Android(安卓)关机流程分析-----（2）JNI和kernel层

最近遇到fastboot关机电流偏高的问题，虽然最后确认是硬件的问题，但还是顺便分析了一下android开关机的流程。总结一下，加深印象，也方便日后查阅。

Android智能手机和平板一般都有Power key，长按Power key弹出关机对话框，选择power off就会让系统关闭。关机动作从按键触发中断，linux kernel层给android framework层返回按键事件进入framework层，再从framework层到kernel层执行关机任务。本文分析过程将分成两篇，（1）Framework层（2）JNI和Kernel层，代码基于自己的android4.3源码。

前面的文章Android 关机流程分析-----（1）Framework层中，分析了framework层的关机流程，本文会继续分析JNI和Kernel层的关机流程。

五、JNI层的接口：android_reboot()

framework层最后执行lowLevelShutdown()，进入nativeShutdown()。在JNI层，有如下对nativeShutdown()以及nativeReboot()的定义。

frameworks/base/services/jni/com_android_server_power_PowerManagerService.cpp

static void nativeShutdown(JNIEnv *env, jclass clazz) {    android_reboot(ANDROID_RB_POWEROFF, 0, 0);}static void nativeReboot(JNIEnv *env, jclass 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 JNINativeMethod gPowerManagerServiceMethods[] = {    /* name, signature, funcPtr */    { "nativeInit", "()V",            (void*) nativeInit },    { "nativeSetPowerState", "(ZZ)V",            (void*) nativeSetPowerState },    { "nativeAcquireSuspendBlocker", "(Ljava/lang/String;)V",            (void*) nativeAcquireSuspendBlocker },    { "nativeReleaseSuspendBlocker", "(Ljava/lang/String;)V",            (void*) nativeReleaseSuspendBlocker },    { "nativeSetInteractive", "(Z)V",            (void*) nativeSetInteractive },    { "nativeSetAutoSuspend", "(Z)V",            (void*) nativeSetAutoSuspend },    { "nativeShutdown", "()V",            (void*) nativeShutdown },    { "nativeReboot", "(Ljava/lang/String;)V",            (void*) nativeReboot },};

在JNI中，framework层中的nativeShutdown()还是被JNI成nativeShutdown()，然后进入android_reboot(ANDROID_RB_POWEROFF, 0, 0)。
android_reboot()在system/core/libcutils/Android_reboot.c中定义如下：

int android_reboot(int cmd, int flags, char *arg){    int ret;    if (!(flags & ANDROID_RB_FLAG_NO_SYNC))        sync();    if (!(flags & ANDROID_RB_FLAG_NO_REMOUNT_RO))        remount_ro();    switch (cmd) {        case ANDROID_RB_RESTART:            ret = reboot(RB_AUTOBOOT);            break;        case ANDROID_RB_POWEROFF:            ret = reboot(RB_POWER_OFF);            break;        case ANDROID_RB_RESTART2:            ret = __reboot(LINUX_REBOOT_MAGIC1, LINUX_REBOOT_MAGIC2,                           LINUX_REBOOT_CMD_RESTART2, arg);            break;        default:            ret = -1;    }    return ret;}

1. 参数cmd是命令行参数，在system/core/include/cutils/Android_reboot.h中定义如下：

/* Commands */#define ANDROID_RB_RESTART  0xDEAD0001#define ANDROID_RB_POWEROFF 0xDEAD0002#define ANDROID_RB_RESTART2 0xDEAD0003/* Flags */#define ANDROID_RB_FLAG_NO_SYNC       0x1#define ANDROID_RB_FLAG_NO_REMOUNT_RO 0x2int android_reboot(int cmd, int flags, char *arg);

分别代表三种不同的shutdown动作。

2. switch(cmd)如果进入ANDROID_RB_POWEROFF，则会执行reboot(RB_POWER_OFF)，即shutdown的linux系统调用。reboot()定义在bionic/libc/bionic/reboot.c中。

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

__reboot是通往linux kernel层的入口，执行__reboot就进入了linux kernel的关机过程。

六、linux kernel的shutdown过程

如前所述，__reboot()进入kernel的关机过程。进入kernel，首先执行SYSCALL_DEFINE4()，根据switch(cmd)语句选择的情况，执行不同的关机动作。

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;lock_kernel();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();unlock_kernel();do_exit(0);panic("cannot halt");case LINUX_REBOOT_CMD_POWER_OFF:kernel_power_off();unlock_kernel();do_exit(0);break;case LINUX_REBOOT_CMD_RESTART2:if (strncpy_from_user(&buffer[0], arg, sizeof(buffer) - 1) < 0) {unlock_kernel();return -EFAULT;}buffer[sizeof(buffer) - 1] = '\0';kernel_restart(buffer);break;#ifdef CONFIG_KEXECcase LINUX_REBOOT_CMD_KEXEC:ret = kernel_kexec();break;#endif#ifdef CONFIG_HIBERNATIONcase LINUX_REBOOT_CMD_SW_SUSPEND:ret = hibernate();break;#endifdefault:ret = -EINVAL;break;}unlock_kernel();return ret;}

如果cmd为LINUX_REBOOT_CMD_POWER_OFF，则进入kernel_power_off()。因为本文主要分析kernel_power_off()，其他情况类似，就不分析了。

/** *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();sysdev_shutdown();printk(KERN_EMERG "Power down.\n");machine_power_off();}

1. kernel_shutdown_prepare()，这个函数中首先会用一个链表的操作block notifier，关于链表的操作可以自己去研究。

然后更新system_state，最后执行device_shutdown()去关闭所有的devices。

static void kernel_shutdown_prepare(enum system_states state){blocking_notifier_call_chain(&reboot_notifier_list,(state == SYSTEM_HALT)?SYS_HALT:SYS_POWER_OFF, NULL);system_state = state;device_shutdown();}

2.disable_nonboot_cpus()，这一步骤和系统suspend时调用的disable_nonboot_cpus()是一样的，即关掉nonboot的cpu。

3. sysdev_shutdown()，系统设备的shutdown。

4.machine_power_off()，这是一个平台相关的关机指针，执行最后的关机动作。在include/kernel/reboot.h中声明此函数为外部的，针对不同的平台，就会编译不同的machine_power_off()函数。

对应找到arch/arm/kernel/process.c的machine_power_off()。

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

如果pm_power_off非空，继续执行pm_power_off()进入PMIC执行硬件的power off。

拿at91sam9260举例，在这里定义了pm_power_off的具体事例。 pm_power_off = at91sam9260_poweroff;

static void at91sam9260_poweroff(void){at91_sys_write(AT91_SHDW_CR, AT91_SHDW_KEY | AT91_SHDW_SHDW);}

这就是power off最后执行的平台相关的操作，接下来的操作就交给硬件了。

分析到这里，两篇Android关机流程分析到此结束。

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