Recovery启动流程(1)--- 应用层到开机进入recovery详解

转载请注明来源：cuixiaolei的技术博客

　进入recovery有两种方式，一种是通过组合键进入recovery，另一种是上层应用设置中执行安装/重置/清除缓存等操作进行recovery。这篇文档主要讲解上层应用是如何进入到recovery的。本文以高通平台为例。

　　

1.app执行安装/重置/清楚缓存操作调用代码文件frameworks/base/core/java/android/os/RecoverySystem.java

不同的操作使用不同的方法：

安装升级包  --------  installPackage

清除用户数据------  rebootWipeUserData

清楚缓存  -----------  rebootWipeCache

上面的所有操作都是往/cache/recovery/command文件中写入不同的命令，在进入recovery后（recovery.cpp）对command的关键字进行判断，执行相应的操作，下文会详细讲解，这里先简单提及。

bootable/recovery/recovery.app
static const struct option OPTIONS[] = {
  { "send_intent", required_argument, NULL, 'i' },
  { "update_package", required_argument, NULL, 'u' },

#ifdef TARGET_USE_REDBEND_FOTA
  { "omadm_package", required_argument, NULL, 'o' },
#endif

  { "wipe_data", no_argument, NULL, 'w' },
  { "wipe_cache", no_argument, NULL, 'c' },
  { "show_text", no_argument, NULL, 't' },
  { "sideload", no_argument, NULL, 's' },
  { "sideload_auto_reboot", no_argument, NULL, 'a' },
  { "just_exit", no_argument, NULL, 'x' },
  { "locale", required_argument, NULL, 'l' },
  { "stages", required_argument, NULL, 'g' },
  { "shutdown_after", no_argument, NULL, 'p' },
  { "reason", required_argument, NULL, 'r' },
  { NULL, , NULL,  },
};

..................

int
main(int argc, char **argv) {
....
get_args(&argc, &argv); //提取cache/recovery/command中的信息
....
while ((arg = getopt_long(argc, argv, "", OPTIONS, NULL)) != -1) {      //解析cache/recovery/command文件中的信息
        printf("***** xiaolei 2\n");
        switch (arg) {
        case 'i': send_intent = optarg; break;
        case 'u': update_package = optarg; break;

        case 'w': should_wipe_data = true; break;
        case 'c': should_wipe_cache = true; break;
        case 't': show_text = true; break;
        case 's': sideload = true; break;
        case 'a': sideload = true; sideload_auto_reboot = true; break;
        case 'x': just_exit = true; break;
        case 'l': locale = optarg; break;
        case 'g': {
            if (stage == NULL || *stage == '\0') {
                char buffer[20] = "1/";
                strncat(buffer, optarg, sizeof(buffer)-3);
                stage = strdup(buffer);
            }
            break;
        }
        case 'p': shutdown_after = true; break;
        case 'r': reason = optarg; break;
        case '?':
            LOGE("Invalid command argument\n");
            continue;
        }
    }
}

本文以installPackage为例：

frameworks/base/core/java/android/os/RecoverySystem.java
public static void installPackage(Context context, File packageFile)
        throws IOException {
        String filename = packageFile.getCanonicalPath();    //获取升级包路径
        String internalPath = Environment.maybeTranslateEmulatedPathToInternal(new File(filename)).getPath();
        FileWriter uncryptFile = new FileWriter(UNCRYPT_FILE);
        try {
            uncryptFile.write(internalPath + "\n");
        } finally {
            uncryptFile.close();
        }
        Log.w(TAG, "!!! REBOOTING TO INSTALL " + filename + " !!!");
        Log.w(TAG, "!!! REBOOTING TO INSTALL REALPATH " + internalPath + " !!!");

        // If the package is on the /data partition, write the block map file
        // If the package is on internal storage sdcard,write the block map file
        // into COMMAND_FILE instead.
        if (filename.startsWith("/data/")                           //加密处理，block.map是解密的映射文件
                ||filename.startsWith("/storage/emulated/0/")) {
            filename = "@/cache/recovery/block.map";
        }

        final String filenameArg = "--update_package=" + filename;     //把“--update_package=path” 通过bootCommand方法写入到cache/recovery/command文件中
        final String localeArg = "--locale=" + Locale.getDefault().toString();  //recovery显示语言
        bootCommand(context, filenameArg, localeArg);
    }

installPackage方法会嗲用bootCommand()方法，此时参数为（context,向cache/recovery/command文件中写入的信息，语言信息）

private static void bootCommand(Context context, String... args) throws IOException {
        RECOVERY_DIR.mkdirs();  // In case we need it
        COMMAND_FILE.delete();  // In case it's not writable
        LOG_FILE.delete();

