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：
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); } 新闻热点
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