Android中Linux suspend/resume流程

Android中Linux suspend/resume流程
首先我们从linux kernel 的suspend说起，不管你是使用echo mem > /sys/power/state 或者使用你的开发板已经拥有的power key 都可以实现系统进入suspend的功能，这是suspend的基础，即控制系统使suspend得到执行的机会，这里相信大家都可以理解，不再过多说明。

那么suspend得到了执行的机会又是怎么一步一步开始往下执行的呢？现在就开始我们的系统的电源管理之旅：

我们就通过echo mem > /sys/power/state这种方式来看，这样更容易被理解，位于/sys/power下面的这个state，做driver不知道那可说不过去，我们就看看这个state是在哪个地方创建的吧

kernel/kernel/power/suspend.c

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static int __init pm_init(void) 
{ 
    int error = pm_start_workqueue(); 
    if (error) 
        return error; 
    hibernate_image_size_init(); 
    hibernate_reserved_size_init(); 
    power_kobj = kobject_create_and_add("power", NULL); 
    if (!power_kobj) 
        return -ENOMEM; 
    return sysfs_create_group(power_kobj, &attr_group); 
} 
 
core_initcall(pm_init);

static int __init pm_init(void)
{
 int error = pm_start_workqueue();
 if (error)
  return error;
 hibernate_image_size_init();
 hibernate_reserved_size_init();
 power_kobj = kobject_create_and_add("power", NULL);
 if (!power_kobj)
  return -ENOMEM;
 return sysfs_create_group(power_kobj, &attr_group);
}

core_initcall(pm_init);
这段代码很少却很重要，我关心的是他确实为我们在sys目录下先建了一个power目录，然后，return时创建了很多接口，其中一个就是state，以下是接口定义

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static struct attribute * g[] = { 
    &state_attr.attr, 
#ifdef CONFIG_PM_TRACE 
    &pm_trace_attr.attr, 
    &pm_trace_dev_match_attr.attr, 
#endif 
#ifdef CONFIG_PM_SLEEP 
    &pm_async_attr.attr, 
    &wakeup_count_attr.attr, 
#ifdef CONFIG_PM_DEBUG 
    &pm_test_attr.attr, 
#endif 
#ifdef CONFIG_USER_WAKELOCK 
    &wake_lock_attr.attr, 
    &wake_unlock_attr.attr, 
#endif 
#endif 
    NULL, 
}; 
 
static struct attribute_group attr_group = { 
    .attrs = g, 
};

static struct attribute * g[] = {
 &state_attr.attr,
#ifdef CONFIG_PM_TRACE
 &pm_trace_attr.attr,
 &pm_trace_dev_match_attr.attr,
#endif
#ifdef CONFIG_PM_SLEEP
 &pm_async_attr.attr,
 &wakeup_count_attr.attr,
#ifdef CONFIG_PM_DEBUG
 &pm_test_attr.attr,
#endif
#ifdef CONFIG_USER_WAKELOCK
 &wake_lock_attr.attr,
 &wake_unlock_attr.attr,
#endif
#endif
 NULL,
};

static struct attribute_group attr_group = {
 .attrs = g,
};上面你可以看到了这些接口了

我们在echo mem > /sys/power/state，或调用的我们的接口函数state_store，suspend也就才真正开始走出第一步

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static ssize_t state_store(struct kobject *kobj, struct kobj_attribute *attr, 
               const char *buf, size_t n) 
{ 
#ifdef CONFIG_SUSPEND 
#ifdef CONFIG_EARLYSUSPEND 
    suspend_state_t state = PM_SUSPEND_ON; 
#else 
    suspend_state_t state = PM_SUSPEND_STANDBY; 
#endif 
    const char * const *s; 
#endif 
    char *p; 
    int len; 
    int error = -EINVAL; 
 
    p = memchr(buf, '\n', n); 
    len = p ? p - buf : n; 
 
    /* First, check if we are requested to hibernate */ 
    if (len == 4 && !strncmp(buf, "disk", len)) { 
        error = hibernate(); 
                goto Exit; 
    } 
 
#ifdef CONFIG_SUSPEND 
    for (s = &pm_states[state]; state < PM_SUSPEND_MAX; s++, state++) { 
        if (*s && len == strlen(*s) && !strncmp(buf, *s, len)) 
            break; 
    } 
    if (state < PM_SUSPEND_MAX && *s) 
#ifdef CONFIG_EARLYSUSPEND 
        if (state == PM_SUSPEND_ON || valid_state(state)) { 
            error = 0; 
            request_suspend_state(state); 
        } 
#else 
        error = enter_state(state); 
#endif 
#endif 
 