        FileWriter command = new FileWriter(COMMAND_FILE);     //向/cache/recovery/command中写入--update_package=path
        try {
            for (String arg : args) {
                if (!TextUtils.isEmpty(arg)) {
                    command.write(arg);
                    command.write("\n");
                }
            }
        } finally {
            command.close();
        }

        // Having written the command file, go ahead and reboot
        PowerManager pm = (PowerManager) context.getSystemService(Context.POWER_SERVICE);
        pm.reboot(PowerManager.REBOOT_RECOVERY);  //PowerManager.REBOOT_RECOVERY的值为字符串"recovery"

        throw new IOException("Reboot failed (no permissions?)");
    }

pm.reboot(PowerManager.REBOOT_RECOVERY); 参数为字符串("recovery")

frameworks/base/core/java/android/os/PowerMangager.java

public void reboot(String reason) {
        try {
            mService.reboot(false, reason, true);
        } catch (RemoteException e) {
        }
    }

 mService.reboot(false, reason, true);    参数为(false,"recovery",true)

frameworks/base/services/core/java/com/android/server/power/PowerManagerService.java

public void reboot(boolean confirm, String reason, boolean wait) {       
            mContext.enforceCallingOrSelfPermission(android.Manifest.permission.REBOOT, null);
            if (PowerManager.REBOOT_RECOVERY.equals(reason)) {
                mContext.enforceCallingOrSelfPermission(android.Manifest.permission.RECOVERY, null);
            }

            final long ident = Binder.clearCallingIdentity();
            try {
                shutdownOrRebootInternal(false, confirm, reason, wait);      
            } finally {
                Binder.restoreCallingIdentity(ident);
            }
        }

shutdownOrRebootInternal(false, confirm, reason, wait);  参数为(false,false,"recovery",true)

frameworks/base/services/core/java/com/android/server/power/PowerManagerService.java

private void shutdownOrRebootInternal(final boolean shutdown, final boolean confirm,
            final String reason, boolean wait) {
        if (mHandler == null || !mSystemReady) {
            throw new IllegalStateException("Too early to call shutdown() or reboot()");
        }

        Runnable runnable = new Runnable() {
            @Override
            public void run() {
                synchronized (this) {
                    if (shutdown) {                   //此处shutdown=false
                        ShutdownThread.shutdown(mContext, confirm);
                    } else {
                        ShutdownThread.reboot(mContext, reason, confirm);   //执行此处代码
                    }
                }
            }
        };

        // ShutdownThread must run on a looper capable of displaying the UI.
        Message msg = Message.obtain(mHandler, runnable);
        msg.setAsynchronous(true);
        mHandler.sendMessage(msg);

        // PowerManager.reboot() is documented not to return so just wait for the inevitable.
        if (wait) {
            synchronized (runnable) {
                while (true) {
                    try {
                        runnable.wait();
                    } catch (InterruptedException e) {
                    }
                }
            }
        }
    }

ShutdownThread.reboot(mContext, reason, confirm); 参数为(mContex,"recovery",false)
此处开了一个线程处理reboot

frameworks/base/services/core/java/com/android/server/power/ShutdownThread.java

public static void reboot(final Context context, String reason, boolean confirm) {        //方法中的变量为全局变量
        mReboot = true;
        mRebootSafeMode = false;
        mRebootUpdate = false;
        mRebootReason = reason;
        shutdownInner(context, confirm);       //此方法是在手机界面上弹出一个确认框，是否重启，此处的代码不再追了
    }

//程序一定会执行run()方法

/***********************

mReboot = true;
mRebootSafeMode = false;
mRebootUpdate = false;
mRebootReason = "recovery";
***********************/
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 : "");     //reason的值为"recovery"
            SystemProperties.set(SHUTDOWN_ACTION_PROPERTY, reason);  //设置系统属性sys.shutdown.requested = "recovery"
        }

