Linux中休眠/喚醒是如何工作的？
休眠/喚醒在嵌入式Linux中是非常重要的部分,嵌入式設(shè)備盡可能的進(jìn)入休眠狀 態(tài)來(lái)延長(zhǎng)電池的續(xù)航時(shí)間.這篇文章就詳細(xì)介紹一下Linux中休眠/喚醒是如何工作 的
我的linux內(nèi)核版本：3.0.31
對(duì)于休眠(suspend)的簡(jiǎn)單介紹
在Linux中,休眠主要分三個(gè)主要的步驟:
1、凍結(jié)用戶態(tài)進(jìn)程和內(nèi)核態(tài)任務(wù)
2、調(diào)用注冊(cè)的設(shè)備的suspend的回調(diào)函數(shù)
3、順序是按照注冊(cè)順序
休眠核心設(shè)備和使CPU進(jìn)入休眠態(tài)凍結(jié)進(jìn)程是內(nèi)核把進(jìn)程列表中所有的進(jìn)程的狀態(tài)都設(shè)置為停止,并且保存下所有進(jìn)程的上下文. 當(dāng)這些進(jìn)程被解凍的時(shí)候,他們是不知道自己被凍結(jié)過(guò)的,只是簡(jiǎn)單的繼續(xù)執(zhí)行.如何讓Linux進(jìn)入休眠呢?用戶可以通過(guò)讀寫sys文件/sys /power/state 是實(shí)現(xiàn)控制系統(tǒng)進(jìn)入休眠. 比如
- # echo mem > /sys/power/state
命令系統(tǒng)進(jìn)入休眠. 也可以使用
- # cat /sys/power/state
來(lái)得到內(nèi)核支持哪幾種休眠方式.
Linux Suspend 的流程
相關(guān)的文件:
你可以通過(guò)訪問(wèn)Linux內(nèi)核網(wǎng)站來(lái)得到源代碼,下面是文件的路徑:
kernel/kernel/power/main.c
kernel/kernel/power/suspend.c
kernel/driver/base/power/main.c
接下來(lái)讓我們?cè)敿?xì)的看一下Linux是怎么休眠/喚醒的. Let ‘s going to see how these happens.
用戶對(duì)于/sys/power/state 的讀寫會(huì)調(diào)用到 main.c中的state_store(), 用戶可以寫入 const char * const pm_state[] 中定義的字符串, 比如”mem”, “standby”.當(dāng)然一般是由suspend和resume的按鍵控制的
然后state_store()會(huì)調(diào)用enter_state(), 它首先會(huì)檢查一些狀態(tài)參數(shù),然后同步文件系統(tǒng). 下面是代碼:
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- /**
- * enter_state - Do common work of entering low-power state.
- * @state: pm_state structure for state we're entering.
- *
- * Make sure we're the only ones trying to enter a sleep state. Fail
- * if someone has beat us to it, since we don't want anything weird to
- * happen when we wake up.
- * Then, do the setup for suspend, enter the state, and cleaup (after
- * we've woken up).
- */
- int enter_state(suspend_state_t state)
- {
- int error;
- if (!valid_state(state))
- return -ENODEV;
- if (!mutex_trylock(&pm_mutex))
- return -EBUSY;
- printk(KERN_INFO "PM: Syncing filesystems ... ");
- sys_sync();
- printk("done.\n");
- pr_debug("PM: Preparing system for %s sleep\n", pm_states[state]);
- error = suspend_prepare();
- if (error)
- goto Unlock;
- if (suspend_test(TEST_FREEZER))
- goto Finish;
- pr_debug("PM: Entering %s sleep\n", pm_states[state]);
- pm_restrict_gfp_mask();
- error = suspend_devices_and_enter(state);
- pm_restore_gfp_mask();
- Finish:
- pr_debug("PM: Finishing wakeup.\n");
- suspend_finish();
- Unlock:
- mutex_unlock(&pm_mutex);
- return error;
- }
準(zhǔn)備, 凍結(jié)進(jìn)程
當(dāng)進(jìn)入到suspend_prepare()中以后, 它會(huì)給suspend分配一個(gè)虛擬終端來(lái)輸出信 息, 然后廣播一個(gè)系統(tǒng)要進(jìn)入suspend的Notify, 關(guān)閉掉用戶態(tài)的helper進(jìn)程, 然后依次調(diào)用suspend_freeze_processes()凍結(jié)所有的進(jìn)程, 這里會(huì)保存所有進(jìn)程當(dāng)前的狀態(tài), 也許有一些進(jìn)程會(huì)拒絕進(jìn)入凍結(jié)狀態(tài), 當(dāng)有這樣的進(jìn)程存在的時(shí)候, 會(huì)導(dǎo)致凍結(jié)失敗,此函數(shù)就會(huì)放棄凍結(jié)進(jìn)程,并且解凍剛才凍結(jié)的所有進(jìn)程.
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- /**
- * suspend_prepare - Do prep work before entering low-power state.
- *
- * This is common code that is called for each state that we're entering.
- * Run suspend notifiers, allocate a console and stop all processes.
