你可以通过访问 Linux内核网站 来得到源代码,下面是文件的路径:

• linux_soruce/kernel/power/main.c
• linux_source/kernel/arch/xxx/mach-xxx/pm.c
• linux_source/driver/base/power/main.c

接下来让我们详细的看一下Linux是怎么休眠/唤醒的. Let 's going to see how these happens.

用户对于/sys/power/state 的读写会调用到 main.c中的state_store(), 用户可以写入 const char * const pm_state[] 中定义的字符串, 比如"mem", "standby".

然后state_store()会调用enter_state(), 它首先会检查一些状态参数,然后同步文件系统. 下面是代码:

/** *      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). */static 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.");

        pr_debug("PM: Preparing system for %s sleep", pm_states[state]);        error = suspend_prepare();        if (error)                goto Unlock;

        if (suspend_test(TEST_FREEZER))                goto Finish;

        pr_debug("PM: Entering %s sleep", pm_states[state]);        error = suspend_devices_and_enter(state);

 Finish:        pr_debug("PM: Finishing wakeup.");        suspend_finish(); Unlock:        mutex_unlock(&pm_mutex);        return error;}

当进入到suspend_prepare()中以后, 它会给suspend分配一个虚拟终端来输出信 息, 然后广播一个系统要进入suspend的Notify, 关闭掉用户态的helper进程, 然后一次调用suspend_freeze_processes()冻结所有的进程, 这里会保存所有进程 当前的状态, 也许有一些进程会拒绝进入冻结状态, 当有这样的进程存在的时候, 会导致冻结失败,此函数就会放弃冻结进程,并且解冻刚才冻结的所有进程.

/** *      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;        unsigned int free_pages;

        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;

        if (suspend_freeze_processes()) {                error = -EAGAIN;                goto Thaw;        }

        free_pages = global_page_state(NR_FREE_PAGES);        if (free_pages < FREE_PAGE_NUMBER) {                pr_debug("PM: free some memory");                shrink_all_memory(FREE_PAGE_NUMBER - free_pages);                if (nr_free_pages() < FREE_PAGE_NUMBER) {                        error = -ENOMEM;                        printk(KERN_ERR "PM: No enough memory");                }        }        if (!error)                return 0;

 Thaw:        suspend_thaw_processes();        usermodehelper_enable(); Finish:        pm_notifier_call_chain(PM_POST_SUSPEND);        pm_restore_console();        return error;}

现在, 所有的进程(也包括workqueue/kthread) 都已经停止了, 内核态人物有 可能在停止的时候握有一些信号量, 所以如果这时候在外设里面去解锁这个信号 量有可能会发生死锁, 所以在外设的suspend()函数里面作lock/unlock锁要非常小心,这里建议设计的时候就不要在suspend()里面等待锁. 而且因为suspend的时候,有一些Log是无法输出的,所以一旦出现问题,非常难调试.

然后kernel在这里会尝试释放一些内存.

最后会调用suspend_devices_and_enter()来把所有的外 设休眠, 在这个函数中, 如果平台注册了suspend_pos(通常是在板级定义中定义和注册), 这里就会调用 suspend_ops->begin(), 然后driver/base/power/main.c 中的 device_suspend()->dpm_suspend() 会被调用,他们会依次调用驱动的suspend() 回调来休眠掉所有的设备.

当所有的设备休眠以后, suspend_ops->prepare()会被调用, 这个函数通常会作 一些准备工作来让板机进入休眠. 接下来Linux,在多核的CPU中的非启动CPU会被关掉, 通过注释看到是避免这些其他的CPU造成race condion,接下来的以后只有一个CPU在运行了.

suspend_ops 是板级的电源管理操作, 通常注册在文件 arch/xxx/mach-xxx/pm.c 中.

接下来, suspend_enter()会被调用, 这个函数会关闭arch irq, 调用 device_power_down(), 它会调用suspend_late()函数, 这个函数是系统真正进入 休眠最后调用的函数, 通常会在这个函数中作最后的检查. 如果检查没问题, 接 下来休眠所有的系统设备和总线, 并且调用 suspend_pos->enter() 来使CPU进入 省电状态. 这时候,就已经休眠了.代码的执行也就停在这里了.

/** *      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, ftrace_save;

        if (!suspend_ops)                return -ENOSYS;

        if (suspend_ops->begin) {                error = suspend_ops->begin(state);                if (error)                        goto Close;        }        suspend_console();        ftrace_save = __ftrace_enabled_save();        suspend_test_start();        error = device_suspend(PMSG_SUSPEND);        if (error) {                printk(KERN_ERR "PM: Some devices failed to suspend");                goto Recover_platform;        }        suspend_test_finish("suspend devices");        if (suspend_test(TEST_DEVICES))                goto Recover_platform;

        if (suspend_ops->prepare) {                error = suspend_ops->prepare();                if (error)                        goto Resume_devices;        }

        if (suspend_test(TEST_PLATFORM))                goto Finish;

        error = disable_nonboot_cpus();        if (!error && !suspend_test(TEST_CPUS))                suspend_enter(state);

        enable_nonboot_cpus(); Finish:        if (suspend_ops->finish)                suspend_ops->finish(); Resume_devices:        suspend_test_start();        device_resume(PMSG_RESUME);        suspend_test_finish("resume devices");        __ftrace_enabled_restore(ftrace_save);        resume_console(); Close:        if (suspend_ops->end)                suspend_ops->end();        return error;

 Recover_platform:        if (suspend_ops->recover)                suspend_ops->recover();        goto Resume_devices;}

如果在休眠中系统被中断或者其他事件唤醒, 接下来的代码就会开始执行, 这个唤醒的顺序是和休眠的循序相反的,所以系统设备和总线会首先唤醒,使能系统中 断, 使能休眠时候停止掉的非启动CPU, 以及调用suspend_ops->finish(), 而且在suspend_devices_and_enter()函数中也会继续唤醒每个设备,使能虚拟终端, 最后调用 suspend_ops->end().

在返回到enter_state()函数中的, 当 suspend_devices_and_enter() 返回以后, 外设已经唤醒了, 但是进程和任务都还是冻结状态, 这里会调用suspend_finish()来解冻这些进程和任务, 而且发出Notify来表示系统已经从suspend状态退出, 唤醒终端.

到这里, 所有的休眠和唤醒就已经完毕了, 系统继续运行了.