 Exit: 
    return error ? error : n; 
}

static ssize_t state_store(struct kobject *kobj, struct kobj_attribute *attr,
      const char *buf, size_t n)
{
#ifdef CONFIG_SUSPEND
#ifdef CONFIG_EARLYSUSPEND
 suspend_state_t state = PM_SUSPEND_ON;
#else
 suspend_state_t state = PM_SUSPEND_STANDBY;
#endif
 const char * const *s;
#endif
 char *p;
 int len;
 int error = -EINVAL;

p = memchr(buf, '\n', n);
 len = p ? p - buf : n;

/* First, check if we are requested to hibernate */
 if (len == 4 && !strncmp(buf, "disk", len)) {
  error = hibernate();
                goto Exit;
 }

#ifdef CONFIG_SUSPEND
 for (s = &pm_states[state]; state < PM_SUSPEND_MAX; s++, state++) {
  if (*s && len == strlen(*s) && !strncmp(buf, *s, len))
   break;
 }
 if (state < PM_SUSPEND_MAX && *s)
#ifdef CONFIG_EARLYSUSPEND
  if (state == PM_SUSPEND_ON || valid_state(state)) {
   error = 0;
   request_suspend_state(state);
  }
#else
  error = enter_state(state);
#endif
#endif

Exit:
 return error ? error : n;
}这里我们echo mem > /sys/power/state， 还有一种echo on > /sys/power/state，接着state_store进入reauest_suspend_state(state)，然后如果是on的话进入late_resume_work（在执行late_resume_work之前会向系统申请main_wake_lock），如果是mem进入early_suspend_work。

reauest_suspend_state函数路径：kernel/kernel/power/earlysuspend.c

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void request_suspend_state(suspend_state_t new_state) 
{ 
    unsigned long irqflags; 
    int old_sleep; 
 
    spin_lock_irqsave(&state_lock, irqflags); 
    old_sleep = state & SUSPEND_REQUESTED; 
    if (debug_mask & DEBUG_USER_STATE) { 
        struct timespec ts; 
        struct rtc_time tm; 
        getnstimeofday(&ts); 
        rtc_time_to_tm(ts.tv_sec, &tm); 
        pr_info("request_suspend_state: %s (%d->%d) at %lld " 
            "(%d-%02d-%02d %02d:%02d:%02d.%09lu UTC)\n", 
            new_state != PM_SUSPEND_ON ? "sleep" : "wakeup", 
            requested_suspend_state, new_state, 
            ktime_to_ns(ktime_get()), 
            tm.tm_year + 1900, tm.tm_mon + 1, tm.tm_mday, 
            tm.tm_hour, tm.tm_min, tm.tm_sec, ts.tv_nsec); 
    } 
    if (!old_sleep && new_state != PM_SUSPEND_ON) { 
        state |= SUSPEND_REQUESTED; 
        queue_work(suspend_work_queue, &early_suspend_work); 
    } else if (old_sleep && new_state == PM_SUSPEND_ON) { 
        state &= ~SUSPEND_REQUESTED; 
        wake_lock(&main_wake_lock); 
        queue_work(suspend_work_queue, &late_resume_work); 
    } 
    requested_suspend_state = new_state; 
    spin_unlock_irqrestore(&state_lock, irqflags); 
}

void request_suspend_state(suspend_state_t new_state)
{
 unsigned long irqflags;
 int old_sleep;