        /*
         * If we are rebooting into safe mode, write a system property
         * indicating so.
         */
        if (mRebootSafeMode) {
            SystemProperties.set(REBOOT_SAFEMODE_PROPERTY, "1");
        }

        Log.i(TAG, "Sending shutdown broadcast...");
        setBootValue("silent_mode",mAudioManager.isSilentMode() ? "1" : "0");
        if(checkAnimationFileExist() && mContext.getResources().
                getBoolean(com.android.internal.R.bool.feature_tctfw_shutdown_animation_on) &&
                !mRebootUpdate) {
            lockDevice();
            showShutdownAnimation();
        }

        // First send the high-level shut down broadcast.
        mActionDone = false;
        Intent intent = new Intent(Intent.ACTION_SHUTDOWN);
        intent.addFlags(Intent.FLAG_RECEIVER_FOREGROUND);
        mContext.sendOrderedBroadcastAsUser(intent,
                UserHandle.ALL, null, br, mHandler, 0, null, null);

        final long endTime = SystemClock.elapsedRealtime() + MAX_BROADCAST_TIME;
        synchronized (mActionDoneSync) {
            while (!mActionDone) {
                long delay = endTime - SystemClock.elapsedRealtime();
                if (delay <= 0) {
                    Log.w(TAG, "Shutdown broadcast timed out");
                    break;
                } else if (mRebootUpdate) {
                    int status = (int)((MAX_BROADCAST_TIME - delay) * 1.0 *
                            BROADCAST_STOP_PERCENT / MAX_BROADCAST_TIME);
                    sInstance.setRebootProgress(status, null);
                }
                try {
                    mActionDoneSync.wait(Math.min(delay, PHONE_STATE_POLL_SLEEP_MSEC));
                } catch (InterruptedException e) {
                }
            }
        }
        if (mRebootUpdate) {
            sInstance.setRebootProgress(BROADCAST_STOP_PERCENT, null);
        }
        Log.i(TAG, "Shutting down activity manager...");

        final IActivityManager am =
            ActivityManagerNative.asInterface(ServiceManager.checkService("activity"));
        if (am != null) {
            try {
                am.shutdown(MAX_BROADCAST_TIME);
            } catch (RemoteException e) {
            }
        }
        if (mRebootUpdate) {
            sInstance.setRebootProgress(ACTIVITY_MANAGER_STOP_PERCENT, null);
        }

        Log.i(TAG, "Shutting down package manager...");

        final PackageManagerService pm = (PackageManagerService)
            ServiceManager.getService("package");
        if (pm != null) {
            pm.shutdown();
        }
        if (mRebootUpdate) {
            sInstance.setRebootProgress(PACKAGE_MANAGER_STOP_PERCENT, null);
        }

        // Shutdown radios.
        shutdownRadios(MAX_RADIO_WAIT_TIME);
        if (mRebootUpdate) {
            sInstance.setRebootProgress(RADIO_STOP_PERCENT, null);
        }

        // Mod-by-yanxi.liu, for sdcard upgrade uncrypt, Defect-1357392
        if (mRebootUpdate) {
            sInstance.setRebootProgress(MOUNT_SERVICE_STOP_PERCENT, null);
            // If it's to reboot to install update, invoke uncrypt via init service.
            uncrypt();
        }
        // Mod-by-yanxi.liu, for sdcard upgrade uncrypt, Defect-1357392

        // Shutdown MountService to ensure media is in a safe state
        IMountShutdownObserver observer = new IMountShutdownObserver.Stub() {
            public void onShutDownComplete(int statusCode) throws RemoteException {
                Log.w(TAG, "Result code " + statusCode + " from MountService.shutdown");
                actionDone();
            }
        };

        Log.i(TAG, "Shutting down MountService");

