以Android5.1的代码来分析。
应用层和框架层实现
上层应用可以通过PowerManager来实现关机、重启、进recovery等功能。比如RecoverySystem 中就是使用PM使系统进入recovery模式：
源码路径：frameworks/base/core/java/android/os/RecoverySystem.java
private static void bootCommand(Context context, String... args) throws IOException { ......
// Having written the command file, go ahead and reboot
PowerManager pm = (PowerManager) context.getSystemService(Context.POWER_SERVICE);
pm.reboot(PowerManager.REBOOT_RECOVERY); ......
}PowerManager会通过aidl与PowerManagerService交互，reboot带reason参数，像重启、关机、恢复出厂设置等等；
源码路径：frameworks/base/core/java/android/os/IPowerManager.aidl
reboot在aidl中的定义
interface IPowerManager{ // WARNING: The first five methods must remain the first five methods because their
// transaction numbers must not change unless IPowerManager.cpp is also updated.
void acquireWakeLock(IBinder lock, int flags, String tag, String packageName, in WorkSource ws,
String historyTag); void acquireWakeLockWithUid(IBinder lock, int flags, String tag, String packageName, int uidtoblame); void releaseWakeLock(IBinder lock, int flags); void updateWakeLockUids(IBinder lock, in int[] uids);
...... void reboot(boolean confirm, String reason, boolean wait); void shutdown(boolean confirm, boolean wait); void crash(String message); void setStayOnSetting(int val); void boostScreenBrightness(long time); // temporarily overrides the screen brightness settings to allow the user to
// see the effect of a settings change without applying it immediately
void setTemporaryScreenBrightnessSettingOverride(int brightness); void setTemporaryScreenAutoBrightnessAdjustmentSettingOverride(float adj); // sets the attention light (used by phone app only)
void setAttentionLight(boolean on, int color);
}PMS中reboot调用shutdownOrRebootInternal
源码路径：frameworks/base/services/core/java/com/android/server/power/PowerManagerService.java
@Override // Binder call
public void reboot(boolean confirm, String reason, boolean wait) {
...... final long ident = Binder.clearCallingIdentity(); try {
shutdownOrRebootInternal(false, confirm, reason, wait);
} finally {
Binder.restoreCallingIdentity(ident);
}
} public static void lowLevelShutdown() {
SystemProperties.set("sys.powerctl", "shutdown");
} public static void lowLevelReboot(String reason) { if (reason == null) {
reason = "";
}
long duration; if (reason.equals(PowerManager.REBOOT_RECOVERY)) {
SystemProperties.set("ctl.start", "pre-recovery"); duration = 300 * 1000L;
} else {
SystemProperties.set("sys.powerctl", "reboot," + reason); duration = 20 * 1000L;
}
try { Thread.sleep(duration);
} catch (InterruptedException e) { Thread.currentThread().interrupt();
}
}shutdownOrRebootInternal根据reason来决定是调用ShutdownThread的reboot或者shutdown:
private void shutdownOrRebootInternal(final boolean shutdown, final boolean confirm, final String reason, boolean wait) {
......
Runnable runnable = new Runnable() { @Override
public void run() { synchronized (this) { if (shutdown) {
ShutdownThread.shutdown(mContext, confirm);
} else {
ShutdownThread.reboot(mContext, reason, confirm);
}
......不管是关机还是重启都是走shutdownInner 源码路径：frameworks/base/services/core/java/com/android/server/power/ShutdownThread.java
public static void reboot(final Context context, String reason, boolean confirm) {
mReboot = true;
mRebootSafeMode = false;
mRebootReason = reason;
shutdownInner(context, confirm);
}
...... public static void shutdown(final Context context, boolean confirm) {
mReboot = false;
mRebootSafeMode = false;
shutdownInner(context, confirm);
}
...... static void shutdownInner(final Context context, boolean confirm) {
......
beginShutdownSequence(context);
}
}看一下beginShutdownSequence
private static void beginShutdownSequence(Context context) {
...... // start the thread that initiates shutdown
sInstance.mHandler = new Handler() {
};
sInstance.start();
}已经start了，我们就去run里面瞧一瞧干了什么事，就对“REBOOT_SAFEMODE_PROPERTY”设置了1或者0，然后调用rebootOrShutdown。
/**
* 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() { ....//关机或者重启前的一些处理
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);
}rebootOrShutdown中会调用PMS的lowLevelReboot或者lowLevelShutdown，这两个函数在上面有展示
public static void rebootOrShutdown(boolean reboot, String reason) { if (reboot) { Log.i(TAG, "Rebooting, reason: " + reason);
PowerManagerService.lowLevelReboot(reason);//重启
Log.e(TAG, "Reboot failed, will attempt shutdown instead");
} else if (SHUTDOWN_VIBRATE_MS > 0) { ....//关机震动相关处理
// Shutdown power
Log.i(TAG, "Performing low-level shutdown...");
PowerManagerService.lowLevelShutdown();//关机
}再回过头去看PowerManagerService里面的lowLevelShutdown，发现关机、重启、recovery等是通过设置Properties来实现控制的:SystemProperties.set("sys.powerctl"......
