一：Busyobx層的分析

這段時間,在忙到一個項目時,需要在busybox中用到reboot命令,開始在busybox中的shell中輸入reboot命令,始終如下的信息,然后就停止在那里了,無法重啟...為了徹底的弄明白這個問題,我在網絡上找了很久,終于有個人寫的一個reboot流程分析,我就借花獻佛.在這里重新分析下busybox是如何運行這個命令,同時又是如何調用到Linux內核中的mach_reset中的arch_reset，當針對不同的ARM芯片時，作為Linux內核開發和驅動開發的朋友，對于這個流程還是一定要了解的。要不，出現問題，又如何找出問題呢。忘記了reboot的打印信息了，如下：

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The?system?is?going?down?NOW?!!??

Sending?SIGTERM?to?all?processes.??

Sending?SIGKILL?to?all?processes.??

Please?stand?by?while?rebooting?the?system.??

Restarting?system.??

.??

通過分析busybox1.20.0的代碼可以看出在init.c中有這樣一行的代碼，如下：

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int?init_main(int?argc,?char?**argv)?MAIN_EXTERNALLY_VISIBLE;??

int?init_main(int?argc?UNUSED_PARAM,?char?**argv)??

{??

static?const?int?magic[]?=?{??

RB_HALT_SYSTEM,??

RB_POWER_OFF,??

RB_AUTOBOOT??

};??

static?const?smallint?signals[]?=?{?SIGUSR1,?SIGUSR2,?SIGTERM?};??

......??

/*?struct?sysinfo?is?linux-specific?*/??

#ifdef?__linux__??

/*?Make?sure?there?is?enough?memory?to?do?something?useful.?*/??

if?(ENABLE_SWAPONOFF)?{?//是否配置了swapoff命令??

struct?sysinfo?info;??

if?(sysinfo(&info)?==?0??

&&?(info.mem_unit???info.mem_unit?:?1)?*?(long?long)info.totalram?

)?{??

message(L_CONSOLE,?"Low?memory,?forcing?swapon");??

/*?swapon?-a?requires?/proc?typically?*/??

new_init_action(SYSINIT,?"mount?-t?proc?proc?/proc",?"");??

/*?Try?to?turn?on?swap?*/??

new_init_action(SYSINIT,?"swapon?-a",?"");??

run_actions(SYSINIT);???/*?wait?and?removing?*/??

}??

}??

#endif??

......??

/*?Make?the?command?line?just?say?"init"??-?thats?all,?nothing?else?*/??

strncpy(argv[0],?"init",?strlen(argv[0]));??

/*?Wipe?argv[1]-argv[N]?so?they?don't?clutter?the?ps?listing?*/??

while?(*++argv)??

memset(*argv,?0,?strlen(*argv));??

/*?Set?up?signal?handlers?*/??

/*?Set?up?signal?handlers?*/??

if?(!DEBUG_INIT)?{??

struct?sigaction?sa;??

bb_signals(0??

+?(1?<

+?(1?<

+?(1?<

,?halt_reboot_pwoff);//看到這個halt_reboot_pwoff??

signal(SIGQUIT,?restart_handler);?/*?re-exec?another?init?*/?//看到這個restart_handler函數，這是我們需要分析的??

/*?Stop?handler?must?allow?only?SIGCONT?inside?itself?*/??

memset(&sa,?0,?sizeof(sa));??

sigfillset(&sa.sa_mask);??

sigdelset(&sa.sa_mask,?SIGCONT);??

sa.sa_handler?=?stop_handler;??

/*?NB:?sa_flags?doesn't?have?SA_RESTART.?

*?It?must?be?able?to?interrupt?wait().?

*/??

sigaction_set(SIGTSTP,?&sa);?/*?pause?*/??

/*?Does?not?work?as?intended,?at?least?in?2.6.20.?

*?SIGSTOP?is?simply?ignored?by?init:?

*/??

sigaction_set(SIGSTOP,?&sa);?/*?pause?*/??

/*?SIGINT?(Ctrl-Alt-Del)?must?interrupt?wait(),?

*?setting?handler?without?SA_RESTART?flag.?