spin_lock_irqsave(&state_lock, irqflags);
 old_sleep = state & SUSPEND_REQUESTED;
 if (debug_mask & DEBUG_USER_STATE) {
  struct timespec ts;
  struct rtc_time tm;
  getnstimeofday(&ts);
  rtc_time_to_tm(ts.tv_sec, &tm);
  pr_info("request_suspend_state: %s (%d->%d) at %lld "
   "(%d-%02d-%02d %02d:%02d:%02d.%09lu UTC)\n",
   new_state != PM_SUSPEND_ON ? "sleep" : "wakeup",
   requested_suspend_state, new_state,
   ktime_to_ns(ktime_get()),
   tm.tm_year + 1900, tm.tm_mon + 1, tm.tm_mday,
   tm.tm_hour, tm.tm_min, tm.tm_sec, ts.tv_nsec);
 }
 if (!old_sleep && new_state != PM_SUSPEND_ON) {
  state |= SUSPEND_REQUESTED;
  queue_work(suspend_work_queue, &early_suspend_work);
 } else if (old_sleep && new_state == PM_SUSPEND_ON) {
  state &= ~SUSPEND_REQUESTED;
  wake_lock(&main_wake_lock);
  queue_work(suspend_work_queue, &late_resume_work);
 }
 requested_suspend_state = new_state;
 spin_unlock_irqrestore(&state_lock, irqflags);
}
这里做的最重要的是就在最下面那两个分支中，决定了我们执行early_suspend_work，还是late_resume_work。这里我们走early_suspend_work这个分支接着往下看。先看看early_suspend_work怎么被调用

queue_work(suspend_work_queue, &early_suspend_work);

这是一个工作队列的调用方法，找到early_suspend_work的定义

static DECLARE_WORK(early_suspend_work, early_suspend);

这里有关于工作队列的方法，不知道就要自己去看看了，所以这里最终调用的其实是early_suspend这个方法

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static void early_suspend(struct work_struct *work) 
{ 
    struct early_suspend *pos; 
    unsigned long irqflags; 
    int abort = 0; 
 
    mutex_lock(&early_suspend_lock); 
    spin_lock_irqsave(&state_lock, irqflags); 
    if (state == SUSPEND_REQUESTED) 
        state |= SUSPENDED; 
    else 
        abort = 1; 
    spin_unlock_irqrestore(&state_lock, irqflags); 
 
    if (abort) { 
        if (debug_mask & DEBUG_SUSPEND) 
            pr_info("early_suspend: abort, state %d\n", state); 
        mutex_unlock(&early_suspend_lock); 
        goto abort; 
    } 
 
    if (debug_mask & DEBUG_SUSPEND) 
        pr_info("early_suspend: call handlers\n"); 
    list_for_each_entry(pos, &early_suspend_handlers, link) { 
        if (pos->suspend != NULL) { 
            if (debug_mask & DEBUG_VERBOSE) 
                pr_info("early_suspend: calling %pf\n", pos->suspend); 
            pos->suspend(pos); 
        } 
    } 
    mutex_unlock(&early_suspend_lock); 
 
    if (debug_mask & DEBUG_SUSPEND) 
        pr_info("early_suspend: sync\n"); 
 
    sys_sync(); 
abort: 
    spin_lock_irqsave(&state_lock, irqflags); 
    if (state == SUSPEND_REQUESTED_AND_SUSPENDED) 
        wake_unlock(&main_wake_lock); 
    spin_unlock_irqrestore(&state_lock, irqflags); 
}

static void early_suspend(struct work_struct *work)
{
 struct early_suspend *pos;
 unsigned long irqflags;
 int abort = 0;

mutex_lock(&early_suspend_lock);
 spin_lock_irqsave(&state_lock, irqflags);
 if (state == SUSPEND_REQUESTED)
  state |= SUSPENDED;
 else
  abort = 1;
 spin_unlock_irqrestore(&state_lock, irqflags);

if (abort) {
  if (debug_mask & DEBUG_SUSPEND)
   pr_info("early_suspend: abort, state %d\n", state);
  mutex_unlock(&early_suspend_lock);
  goto abort;
 }

if (debug_mask & DEBUG_SUSPEND)
  pr_info("early_suspend: call handlers\n");
 list_for_each_entry(pos, &early_suspend_handlers, link) {
  if (pos->suspend != NULL) {
   if (debug_mask & DEBUG_VERBOSE)
    pr_info("early_suspend: calling %pf\n", pos->suspend);
   pos->suspend(pos);
  }
 }
 mutex_unlock(&early_suspend_lock);

if (debug_mask & DEBUG_SUSPEND)
  pr_info("early_suspend: sync\n");

sys_sync();
abort:
 spin_lock_irqsave(&state_lock, irqflags);
 if (state == SUSPEND_REQUESTED_AND_SUSPENDED)
  wake_unlock(&main_wake_lock);
 spin_unlock_irqrestore(&state_lock, irqflags);
}

early_suspend()这个函数里会遍历early_suspend_handlers，依次执行里面的early_suspend函数,执行完所有的early_suspend后，释放main_wake_lock，进入wake_unlock函数。

wake_unlock(&main_wake_lock);