- */
- static int suspend_prepare(void)
- {
- int error;
- if (!suspend_ops || !suspend_ops->enter)
- return -EPERM;
- pm_prepare_console();
- error = pm_notifier_call_chain(PM_SUSPEND_PREPARE);
- if (error)
- goto Finish;
- error = usermodehelper_disable();
- if (error)
- goto Finish;
- error = suspend_freeze_processes();
- if (!error)
- return 0;
- suspend_thaw_processes();
- usermodehelper_enable();
- Finish:
- pm_notifier_call_chain(PM_POST_SUSPEND);
- pm_restore_console();
- return error;
- }
讓外設(shè)進(jìn)入休眠
現(xiàn)在, 所有的進(jìn)程(也包括workqueue/kthread) 都已經(jīng)停止了, 內(nèi)核態(tài)人物有 可能在停止的時(shí)候握有一些信號(hào)量, 所以如果這時(shí)候在外設(shè)里面去解鎖這個(gè)信號(hào) 量有可能會(huì)發(fā)生死鎖, 所以在外設(shè)的suspend()函數(shù)里面作lock/unlock鎖要非常 小心,這里建議設(shè)計(jì)的時(shí)候就不要在suspend()里面等待鎖. 而且因?yàn)閟uspend的時(shí)候,有一些Log是無(wú)法輸出的,所以一旦出現(xiàn)問(wèn)題,非常難調(diào)試.
然后kernel在這里會(huì)嘗試釋放一些內(nèi)存.
最后會(huì)調(diào)用suspend_devices_and_enter()來(lái)把所有的外設(shè)休眠, 在這個(gè)函數(shù)中, 如果平臺(tái)注冊(cè)了suspend_pos(通常是在板級(jí)定義中定義和注冊(cè)), 這里就會(huì)調(diào)用 suspend_ops->begin(), 然后driver/base/power/main.c 中的 device_suspend()->dpm_suspend() 會(huì)被調(diào)用,他們會(huì)依次調(diào)用驅(qū)動(dòng)的suspend() 回調(diào)來(lái)休眠掉所有的設(shè)備.
當(dāng)所有的設(shè)備休眠以后, suspend_ops->prepare()會(huì)被調(diào)用, 這個(gè)函數(shù)通常會(huì)作 一些準(zhǔn)備工作來(lái)讓板機(jī)進(jìn)入休眠. 接下來(lái)Linux,在多核的CPU中的非啟動(dòng)CPU會(huì)被關(guān)掉, 通過(guò)注釋看到是避免這些其他的CPU造成race condion,接下來(lái)的以后只有一個(gè)CPU在運(yùn)行了.
suspend_ops 是板級(jí)的電源管理操作, 通常注冊(cè)在文件 arch/xxx/mach-xxx/pm.c 中.
接下來(lái), suspend_enter()會(huì)被調(diào)用, 這個(gè)函數(shù)會(huì)關(guān)閉arch irq, 調(diào)用 device_power_down(), 它會(huì)調(diào)用suspend_late()函數(shù), 這個(gè)函數(shù)是系統(tǒng)真正進(jìn)入 休眠最后調(diào)用的函數(shù), 通常會(huì)在這個(gè)函數(shù)中作最后的檢查. 如果檢查沒問(wèn)題, 接 下來(lái)休眠所有的系統(tǒng)設(shè)備和總線, 并且調(diào)用 suspend_pos->enter() 來(lái)使CPU進(jìn)入 省電狀態(tài). 這時(shí)候,就已經(jīng)休眠了.代碼的執(zhí)行也就停在這里了.
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- /**
- * suspend_devices_and_enter - suspend devices and enter the desired system
- * sleep state.
- * @state: state to enter
- */
- int suspend_devices_and_enter(suspend_state_t state)
- {
- int error;
- if (!suspend_ops)
- return -ENOSYS;
- trace_machine_suspend(state);
- if (suspend_ops->begin) {
- error = suspend_ops->begin(state);
- if (error)
- goto Close;
- }
- suspend_console();
- suspend_test_start();
- error = dpm_suspend_start(PMSG_SUSPEND);
- if (error) {
- printk(KERN_ERR "PM: Some devices failed to suspend\n");
- goto Recover_platform;
- }
- suspend_test_finish("suspend devices");
- if (suspend_test(TEST_DEVICES))
- goto Recover_platform;
- error = suspend_enter(state);
- Resume_devices:
- suspend_test_start();
- dpm_resume_end(PMSG_RESUME);
- suspend_test_finish("resume devices");
- resume_console();
- Close:
- if (suspend_ops->end)
- suspend_ops->end();
- trace_machine_suspend(PWR_EVENT_EXIT);
- return error;
- Recover_platform:
- if (suspend_ops->recover)
- suspend_ops->recover();
- goto Resume_devices;
- }
RESUME
如果在休眠中系統(tǒng)被中斷或者其他事件喚醒, 接下來(lái)的代碼就會(huì)開始執(zhí)行, 這個(gè) 喚醒的順序是和休眠的循序相反的,所以系統(tǒng)設(shè)備和總線會(huì)首先喚醒,使能系統(tǒng)中 斷, 使能休眠時(shí)候停止掉的非啟動(dòng)CPU, 以及調(diào)用suspend_ops->finish(), 而且 在suspend_devices_and_enter()函數(shù)中也會(huì)繼續(xù)喚醒每個(gè)設(shè)備,使能虛擬終端, 最后調(diào)用 suspend_ops->end().
在返回到enter_state()函數(shù)中的, 當(dāng) suspend_devices_and_enter() 返回以后, 外設(shè)已經(jīng)喚醒了, 但是進(jìn)程和任務(wù)都還是凍結(jié)狀態(tài), 這里會(huì)調(diào)用suspend_finish()來(lái)解凍這些進(jìn)程和任務(wù), 而且發(fā)出Notify來(lái)表示系統(tǒng)已經(jīng)從suspend狀態(tài)退出, 喚醒終端.
到這里, 所有的休眠和喚醒就已經(jīng)完畢了, 系統(tǒng)繼續(xù)運(yùn)行了.