*/??

bb_signals_recursive_norestart((1?<

}??

......??

}??

單獨拿出halt_reboot_pwoff和restart_handler這個函數來看看

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/*?The?SIGUSR[12]/SIGTERM?handler?*/??

static?void?halt_reboot_pwoff(int?sig)?NORETURN;??

static?void?halt_reboot_pwoff(int?sig)??

{??

const?char?*m;??

unsigned?rb;??

/*?We?may?call?run()?and?it?unmasks?signals,?

*?including?the?one?masked?inside?this?signal?handler.?

*?Testcase?which?would?start?multiple?reboot?scripts:?

*??while?true;?do?reboot;?done?

*?Preventing?it:?

*/??

reset_sighandlers_and_unblock_sigs();??

run_shutdown_and_kill_processes();??

m?=?"halt";??

rb?=?RB_HALT_SYSTEM;??

if?(sig?==?SIGTERM)?{??

m?=?"reboot";??

rb?=?RB_AUTOBOOT;??

}?else?if?(sig?==?SIGUSR2)?{??

m?=?"poweroff";??

rb?=?RB_POWER_OFF;??

}??

message(L_CONSOLE,?"Requesting?system?%s",?m);??

pause_and_low_level_reboot(rb);??

/*?not?reached?*/??

}??

restart_handler函數如下：

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/*?Handler?for?QUIT?-?exec?"restart"?action,?

*?else?(no?such?action?defined)?do?nothing?*/??

static?void?restart_handler(int?sig?UNUSED_PARAM)??

{??

struct?init_action?*a;??

for?(a?=?init_action_list;?a;?a?=?a->next)?{??

if?(!(a->action_type?&?RESTART))??

continue;??

/*?Starting?from?here,?we?won't?return.?

*?Thus?don't?need?to?worry?about?preserving?errno?

*?and?such.?

*/??

reset_sighandlers_and_unblock_sigs();??

run_shutdown_and_kill_processes();??

#ifdef?RB_ENABLE_CAD??

/*?Allow?Ctrl-Alt-Del?to?reboot?the?system.?

*?This?is?how?kernel?sets?it?up?for?init,?we?follow?suit.?

*/??

reboot(RB_ENABLE_CAD);?/*?misnomer?*/??

#endif??

if?(open_stdio_to_tty(a->terminal))?{??

dbg_message(L_CONSOLE,?"Trying?to?re-exec?%s",?a->command);??

/*?Theoretically?should?be?safe.?

*?But?in?practice,?kernel?bugs?may?leave?

*?unkillable?processes,?and?wait()?may?block?forever.?

*?Oh?well.?Hoping?"new"?init?won't?be?too?surprised?

*?by?having?children?it?didn't?create.?

*/??

//while?(wait(NULL)?>?0)??

//??continue;??

init_exec(a->command);??

}??

/*?Open?or?exec?failed?*/??

pause_and_low_level_reboot(RB_HALT_SYSTEM);??

/*?not?reached?*/??

}??

}??

通過分析，我們看到他們都會有調用這兩個函數：reset_sighandlers_and_unblock_sigs();以及?run_shutdown_and_kill_processes();，我們重點關注如下這個函數：

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static?void?run_shutdown_and_kill_processes(void)??

{??

/*?Run?everything?to?be?run?at?"shutdown".??This?is?done?_prior_?

*?to?killing?everything,?in?case?people?wish?to?use?scripts?to?

*?shut?things?down?gracefully...?*/??

run_actions(SHUTDOWN);??

message(L_CONSOLE?|?L_LOG,?"The?system?is?going?down?NOW!");??

/*?Send?signals?to?every?process?_except_?pid?1?*/??

kill(-1,?SIGTERM);??

message(L_CONSOLE?|?L_LOG,?"Sent?SIG%s?to?all?processes",?"TERM");??

sync();??

sleep(1);??

kill(-1,?SIGKILL);??

message(L_CONSOLE,?"Sent?SIG%s?to?all?processes",?"KILL");??

sync();??

/*sleep(1);?-?callers?take?care?about?making?a?pause?*/??

