Android 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 0x2 int 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_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;
}
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。