        // Set initial variables and time out time.
        mActionDone = false;
        final long endShutTime = SystemClock.elapsedRealtime() + MAX_SHUTDOWN_WAIT_TIME;
        synchronized (mActionDoneSync) {
            try {
                final IMountService mount = IMountService.Stub.asInterface(
                        ServiceManager.checkService("mount"));
                if (mount != null) {
                    mount.shutdown(observer);
                } else {
                    Log.w(TAG, "MountService unavailable for shutdown");
                }
            } catch (Exception e) {
                Log.e(TAG, "Exception during MountService shutdown", e);
            }
            while (!mActionDone) {
                long delay = endShutTime - SystemClock.elapsedRealtime();
                if (delay <= 0) {
                    Log.w(TAG, "Shutdown wait timed out");
                    break;
                } else if (mRebootUpdate) {
                    int status = (int)((MAX_SHUTDOWN_WAIT_TIME - delay) * 1.0 *
                            (MOUNT_SERVICE_STOP_PERCENT - RADIO_STOP_PERCENT) /
                            MAX_SHUTDOWN_WAIT_TIME);
                    status += RADIO_STOP_PERCENT;
                    sInstance.setRebootProgress(status, null);
                }
                try {
                    mActionDoneSync.wait(Math.min(delay, PHONE_STATE_POLL_SLEEP_MSEC));
                } catch (InterruptedException e) {
                }
            }
        }
        // add by feikuang for defect 1453123 start
        isVibrate = mContext.getResources().getBoolean(com.android.internal.R.bool.config_isVibrate);
        Log.i(TAG,"isVibrate " + isVibrate);
        // add by feikuang for defect 1453123 end
        waitShutDownAnimationCompleted();
        rebootOrShutdown(mContext, mReboot, mRebootReason);        //程序执行到这里
    }

rebootOrShutdown(mContext, mReboot, mRebootReason); 参数为(mContext,true,"recovery")

frameworks/base/services/core/java/com/android/server/power/ShutdownThread.java
public static void rebootOrShutdown(final Context context, boolean reboot, String reason) {
        deviceRebootOrShutdown(reboot, reason);
        //[FEATURE]-Add-BEGIN by TSNJ.shu.wang,11/06/2015,TASK-871146
        String bootAlarms = SystemProperties.get("sys.boot.alarm");
        boolean isBootAlarms = bootAlarms != null && bootAlarms.equals("1");
        if (reboot) {
            Log.i(TAG, "Rebooting, reason: " + reason);
            PowerManagerService.lowLevelReboot(reason);           //程序执行到这里
            Log.e(TAG, "Reboot failed, will attempt shutdown instead");
        } else if (SHUTDOWN_VIBRATE_MS > 0 && context != null && !isBootAlarms && isVibrate) {
        //[FEATURE]-Add-END by TSNJ.shu.wang,11/06/2015,TASK-871146
            // vibrate before shutting down
            Vibrator vibrator = new SystemVibrator(context);
            try {
                vibrator.vibrate(SHUTDOWN_VIBRATE_MS, VIBRATION_ATTRIBUTES);
            } 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();
    }

PowerManagerService.lowLevelReboot(reason);  参数为"recovery"

frameworks/base/services/core/java/com/android/server/power/PowerManagerService.java
public static void lowLevelReboot(String reason) {
        if (reason == null) {
            reason = "";
        }
        if (reason.equals(PowerManager.REBOOT_RECOVERY)) {
            // If we are rebooting to go into recovery, instead of
            // setting sys.powerctl directly we'll start the
            // pre-recovery service which will do some preparation for
            // recovery and then reboot for us.
            SystemProperties.set("ctl.start", "pre-recovery");    //到这里可以知道，一个新的进程会被enable
        } else {
            SystemProperties.set("sys.powerctl", "reboot," + reason);
        }
        try {
            Thread.sleep(20 * 1000L);
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        }
        Slog.wtf(TAG, "Unexpected return from lowLevelReboot!");
    }

SystemProperties.set("ctl.start", "pre-recovery");    //到这里可以知道，一个新的进程会被enable

system/core/rootdir/init.rc
service pre-recovery /system/bin/uncrypt --reboot  
    class main
    disabled
    oneshot

service pre-recovery /system/bin/uncrypt --reboot    system/bin/uncrypt 这个程序会被执行，传入的参数是--reboot
uncrypt的源码位于bootable/recovery/uncrypt/uncrypt.cpp，下面对uncrypt进行分析

bootable/recovery/uncrypt/uncrypt.cpp
int main(int argc, char** argv) {          //此处argc为2  argv[1]="--reboot"
    const char* input_path;
    const char* map_file;

    if (argc != 3 && argc != 1 && (argc == 2 && strcmp(argv[1], "--reboot") != 0)) {
        fprintf(stderr, "usage: %s [--reboot] [<transform_path> <map_file>]\n", argv[0]);
        return 2;
    }

