Android关机流程解析---从reboot说起

转自：http://blog.sina.com.cn/s/blog_6695f9eb0101hse4.html

Android关机流程解析

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。

--------------------------------上层空间--------------------------------

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

[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) { 
    }    
}  

   
    public void reboot(String reason)
    {  
        try {
            mService.reboot(reason);
        } catch (RemoteException e) {
        }  
    }
mService为IPowerManager Binder接口服务。

[java] 
 
public PowerManager(IPowerManager service, Handler handler) 
{ 
    mService = service; 
    mHandler = handler; 
} 

   
    public PowerManager(IPowerManager service, Handler handler)
    {
        mService = service;
        mHandler = handler;
    }

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

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

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

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

[java] 
 
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) {  
            }     
        }     
    }     
} 

   
    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

[java]
 
public static void reboot(final Context context, String reason, boolean confirm) { 
    mReboot = true; 
    mRebootSafeMode = false; 
    mRebootReason = reason; 
    shutdownInner(context, confirm); 
} 

   
    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。

[java]
  
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(); 
        } 
    }; 
 
      
    {    
        String reason = (mReboot ? "1" : "0") + (mRebootReason != null ? mRebootReason : ""); 
        SystemProperties.set(SHUTDOWN_ACTION_PROPERTY, reason); 
    } 
 
      
    if (mRebootSafeMode) { 
        SystemProperties.set(REBOOT_SAFEMODE_PROPERTY, "1"); 
    } 
    ... 
    rebootOrShutdown(mReboot, mRebootReason); 
}  

   
    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();
            }
        };

       
        {  
            String reason = (mReboot ? "1" : "0") + (mRebootReason != null ? mRebootReason : "");
            SystemProperties.set(SHUTDOWN_ACTION_PROPERTY, reason);
        }

       
        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。

[java] 
     
    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(); 
    } 
} 

   
    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。

[jav
 
public static void shutdown(final Context context, boolean confirm) { 
    mReboot = false; 
    mRebootSafeMode = false; 
    shutdownInner(context, confirm); 
} 

   
    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

[java]  ?
 
public static void lowLevelReboot(String reason) throws IOException { 
    nativeReboot(reason); 
}   
 
 
public static void lowLevelShutdown() { 
    nativeShutdown(); 
}  

   
    public static void lowLevelReboot(String reason) throws IOException {
        nativeReboot(reason);
    } 

   
    public static void lowLevelShutdown() {
        nativeShutdown();
    }

很熟悉的字样native，是JNI调用了：

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

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

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

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

static JNINativeMethod gPowerManagerServiceMethods[] = {
   
    ...
    { "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：

[cpp] 
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); 
} 

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

[cpp] 
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; 
} 

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：

[cpp] 
#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 

#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：

[cpp]
#define LINUX_REBOOT_MAGIC1 0xfee1dead  
#define LINUX_REBOOT_MAGIC2 672274793  
 
#define LINUX_REBOOT_MAGIC2A 85072278  
#define LINUX_REBOOT_MAGIC2B 369367448  
#define LINUX_REBOOT_MAGIC2C 537993216  
#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 

#define LINUX_REBOOT_MAGIC1 0xfee1dead
#define LINUX_REBOOT_MAGIC2 672274793

#define LINUX_REBOOT_MAGIC2A 85072278
#define LINUX_REBOOT_MAGIC2B 369367448
#define LINUX_REBOOT_MAGIC2C 537993216
#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

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

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。

--------------------------------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）

[plain]
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) 

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：

[plain]
#define __NR_reboot                       (__NR_SYSCALL_BASE + 88) 

#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

[cpp] 
 
#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) 

#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：

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

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：

[cpp]
#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__)) 

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

#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：
[cpp] 
 
SYSCALL_DEFINE4(reboot, int, magic1, int, magic2, unsigned int, cmd, 
        void __user *, arg) 
{ 
    char buffer[256]; 
    int ret = 0; 
 
     
    if (!capable(CAP_SYS_BOOT)) 
        return -EPERM; 
 
     
    if (magic1 != LINUX_REBOOT_MAGIC1 || 
        (magic2 != LINUX_REBOOT_MAGIC2 && 
                    magic2 != LINUX_REBOOT_MAGIC2A && 
            magic2 != LINUX_REBOOT_MAGIC2B && 
                    magic2 != LINUX_REBOOT_MAGIC2C)) 
        return -EINVAL; 
 
     
    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; 
} 

SYSCALL_DEFINE4(reboot, int, magic1, int, magic2, unsigned int, cmd,
  void __user *, arg)
{
 char buffer[256];
 int ret = 0;

 
 if (!capable(CAP_SYS_BOOT))
  return -EPERM;

 
 if (magic1 != LINUX_REBOOT_MAGIC1 ||
     (magic2 != LINUX_REBOOT_MAGIC2 &&
                 magic2 != LINUX_REBOOT_MAGIC2A &&
   magic2 != LINUX_REBOOT_MAGIC2B &&
                 magic2 != LINUX_REBOOT_MAGIC2C))
  return -EINVAL;

 
 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：

[cpp] 
 
if (!capable(CAP_SYS_BOOT)) 
    return -EPERM; 

   
    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：

[cpp] 
 
if (magic1 != LINUX_REBOOT_MAGIC1 || 
    (magic2 != LINUX_REBOOT_MAGIC2 && 
                magic2 != LINUX_REBOOT_MAGIC2A && 
        magic2 != LINUX_REBOOT_MAGIC2B && 
                magic2 != LINUX_REBOOT_MAGIC2C)) 
    return -EINVAL; 

   
    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：

[cpp] 
 
if ((cmd == LINUX_REBOOT_CMD_POWER_OFF) && !pm_power_off) 
    cmd = LINUX_REBOOT_CMD_HALT; 

   
    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：

[cpp]
 
void (*pm_power_off)(void); 
EXPORT_SYMBOL(pm_power_off); 

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：

[cpp] 
pm_power_off = msm_pm_power_off; 

     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：

[cpp] 
      
         
#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 

    
       
#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：

[cpp] 
 
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); 

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()：

[cpp] 
void machine_power_off(void) 
{    
    machine_shutdown(); 
    if (pm_power_off) 
        pm_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：

[cpp] 
 
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); 

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。

[cpp]
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); 

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：

[cpp] 
arm_pm_restart = msm_pm_restart; 

arm_pm_restart = msm_pm_restart;
赋值的函数为msm_pm_init， 其调用为

[cpp] 
late_initcall_sync(msm_pm_init); 

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

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。

[cpp] v
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(); 
} 

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)

 

[cpp] 
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); 
... 
} 

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。

 

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
[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); 
} 

        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);
        }
    }

这样就可以了，不用多说了吧。