这里还是说一下吧，这个main_wake_lock是个什么东西，路径：kernel/kernel/power/wakelock.c

struct wake_lock main_wake_lock;

看他的初始化

wake_lock_init(&main_wake_lock, WAKE_LOCK_SUSPEND, "main");

wake_lock(&main_wake_lock);

首先初始化，然后lock，等待unlock

对于一个lock进入wake_unlock，首先会将lock从原链表中删除(active_wake_locks)，然后加入inactive_locks链表中。

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void wake_unlock(struct wake_lock *lock) 
{ 
    int type; 
    unsigned long irqflags; 
    spin_lock_irqsave(&list_lock, irqflags); 
    type = lock->flags & WAKE_LOCK_TYPE_MASK; 
#ifdef CONFIG_WAKELOCK_STAT 
    wake_unlock_stat_locked(lock, 0); 
#endif 
    if (debug_mask & DEBUG_WAKE_LOCK) 
        pr_info("wake_unlock: %s\n", lock->name); 
    lock->flags &= ~(WAKE_LOCK_ACTIVE | WAKE_LOCK_AUTO_EXPIRE); 
    list_del(&lock->link); 
    list_add(&lock->link, &inactive_locks); 
    if (type == WAKE_LOCK_SUSPEND) { 
        long has_lock = has_wake_lock_locked(type); 
        if (has_lock > 0) { 
            if (debug_mask & DEBUG_EXPIRE) 
                pr_info("wake_unlock: %s, start expire timer, " 
                    "%ld\n", lock->name, has_lock); 
            mod_timer(&expire_timer, jiffies + has_lock); 
        } else { 
            if (del_timer(&expire_timer)) 
                if (debug_mask & DEBUG_EXPIRE) 
                    pr_info("wake_unlock: %s, stop expire " 
                        "timer\n", lock->name); 
            if (has_lock == 0) 
                queue_work(suspend_work_queue, &suspend_work); 
        } 
        if (lock == &main_wake_lock) { 
            if (debug_mask & DEBUG_SUSPEND) 
                print_active_locks(WAKE_LOCK_SUSPEND); 
#ifdef CONFIG_WAKELOCK_STAT 
            update_sleep_wait_stats_locked(0); 
#endif 
        } 
    } 
    spin_unlock_irqrestore(&list_lock, irqflags); 
}

void wake_unlock(struct wake_lock *lock)
{
 int type;
 unsigned long irqflags;
 spin_lock_irqsave(&list_lock, irqflags);
 type = lock->flags & WAKE_LOCK_TYPE_MASK;
#ifdef CONFIG_WAKELOCK_STAT
 wake_unlock_stat_locked(lock, 0);
#endif
 if (debug_mask & DEBUG_WAKE_LOCK)
  pr_info("wake_unlock: %s\n", lock->name);
 lock->flags &= ~(WAKE_LOCK_ACTIVE | WAKE_LOCK_AUTO_EXPIRE);
 list_del(&lock->link);
 list_add(&lock->link, &inactive_locks);
 if (type == WAKE_LOCK_SUSPEND) {
  long has_lock = has_wake_lock_locked(type);
  if (has_lock > 0) {
   if (debug_mask & DEBUG_EXPIRE)
    pr_info("wake_unlock: %s, start expire timer, "
     "%ld\n", lock->name, has_lock);
   mod_timer(&expire_timer, jiffies + has_lock);
  } else {
   if (del_timer(&expire_timer))
    if (debug_mask & DEBUG_EXPIRE)
     pr_info("wake_unlock: %s, stop expire "
      "timer\n", lock->name);
   if (has_lock == 0)
    queue_work(suspend_work_queue, &suspend_work);
  }
  if (lock == &main_wake_lock) {
   if (debug_mask & DEBUG_SUSPEND)
    print_active_locks(WAKE_LOCK_SUSPEND);
#ifdef CONFIG_WAKELOCK_STAT
   update_sleep_wait_stats_locked(0);
#endif
  }
 }
 spin_unlock_irqrestore(&list_lock, irqflags);
}
对于释放锁，上面两个过程就结束了，但是如果这个锁的类型是WAKE_LOCK_SUSPEND，那么还需要执行一些操作，判断是否可以进入睡眠。首先调has_wake_lock_locked(type)去查找是否还有这种类型的锁,会遍历active_wake_locks[type]链表，如果在这个链表中一检测中有锁，而且该锁不是超时锁，那么就返回-1。如果是超时锁，且已经超时了，那就去释放这个锁，如果没超时就得到一个max_timeout，然后返回max_timeout。接着就会回到wake_unlock函数中,调用mod_timer(&expire_timer,jiffies +has_lock);has_lock就是前面返回的max_timeout，这句话的意思就是向系统中再添加定时器,定时时间就是最大的超时时间.expire_timer的操作函数是expire_wake_locks,这里会去检测还有没有锁，没有的话就进入suspend_work，执行suspend，进入睡眠流程。上面wake_unlock中如果没有检测到锁，也会执行suspend。在suspend函数中又会通过has_wake_lock去检测有没有锁，有锁就直接返回。