    // When uncrypt is started with "--reboot", it wipes misc and reboots.
    // Otherwise it uncrypts the package and writes the block map.
    if (argc == 2) {   //程序执行到此处
        if (read_fstab() == NULL) {  
            return 1;
        }
        wipe_misc();        //擦出misc分区内容
        reboot_to_recovery();  //重启到recovery
    } else {
        // The pipe has been created by the system server.
        int status_fd = open(status_file.c_str(), O_WRONLY | O_CREAT | O_SYNC, S_IRUSR | S_IWUSR);
        if (status_fd == -1) {
            ALOGE("failed to open pipe \"%s\": %s\n", status_file.c_str(), strerror(errno));
            return 1;
        }
        std::string package;
        if (argc == 3) {
            // when command-line args are given this binary is being used
            // for debugging.
            input_path = argv[1];
            map_file = argv[2];
        } else {
            if (!find_uncrypt_package(package)) {
                android::base::WriteStringToFd("-1\n", status_fd);
                close(status_fd);
                return 1;
            }
            input_path = package.c_str();
            map_file = cache_block_map.c_str();
        }

        int status = uncrypt(input_path, map_file, status_fd);
        if (status != 0) {
            android::base::WriteStringToFd("-1\n", status_fd);
            close(status_fd);
            return 1;
        }

        android::base::WriteStringToFd("100\n", status_fd);
        close(status_fd);
    }

    return 0;
}

reboot_to_recovery();

bootable/recovery/uncrpty/uncrpty.cpp
static void reboot_to_recovery() {
    ALOGI("rebooting to recovery");
    property_set("sys.powerctl", "reboot,recovery");
    sleep(10);
    ALOGE("reboot didn't succeed?");
}

property_set("sys.powerctl", "reboot,recovery");     sys.powerctl属性出发开关在init.rc中

system/core/rootdir/init.rc
on property:sys.powerctl=*
    powerctl ${sys.powerctl}

system/core/init/keywords.h
KEYWORD(powerctl,    COMMAND, 1, do_powerctl

system/core/init/builtins.cpp               
int do_powerctl(int nargs, char **args)           //传入的参数为字符串"reboot,recovery"
{
    char command[PROP_VALUE_MAX];
    int res;
    int len = 0;
    int cmd = 0;
    const char *reboot_target;

    res = expand_props(command, args[1], sizeof(command));
    if (res) {
        ERROR("powerctl: cannot expand '%s'\n", args[1]);
        return -EINVAL;
    }

    if (strncmp(command, "shutdown", 8) == 0) {
        cmd = ANDROID_RB_POWEROFF;
        len = 8;
    } else if (strncmp(command, "reboot", 6) == 0) {    //程序走到这，cmd=ANDROID_RB_RESTART2
        cmd = ANDROID_RB_RESTART2;
        len = 6;
    } else {
        ERROR("powerctl: unrecognized command '%s'\n", command);
        return -EINVAL;
    }

    if (command[len] == ',') {
        char prop_value[PROP_VALUE_MAX] = {0};
        reboot_target = &command[len + 1];    //设置reboot_target = recovery

        if ((property_get("init.svc.recovery", prop_value) == 0) &&
            (strncmp(reboot_target, "keys", 4) == 0)) {
            ERROR("powerctl: permission denied\n");
            return -EINVAL;
        }
    } 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(cmd, 0, reboot_target);    参数为(ANDROID_RB_RESTART2, 0, "recovery")

system/core/libcutils/android_reboot.c
int android_reboot(int cmd, int flags UNUSED, const 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:             //程序跑到这里其中arg="recovery"
            ret = syscall(__NR_reboot, LINUX_REBOOT_MAGIC1, LINUX_REBOOT_MAGIC2,
                           LINUX_REBOOT_CMD_RESTART2, arg);
            break;

        default:
            ret = -1;
    }

    return ret;
}

ret = syscall(__NR_reboot, LINUX_REBOOT_MAGIC1, LINUX_REBOOT_MAGIC2,
                           LINUX_REBOOT_CMD_RESTART2, arg);

kernel/include/uapi/asm-generic/unistd.h
#define __NR_reboot 142
__SYSCALL(__NR_reboot, sys_reboot）