native实现
Android系统开始会执行init，而init会去解析执行init.rc中的内容。其中就有对于Properties “sys.powerctl ”的处理（当“sys.powerctl 发生改变时触发进行powerctl 操作）。 位于源码：system/core/rootdir/init.rc
on property:sys.powerctl=*
powerctl ${sys.powerctl}而powerctl 是init的内嵌命令,在init源码中： 源码路径：system/core/init/builtins.c
int do_powerctl(int nargs, char **args)
{
...... if (strncmp(command, "shutdown", 8) == 0) {
cmd = ANDROID_RB_POWEROFF; len = 8;
} else if (strncmp(command, "reboot", 6) == 0) {
cmd = ANDROID_RB_RESTART2; len = 6;
} else {
ERROR("powerctl: unrecognized command '%s'\n", command); return -EINVAL;
} if (command[len] == ',') {
reboot_target = &command[len + 1];
} else if (command[len] == '\0') {
reboot_target = "";
} else {
ERROR("powerctl: unrecognized reboot target '%s'\n", &command[len]); return -EINVAL;
} return android_reboot(cmd, 0, reboot_target);
}android_reboot系统调用reboot，如果是recovery、wipe data等操作还需要额外写参数到misc分区，以便开机时区分正常进Android系统还是recovery、efex。
源码路径：system/core/libcutils/android_reboot.c
int android_reboot(int cmd, int flags UNUSED, char *arg)
{ int ret;
sync();
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 = syscall(__NR_reboot, LINUX_REBOOT_MAGIC1, LINUX_REBOOT_MAGIC2,
// LINUX_REBOOT_CMD_RESTART2, arg);
write_misc(arg);
ret = reboot(RB_AUTOBOOT); break;
default:
ret = -1;
} return ret;
}write_misc源码路径：system/core/libcutils/misc_rw.c有兴趣的可以研究一下。
static const char *MISC_DEVICE = "/dev/block/by-name/misc";/* Bootloader Message
*/struct bootloader_message {
char command[32]; char status[32]; char recovery[1024];
};
....../* force the next boot to recovery/efex */int write_misc(char *reason) { struct bootloader_message boot, temp;
memset(&boot, 0, sizeof(boot)); if (!strcmp("recovery", reason)) {
reason = "boot-recovery";
} strcpy(boot.command, reason); if (set_bootloader_message_block(&boot, MISC_DEVICE) ) return -1; //read for compare
memset(&temp, 0, sizeof(temp)); if (get_bootloader_message_block(&temp, MISC_DEVICE)) return -1; if( memcmp(&boot, &temp, sizeof(boot)) ) return -1; return 0;
}static int set_bootloader_message_block(const struct bootloader_message *in, const char* device) {
FILE* f = fopen(device, "wb"); if (f == NULL) {
LOGE("Can't open %s\n(%s)\n", device, strerror(errno)); return -1;
} int count = fwrite(in, sizeof(*in), 1, f); if (count != 1) {
LOGE("Failed writing %s\n(%s)\n", device, strerror(errno)); return -1;
}
sync(); if (fclose(f) != 0) {
LOGE("Failed closing %s\n(%s)\n", device, strerror(errno)); return -1;
} return 0;
}那么问题来了，开机时怎么判断是该正常开机、还是进入recovery呢？ 一般是在u-boot中读取misc分区的数据，然后进行解析后对boot env "bootcmd"进行参数配置，这样就可以引导进入不同的系统了。 例如全志平台u-boot： board/sunxi/common/board_common.c
int update_bootcmd(void)
{
......
misc_message = (struct bootloader_message *)misc_args;
memset(misc_args, 0x0, 2048);
memset(misc_fill, 0xff, 2048);
mode = detect_other_boot_mode();
switch(mode)
{
...... case ANDROID_NULL_MODE:
{
......
else
{
misc_offset = sunxi_partition_get_offset_byname("misc");
......
printf("misc partition found\n"); //read misc partition data
sunxi_flash_read(misc_offset, 2048/512, misc_args);
......
else if(!strcmp(misc_message->command, "boot-recovery"))
......
puts("Recovery detected, will boot recovery\n");
sunxi_str_replace(boot_commond, "boot_normal", "boot_recovery");
} /* android recovery will clean the misc */
}
else if(!strcmp(misc_message->command, "bootloader"))
{
puts("Fastboot detected, will boot fastboot\n");
sunxi_str_replace(boot_commond, "boot_normal", "boot_fastboot"); if(misc_offset)
sunxi_flash_write(misc_offset, 2048/512, misc_fill);
}
else if(!strcmp(misc_message->command, "usb-recovery"))
{
puts("Recovery detected, will usb recovery\n");
sunxi_str_replace(boot_commond, "boot_normal", "boot_recovery");
}
else if(!strcmp(misc_message->command ,"debug_mode"))
{
puts("debug_mode detected ,will enter debug_mode"); if(0 == debug_mode_set())
{
debug_mode_update_info();
}
sunxi_flash_write(misc_offset,2048/512,misc_fill);
......
setenv("bootcmd", boot_commond);
printf("to be run cmd=%s\n", boot_commond);
return 0;
}boot_recovery的evn：
"boot_recovery=sunxi_flash read 4007f800 recovery;boota 4007f800\0"
boot_recovery将recovery加载到内存中并引导，然后就这样就进入recovery中了。
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