queue_work(suspend_work_queue, &suspend_work);

又是一个工作队列，看看他的定义，找到他的处理过程

static DECLARE_WORK(suspend_work, suspend);

所以他真正执行的是suspend这个方法

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static void suspend(struct work_struct *work) 
{ 
    int ret; 
    int entry_event_num; 
    struct timespec ts_entry, ts_exit; 
 
    if (has_wake_lock(WAKE_LOCK_SUSPEND)) { 
        if (debug_mask & DEBUG_SUSPEND) 
            pr_info("suspend: abort suspend\n"); 
        return; 
    } 
 
    entry_event_num = current_event_num; 
    sys_sync(); 
    if (debug_mask & DEBUG_SUSPEND) 
        pr_info("suspend: enter suspend\n"); 
    getnstimeofday(&ts_entry); 
    ret = pm_suspend(requested_suspend_state); 
    getnstimeofday(&ts_exit); 
 
    if (debug_mask & DEBUG_EXIT_SUSPEND) { 
        struct rtc_time tm; 
        rtc_time_to_tm(ts_exit.tv_sec, &tm); 
        pr_info("suspend: exit suspend, ret = %d " 
            "(%d-%02d-%02d %02d:%02d:%02d.%09lu UTC)\n", ret, 
            tm.tm_year + 1900, tm.tm_mon + 1, tm.tm_mday, 
            tm.tm_hour, tm.tm_min, tm.tm_sec, ts_exit.tv_nsec); 
    } 
 
    if (ts_exit.tv_sec - ts_entry.tv_sec <= 1) { 
        ++suspend_short_count; 
 
        if (suspend_short_count == SUSPEND_BACKOFF_THRESHOLD) { 
            suspend_backoff(); 
            suspend_short_count = 0; 
        } 
    } else { 
        suspend_short_count = 0; 
    } 
 
    if (current_event_num == entry_event_num) { 
        if (debug_mask & DEBUG_SUSPEND) 
            pr_info("suspend: pm_suspend returned with no event\n"); 
        wake_lock_timeout(&unknown_wakeup, HZ / 2); 
    } 
}

static void suspend(struct work_struct *work)
{
 int ret;
 int entry_event_num;
 struct timespec ts_entry, ts_exit;

if (has_wake_lock(WAKE_LOCK_SUSPEND)) {
  if (debug_mask & DEBUG_SUSPEND)
   pr_info("suspend: abort suspend\n");
  return;
 }

entry_event_num = current_event_num;
 sys_sync();
 if (debug_mask & DEBUG_SUSPEND)
  pr_info("suspend: enter suspend\n");
 getnstimeofday(&ts_entry);
 ret = pm_suspend(requested_suspend_state);
 getnstimeofday(&ts_exit);

if (debug_mask & DEBUG_EXIT_SUSPEND) {
  struct rtc_time tm;
  rtc_time_to_tm(ts_exit.tv_sec, &tm);
  pr_info("suspend: exit suspend, ret = %d "
   "(%d-%02d-%02d %02d:%02d:%02d.%09lu UTC)\n", ret,
   tm.tm_year + 1900, tm.tm_mon + 1, tm.tm_mday,
   tm.tm_hour, tm.tm_min, tm.tm_sec, ts_exit.tv_nsec);
 }

if (ts_exit.tv_sec - ts_entry.tv_sec <= 1) {
  ++suspend_short_count;