__NR_reboot被映射到sys_reboot上,会执行SYSCALL_DEFINE4(),为什么会这么执行还不清楚。

kernel/kernel/sys.c
SYSCALL_DEFINE4(reboot, int, magic1, int, magic2, unsigned int, cmd,
        void __user *, arg)                                                 //对应LINUX_REBOOT_MAGIC1, LINUX_REBOOT_MAGIC2, LINUX_REBOOT_CMD_RESTART2, "recovery"
{
    struct pid_namespace *pid_ns = task_active_pid_ns(current);
    char buffer[256];
    int ret = 0;

    /* We only trust the superuser with rebooting the system. */
    if (!ns_capable(pid_ns->user_ns, 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;

    /*
     * If pid namespaces are enabled and the current task is in a child
     * pid_namespace, the command is handled by reboot_pid_ns() which will
     * call do_exit().
     */
    ret = reboot_pid_ns(pid_ns, cmd);
    if (ret)
        return ret;

    /* 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);              //程序会跑到这里
        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;
}

kernel_restart(buffer); 参数为("recovery")

kernel/kernel/sys.c
void kernel_restart(char *cmd)
{    
    kernel_restart_prepare(cmd);       
    migrate_to_reboot_cpu();
    syscore_shutdown();
    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);
}

machine_restart(cmd); 参数为("recovery")

kernel/arch/arm/kernel/process.c
void machine_restart(char *cmd)
{
    preempt_disable();
    smp_send_stop();

    /* Flush the console to make sure all the relevant messages make it
     * out to the console drivers */
    arm_machine_flush_console();

    arm_pm_restart(reboot_mode, cmd);

    /* Give a grace period for failure to restart of 1s */
    mdelay(1000);

    /* Whoops - the platform was unable to reboot. Tell the user! */
    printk("Reboot failed -- System halted\n");
    local_irq_disable();
    while (1);
}

arm_pm_restart(reboot_mode, cmd);

kernel/drivers/power/reset/msm-poweroff.c
static int msm_restart_probe(struct platform_device *pdev)
{
 .....
arm_pm_restart = do_msm_restart;
.....
}

kernel/drivers/power/reset/msm-poweroff.c
static void do_msm_restart(enum reboot_mode reboot_mode, const char *cmd)
{
    int ret;
    struct scm_desc desc = {
        .args[0] = 1,
        .args[1] = 0,
        .arginfo = SCM_ARGS(2),
    };

    pr_notice("Going down for restart now\n");

    msm_restart_prepare(cmd);   //程序走到这里

#ifdef CONFIG_MSM_DLOAD_MODE
    /*
     * Trigger a watchdog bite here and if this fails,
     * device will take the usual restart path.
     */

    if (WDOG_BITE_ON_PANIC && in_panic)
        msm_trigger_wdog_bite();
#endif

    /* Needed to bypass debug image on some chips */
    if (!is_scm_armv8())
        ret = scm_call_atomic2(SCM_SVC_BOOT,
                   SCM_WDOG_DEBUG_BOOT_PART, 1, 0);
    else
        ret = scm_call2_atomic(SCM_SIP_FNID(SCM_SVC_BOOT,
              SCM_WDOG_DEBUG_BOOT_PART), &desc);
    if (ret)
        pr_err("Failed to disable secure wdog debug: %d\n", ret);

    halt_spmi_pmic_arbiter();
    deassert_ps_hold();

    mdelay(10000);
}

msm_restart_prepare(cmd);  参数为("recovery")

kernel/drivers/power/reset/msm-poweroff.c
static void msm_restart_prepare(const char *cmd)
{
#ifdef CONFIG_MSM_SUBSYSTEM_RESTART
    extern char panic_subsystem[];
#endif /* CONFIG_MSM_SUBSYSTEM_RESTART */
    bool need_warm_reset = false;

#ifdef CONFIG_MSM_DLOAD_MODE

    /* Write download mode flags if we're panic'ing
     * Write download mode flags if restart_mode says so
     * Kill download mode if master-kill switch is set
     */

    set_dload_mode(download_mode &&
            (in_panic || restart_mode == RESTART_DLOAD));
#endif

    if (qpnp_pon_check_hard_reset_stored()) {
        /* Set warm reset as true when device is in dload mode
         *  or device doesn't boot up into recovery, bootloader or rtc.
         */
        if (get_dload_mode() ||
            ((cmd != NULL && cmd[0] != '\0') &&
            strcmp(cmd, "recovery") &&
            strcmp(cmd, "bootloader") &&
            strcmp(cmd, "rtc")))
            need_warm_reset = true;
    } else {
        need_warm_reset = (get_dload_mode() ||
                (cmd != NULL && cmd[0] != '\0'));
    }

