換為掛起:

/* 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中可以找到它的定義:

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

好的,只是一個函數指針,而且做了全局操作,整個kernel都可以調用它。以高通msm7x30為例,在arch/arm/mach-msm/pm2.c中對這個函數指針進行了賦值:

pm_power_off = msm_pm_power_off; 

msm_pm_power_off的具體實現就不再跟蹤了,各家的都不一樣,跟下去沒有太大意義。現在只要知道,我分析的這個kernel是給了這個函數指針賦值的,所以不為空,關機命令將正常執行。
接下來就是這個函數的正題了,對用戶命令進行解析操作,同時這個過程是用reboot_mutex互斥鎖來進行保護的,以保證同一時間只可能有一個解析過程,避免沖突。
下邊貼出所有關機重啟相關的命令定義:

/* 
 * 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 

注釋中的說明很詳細了,比較陌生的就是關於CAD,其實就是用來想用Ctrl+Alt+Del操作的;然後SW_SYSPEND是軟件休眠;KEXEC就太高端了,屬於內核的一個補丁,用來利用老內核重啟,詳細資料:http://www.ibm.com/developerworks/cn/linux/l-kexec/?ca=dwcn-newsletter-linux
以上這些只有前六個命令被Android系統所使用,為什麼這麼說,可以去看bionic/libc/include/sys/reboot.h,上邊已經貼出了。LINUX_REBOOT_CMD_HALT雖有定義,但是也沒有發現Android系統中哪裡有調用,有高手找到的話,希望能夠告知一下。最終的最終,能夠用到的就只有三個:

  • RESTART
  • POWER_OFF
  • RESTART2

10.最終實現
重啟調用的是kernel_restart,區別是參數是不是空,關機則調用kernel_power_off(),先看關機:

/** 
 * 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); 

最了一系列准備工作,最終調用machine_power_off():

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

之前找尋的pm_power_off在這裡就有用處了,是關機的最後一步操作。關機完成,之後看下重啟操作:

/** 
 * 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); 

同樣的套路,也是會進行一些准備工作,之後調用machine_restart(cmd), 如果是普通重啟,那麼中個cmd就為NULL,如果是特殊重啟,那麼這個cmd就是一層一層傳遞下來得那個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); 

而還記得剛才的pm2.c嗎?在那裡同樣對arm_pm_restart進行了指針賦值:

arm_pm_restart = msm_pm_restart; 

賦值的函數為msm_pm_init, 其調用為

late_initcall_sync(msm_pm_init); 

late_initcall_sync的啟動優先級是最低的,為7。module_init其實是6的優先級,數字越大優先級越低。所以,這樣推斷的話,最終arm_pm_restart這個函數指針會指向msm_pm_restart。關於msm_pm_restart的具體實現也不細看了,跟前邊說的一樣,都是各家不一樣,就幾行代碼:

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

但是細心的朋友可能會發現這裡有一個restart_reason,這個並不是傳遞下來的參數。事實上,這個值已經在之前kernel_restart_prepare(cmd)的時候就已經設置好了。

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機制,這個操作在之前的shutdown關機操作中也有,且是同一個list,都是reboot_notifier_list。也很容易理解,就是將注冊在reboot_notifier_list上的函數傳入相關參數後執行,作為了解,看一下具體是怎麼使用的:(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,萬事大吉,在kernel_restart_prepare的時候msm_reboot_call會被首先調用,這個函數的作用就是根據用戶命令給restart_reason賦值,從而在之後調用msm_pm_restart的時候使用。這裡我們發現在reboot的時候可以帶的參數不僅有recovery,bootloader,還有eraseflash和oem-???,字面上看應該是用來擦除ROM和解鎖之類的操作了。

三、關機怎麼用?
本文的分析是由Android給出的reboot接口開始的,但是分析來分析去,回頭想一想會發現,Android給出的接口reboot就真的只能重啟而已,不能進行關機操作,可以在跟蹤這個流程的過程中會發現,確實是有存在關機的相關接口的。那麼關機該怎麼用呢?
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); 
} 

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