}??

嘿嘿，終于看到了上面的打印信息：The system is going down NOW !! 以及Sending SIGTERM to all processes. 同時在上面的halt_reboot_pwoff和restart_handler中都會調用這樣一個函數，如下：

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static?void?pause_and_low_level_reboot(unsigned?magic)?NORETURN;??

static?void?pause_and_low_level_reboot(unsigned?magic)??

{??

pid_t?pid;??

/*?Allow?time?for?last?message?to?reach?serial?console,?etc?*/??

sleep(1);??

/*?We?have?to?fork?here,?since?the?kernel?calls?do_exit(EXIT_SUCCESS)?

*?in?linux/kernel/sys.c,?which?can?cause?the?machine?to?panic?when?

*?the?init?process?exits...?*/??

pid?=?vfork();??

if?(pid?==?0)?{?/*?child?*/??

reboot(magic);??

_exit(EXIT_SUCCESS);??

}??

while?(1)??

sleep(1);??

}??

看到了嗎？有一個reboot(magic)函數，對于vfork函數，請參考fork函數。這里不多說了.... 我們現在來看看reboot.h文件，如下：

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/*?

*?Definitions?related?to?the?reboot()?system?call,?

*?shared?between?init.c?and?halt.c.?

*/??

#include???

#ifndef?RB_HALT_SYSTEM??

#?if?defined(__linux__)??

#??define?RB_HALT_SYSTEM??0xcdef0123??

#??define?RB_ENABLE_CAD???0x89abcdef??

#??define?RB_DISABLE_CAD??0??

#??define?RB_POWER_OFF????0x4321fedc??

#??define?RB_AUTOBOOT?????0x01234567??

#?elif?defined(RB_HALT)??

#??define?RB_HALT_SYSTEM??RB_HALT??

#?endif??

#endif??

/*?Stop?system?and?switch?power?off?if?possible.??*/??

#ifndef?RB_POWER_OFF??

#?if?defined(RB_POWERDOWN)??

#??define?RB_POWER_OFF??RB_POWERDOWN??

#?elif?defined(__linux__)??

#??define?RB_POWER_OFF??0x4321fedc??

#?else??

#??warning?"poweroff?unsupported,?using?halt?as?fallback"??

#??define?RB_POWER_OFF??RB_HALT_SYSTEM??

#?endif??

#endif??

而在linux的內核中的定義如下：

busybox和linux內核中的REBOOT的定義值是一樣的。看到了沒有了。這個很重要的哦，否則busybox是無法調用linux內核的reboot函數。

二：Linux內核層的分析

Linux內核是如何銜接busybox的reboot函數的呢，如下代碼：

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/*?

*?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;??

lock_kernel();??

switch?(cmd)?{??

case?LINUX_REBOOT_CMD_RESTART:??

kernel_restart(NULL);?//這個就是重新啟動Linx的命令??

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)?

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;??

}??

繼續跟蹤kernel_restart()函數，如下：

最終會調用一個machine_restart(cmd)函數，這個是跟具體的芯片有很大的關系的，我們進一步的分析如下：

看到了嗎，最終是調用arch_reset來復位整個系統的。同時我們也看到了S3C2440的reset的函數如下：

在arm_pm_restart = s3c24xx_pm_restart()函數，最終也是調用arm_machine_restart(mod, cmd)來實現的。而在arm_machine_restart()函數中，最終也是調用arch_reset()函數來實現，而這個函數是在哪里呢。在S3C2440沒有看到arch_reset函數的實現，因此從S3C2410中找到了如下的代碼，請繼續看下面的代碼：

終于看到了arch_reset函數，最終是采用S3C2410或者S3C2440的WatchDog來實現reboot的命令的。大家可以想想，busybox的poweroff命令，是如何實現通過Linux系統關閉整個系統的電源呢，其實很簡單，只需要實現下面的函數中的pm_power_off的回調函數即可。

我們可以通過一個GPIO來控制整個系統的電源，而通過上面的pm_power_off的回調函數來實現，只需要在pm_power_off函數對GPIO進行操作就可以了。你看不是很簡單嗎?

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