if (suspend_short_count == SUSPEND_BACKOFF_THRESHOLD) {
   suspend_backoff();
   suspend_short_count = 0;
  }
 } else {
  suspend_short_count = 0;
 }

if (current_event_num == entry_event_num) {
  if (debug_mask & DEBUG_SUSPEND)
   pr_info("suspend: pm_suspend returned with no event\n");
  wake_lock_timeout(&unknown_wakeup, HZ / 2);
 }
}

suspend函数中，通过pm_suspend(requested_suspend_state)进入suspend操作。这个里面也有唤醒操作，只有等唤醒后才会跳出pm_suspend，跳出后会打印log：suspend:exit suspend, ret =pm_suspend就是判断传入的state是否符合suspend,符合就调用enter_state(state)，到现在开始才进入了linux标准的suspend流程。

pm_suspend的路径：kernel/kernel/power/suspend.c

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int pm_suspend(suspend_state_t state) 
{ 
    if (state > PM_SUSPEND_ON && state < PM_SUSPEND_MAX) 
        return enter_state(state); 
    return -EINVAL; 
} 
EXPORT_SYMBOL(pm_suspend);

int pm_suspend(suspend_state_t state)
{
 if (state > PM_SUSPEND_ON && state < PM_SUSPEND_MAX)
  return enter_state(state);
 return -EINVAL;
}
EXPORT_SYMBOL(pm_suspend);
enter_state这个函数主要有三个函数调用，分别是suspend_prepare,suspend_devices_and_enter,suspend_finish。

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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; 
}

/**
 * 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;
}

suspend_prepare做一些睡眠的准备工作

suspend_devices_and_enter就是真正的设备进入睡眠

suspend_finish唤醒后进行的操作。

下面来一个一个分析：

suspend_prepare中首先通过pm_prepare_console，给suspend分配一个虚拟终端来输出信息；接着通过pm_notifier_call_chain来广播一个系统进入suspend的通报；关闭用户态的helper进程；最后通过suspend_freeze_processes来冻结用户态进程，最后会尝试释放一些内存。在suspend_freeze_processes()函数中调用了freeze_processes()函数，而freeze_processes()函数中又调用了try_to_freeze_tasks()来完成冻结任务。在冻结过程中,会判断当前进程是否有wake_lock,若有,则冻结失败，函数会放弃冻结。

执行完上面的操作后再次回到enter_state函数中，下面开始调用suspend_devices_and_enter()函数让外设进入休眠。在suspend_devices_and_enter()中首先调用关于平台的suspend_ops->begin，接着通过suspend_console来关闭console，也可以通过改变一个flag来使这个函数无效。接着调用dpm_suspend_start。dpm_suspend_start中会执行device_prepare和device_suspend,这两个函数都是调用pm接口里的prepare和suspend函数(其实这里就开始通过总线的接口来执行驱动的suspend函数了，通过bus->pm->suspend)。接着回到suspend_devices_and_enter中调用suspend_enter(state);在suspend_enter中，首先调用平台相关的suspend_ops->prepare，接着执行dpm_suspend_noirq()调用pm接口里的pm->suspend_noirq，回到suspend_enter，接着调用suspend_ops->prepare_late，接下来多cpu中非启动的cpu通过函数disable_nonboot_cpus()被关闭，然后通过调用arch_suspend_disable_irqs()关闭本地中断。再后来才到睡眠设备的操作，sysdev_suspend(PMSG_SUSPEND)，这样就会进入sysdev_driver.suspend阶段。最后调用suspend_ops->enter()，这里就开始执行到睡眠的最后一步了，执行平台相关的睡眠。在平台睡眠的代码中主要是通过suspend_in_iram(suspend_param1)来执行一段汇编代码，最终在汇编中睡死。唤醒的步骤与睡眠的步骤相反，cpu有电后会首先从汇编中起来，接着回到suspend_enter函数中，执行suspend_ops->enter()返回后的一些唤醒代码，这边就不再去说了，基本是按照上面的逆序来操作的。

上面的过程在我看来还是很复杂的，power management 要好好研究一下了

resume的过程

唤醒的时候，程序从suspend_devices_and_enter函数中出来后，开始执行suspend_finish，接着就会从enter_state中退出来，返回pm_suspend，然后又从pm_suspend返回到wakelock.c中的suspend()，在这里接下来就会打印出”suspend:exit suspend, ret“这些log。