關於wake_lock
在上文中，已經介紹了wakelock機制，下面從代碼的角度進行介紹。
wakelock有2種類型，常用爲WAKE_LOCK_SUSPEND，作用是防止系統進入睡眠。WAKE_LOCK_IDLE
這種鎖不會影響到系統進入休眠，但是如果這種鎖被持有，那麼系統將無法進入idle空閒模式。
enum {
WAKE_LOCK_SUSPEND,
WAKE_LOCK_IDLE,
WAKE_LOCK_TYPE_COUNT
};
Wakelock有加鎖和解鎖2種操作，加鎖有2種方式，第一種是永久加鎖（wake_lock），這種鎖必須手動的解鎖；另一種是超時鎖（wake_lock_timeout），這種鎖在過去指定時間後，會自動解鎖。
void wake_lock(struct wake_lock *lock)
{
wake_lock_internal(lock, 0, 0);
}
void wake_lock_timeout(struct wake_lock *lock, long timeout)
{
wake_lock_internal(lock, timeout, 1);
}
對於wakelock，timeout = has_timeout = 0；直接加鎖後，然後退出；
static void wake_lock_internal(
struct wake_lock *lock, long timeout, int has_timeout)
{
int type;
unsigned long irqflags;
long expire_in;
spin_lock_irqsave(&list_lock, irqflags);
type = lock->flags & WAKE_LOCK_TYPE_MASK;
BUG_ON(type >= WAKE_LOCK_TYPE_COUNT);
BUG_ON(!(lock->flags & WAKE_LOCK_INITIALIZED));
#ifdef CONFIG_WAKELOCK_STAT
if (type == WAKE_LOCK_SUSPEND && wait_for_wakeup) {
if (debug_mask & DEBUG_WAKEUP)
pr_info("wakeup wake lock: %s\n", lock->name);
wait_for_wakeup = 0;
lock->stat.wakeup_count++;
}
if ((lock->flags & WAKE_LOCK_AUTO_EXPIRE) &&
(long)(lock->expires - jiffies) <= 0) {
wake_unlock_stat_locked(lock, 0);
lock->stat.last_time = ktime_get();
}
#endif
if (!(lock->flags & WAKE_LOCK_ACTIVE)) {
lock->flags |= WAKE_LOCK_ACTIVE;
#ifdef CONFIG_WAKELOCK_STAT
lock->stat.last_time = ktime_get();
#endif
}
list_del(&lock->link);
if (has_timeout) {
if (debug_mask & DEBUG_WAKE_LOCK)
pr_info("wake_lock: %s, type %d, timeout %ld.lu\n",
lock->name, type, timeout / HZ,
(timeout % HZ) * MSEC_PER_SEC / HZ);
lock->expires = jiffies + timeout;
lock->flags |= WAKE_LOCK_AUTO_EXPIRE;
list_add_tail(&lock->link, &active_wake_locks[type]);
} else {
if (debug_mask & DEBUG_WAKE_LOCK)
pr_info("wake_lock: %s, type %d\n", lock->name, type);
lock->expires = LONG_MAX;
lock->flags &= ~WAKE_LOCK_AUTO_EXPIRE;
list_add(&lock->link, &active_wake_locks[type]);
}
if (type == WAKE_LOCK_SUSPEND) {
current_event_num++;
#ifdef CONFIG_WAKELOCK_STAT
if (lock == &main_wake_lock)
update_sleep_wait_stats_locked(1);
else if (!wake_lock_active(&main_wake_lock))
update_sleep_wait_stats_locked(0);
#endif
if (has_timeout)
expire_in = has_wake_lock_locked(type);
else
expire_in = -1;
if (expire_in > 0) {
if (debug_mask & DEBUG_EXPIRE)
pr_info("wake_lock: %s, start expire timer, "
"%ld\n", lock->name, expire_in);
mod_timer(&expire_timer, jiffies + expire_in);
} else {
if (del_timer(&expire_timer))
if (debug_mask & DEBUG_EXPIRE)
pr_info("wake_lock: %s, stop expire timer\n",
lock->name);
if (expire_in == 0)
queue_work(suspend_work_queue, &suspend_work);
}
}
spin_unlock_irqrestore(&list_lock, irqflags);
}
而對於wake_lock_timeout，在經過timeout時間後，才加鎖。再判斷當前持有wakelock時，啓動另一個定時器，在expire_timer的回調函數中再次判斷是否持有wakelock。
static void expire_wake_locks(unsigned long data)
{
long has_lock;
unsigned long irqflags;
if (debug_mask & DEBUG_EXPIRE)
pr_info("expire_wake_locks: start\n");
spin_lock_irqsave(&list_lock, irqflags);
if (debug_mask & DEBUG_SUSPEND)
print_active_locks(WAKE_LOCK_SUSPEND);
has_lock = has_wake_lock_locked(WAKE_LOCK_SUSPEND);
if (debug_mask & DEBUG_EXPIRE)
pr_info("expire_wake_locks: done, has_lock %ld\n", has_lock);
if (has_lock == 0)
queue_work(suspend_work_queue, &suspend_work);
spin_unlock_irqrestore(&list_lock, irqflags);
}
static DEFINE_TIMER(expire_timer, expire_wake_locks, 0, 0);
在wakelock中，有2個地方可以讓系統從early_suspend進入suspend狀態。分別是：
l 在wake_unlock中，解鎖之後，若沒有其他的wakelock，則進入suspend。
l 在超時鎖的定時器超時後，定時器的回調函數，會判斷有沒有其他的wakelock，若沒有，則進入suspend。