    /* Hard reset the PMIC unless memory contents must be maintained. */
    if (need_warm_reset || restart_mode == RESTART_DLOAD) {// MODIFIED by xin.peng, 2016-03-21, BUG-1761259
        qpnp_pon_system_pwr_off(PON_POWER_OFF_WARM_RESET);
    } else {
        qpnp_pon_system_pwr_off(PON_POWER_OFF_HARD_RESET);
    }

    if (cmd != NULL) {
        if (!strncmp(cmd, "bootloader", 10)) {
            qpnp_pon_set_restart_reason(
                PON_RESTART_REASON_BOOTLOADER);
            __raw_writel(0x77665500, restart_reason);
        } else if (!strncmp(cmd, "recovery", 8)) {
            qpnp_pon_set_restart_reason(
                PON_RESTART_REASON_RECOVERY);
            __raw_writel(0x77665502, restart_reason);          //********************

　　else if (!strcmp(cmd, "rtc")) {
            qpnp_pon_set_restart_reason(
                PON_RESTART_REASON_RTC);
            __raw_writel(0x77665503, restart_reason);
        } else if (!strncmp(cmd, "oem-", 4)) {
            unsigned long code;
            int ret;
            ret = kstrtoul(cmd + 4, 16, &code);
            if (!ret)
                __raw_writel(0x6f656d00 | (code & 0xff),
                         restart_reason);
                if (download_mode && (code & 0xff) == 0x3a)
                    set_dload_mode(1);
        } else if (!strncmp(cmd, "edl", 3)) {
            enable_emergency_dload_mode();
        } else {
            __raw_writel(0x77665501, restart_reason);
        }
    }

#ifdef CONFIG_MSM_SUBSYSTEM_RESTART
    if (in_panic) {
        printk(KERN_ERR "subsystem %s crash\n", panic_subsystem);
        if (!memcmp(panic_subsystem, "modem", 5)) {
            __raw_writel(0x6f656dc1, restart_reason);
        } else if (!memcmp(panic_subsystem, "wcnss", 5)) {
            __raw_writel(0x6f656dc2, restart_reason);
        } else if (!memcmp(panic_subsystem, "adsp", 4) || !memcmp(panic_subsystem, "ADSP", 4)) {
            __raw_writel(0x6f656dc3, restart_reason);
        } else if (!memcmp(panic_subsystem, "venus", 5)) {
            __raw_writel(0x6f656dc4, restart_reason);
        } else {
            __raw_writel(0x6f656dc0, restart_reason);
        }
        if (download_mode)
            set_dload_mode(1);
    }
#endif /* CONFIG_MSM_SUBSYSTEM_RESTART */

//add begin by liping.gao,Powering on mode switch to 9008 mode,task:665748
    if (restart_mode == RESTART_DLOAD) {
        enable_emergency_dload_mode();
    }
//add end by liping.gao
    flush_cache_all();

    /*outer_flush_all is not supported by 64bit kernel*/
#ifndef CONFIG_ARM64
    outer_flush_all();
#endif

}

__raw_writel(0x77665502, restart_reason);          //在地址上0x77665502写入"recovery"

应用执行升级/清缓存/重置等操作导致底层在地址0x77665502写入"recovery"

当开机以后，在lk阶段会判断这个地址的值，如果是recovery，会设置boot_into_recovery=1,然后读取recovery.img镜像，把recovery.img的地址和ramdisk等信息作为参数启动kernel，从而进入recovery模式，下面进行简单的分析。

lk阶段：

bootable/bootloader/lk/app/aboot/aboot.c
......
#define RECOVERY_MODE        0x77665502
......
reboot_mode = check_reboot_mode();
hard_reboot_mode = check_hard_reboot_mode();
if (reboot_mode == RECOVERY_MODE ||
        hard_reboot_mode == RECOVERY_HARD_RESET_MODE) {
        boot_into_recovery = 1;
    }
.....