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Wakeup count是的组成部分，用于解决“system suspend和system wakeup events之间的同步问题”。本文将结合“”和“”两篇文章，分析wakeup count的功能、实现逻辑、背后的思考，同时也是对这两篇文章的复习和总结。
wakeup count的实现位于wakeup events framework中（drivers/base/power/wakeup.c），主要为两个模块提供接口：通过PM core向用户空间提供sysfs接口；直接向autosleep（请参考下一篇文章）提供接口。
wakeup count的功能是suspend同步，实现思路是这样的：
1）任何想发起电源状态切换的实体（可以是用户空间电源管理进程，也可以是内核线程，简称C），在发起状态切换前，读取系统的wakeup counts（该值记录了当前的wakeup event总数），并将读取的counts告知wakeup events framework。
2）wakeup events framework记录该counts到一个全局变量中（saved_count）。
3）随后C发起电源状态切换（如STR），执行suspend过程。
4）在suspend的过程中，wakeup events framework照旧工作（直到系统中断被关闭），上报wakeup events，增加wakeup events counts。
5）suspend执行的一些时间点（可参考“”），会调用wakeup events framework提供的接口（pm_wakeup_pending），检查是否有wakeup没有处理。
6）检查逻辑很简单，就是比较当前的wakeup counts和saved wakeup counts（C发起电源状态切换时的counts），如果不同，就要终止suspend过程。
在进行代码分析之前，我们先用伪代码的形式，写出一个利用wakeup count进行suspend操作的例子，然后基于该例子，分析相关的实现。
1: do { 2: ret = read(&cnt, "/sys/power/wakeup_count");
3: if (ret) { 4: ret = write(cnt, "/sys/power/wakeup_count");
5: } else { 6: countine;
7: }
8: } while (!ret);
9:
10: write("mem", "/sys/power/state"); 11:
12: /* goto here after wakeup */
例子很简单：
a）读取wakeup count值，如果成功，将读取的值回写。否则说明有正在处理的wakeup events，continue。
b）回写后，判断返回值是否成功，如果不成功（说明读、写的过程中产生了wakeup events），继续读、写，直到成功。成功后，可以触发电源状态切换。
wakeup_count文件是在kernel/power/main.c中，利用power_attr注册的，如下（大家可以仔细研读一下那一大段注释，内核很多注释写的非常好，而好的注释，就是软件功力的体现）：
1: /*
2: * The 'wakeup_count' attribute, along with the functions defined in
3: * drivers/base/power/wakeup.c, provides a means by which wakeup events can be
4: * handled in a non-racy way.
5: *
6: * If a wakeup event occurs when the system is in a sleep state, it simply is
7: * woken up. In turn, if an event that would wake the system up from a sleep
8: * state occurs when it is undergoing a transition to that sleep state, the
9: * transition should be aborted. Moreover, if such an event occurs when the
10: * system is in the working state, an attempt to start a transition to the
11: * given sleep state should fail during certain period after the detection of
12: * the event. Using the 'state' attribute alone is not sufficient to satisfy
13: * these requirements, because a wakeup event may occur exactly when 'state'
14: * is being written to and may be delivered to user space right before it is
15: * frozen, so the event will remain only partially processed until the system is
16: * woken up by another event. In particular, it won't cause the transition to
17: * a sleep state to be aborted.
18: *
19: * This difficulty may be overcome if user space uses 'wakeup_count' before
20: * writing to 'state'. It first should read from 'wakeup_count' and store
21: * the read value. Then, after carrying out its own preparations for the system
22: * transition to a sleep state, it should write the stored value to
23: * 'wakeup_count'. If that fails, at least one wakeup event has occurred since
24: * 'wakeup_count' was read and 'state' should not be written to. Otherwise, it
25: * is allowed to write to 'state', but the transition will be aborted if there
26: * are any wakeup events detected after 'wakeup_count' was written to.
27: */
28:
29: static ssize_t wakeup_count_show(struct kobject *kobj,
30: struct kobj_attribute *attr,
31: char *buf)
32: { 33: unsigned int val;
34:
35: return pm_get_wakeup_count(&val, true) ?
36: sprintf(buf, "%u\n", val) : -EINTR;
37: }
38:
39: static ssize_t wakeup_count_store(struct kobject *kobj,
40: struct kobj_attribute *attr,
41: const char *buf, size_t n)
42: { 43: unsigned int val;
44: int error;
45:
46: error = pm_autosleep_lock();
47: if (error)
48: return error;
49:
50: if (pm_autosleep_state() > PM_SUSPEND_ON) { 51: error = -EBUSY;
52: goto out;
53: }
54:
55: error = -EINVAL;
56: if (sscanf(buf, "%u", &val) == 1) { 57: if (pm_save_wakeup_count(val))
58: error = n;
59: }
60:
61: out:
62: pm_autosleep_unlock();
63: return error;
64: }
65:
66: tr(wakeup_count);
实现很简单：read时，直接调用pm_get_wakeup_count（注意第2个参数）；write时，直接调用pm_save_wakeup_count（注意用户空间的wakeup count功能和auto sleep互斥，会在下篇文章解释原因）。这两个接口均是wakeup events framework提供的接口，跟着代码往下看吧。
pm_get_wakeup_count的实现如下：
1: bool pm_get_wakeup_count(unsigned int *count, bool block)
2: { 3: unsigned int cnt, inpr;
4:
5: if (block) { 6: DEFINE_WAIT(wait);
7:
8: for (;;) { 9: prepare_to_wait(&wakeup_count_wait_queue, &wait,
10: TASK_INTERRUPTIBLE);
11: split_counters(&cnt, &inpr);
12: if (inpr == 0 || signal_pending(current))
13: break;
14:
15: schedule();
16: }
17: finish_wait(&wakeup_count_wait_queue, &wait);
18: }
19:
20: split_counters(&cnt, &inpr);
21: *count = cnt;
22: return !inpr;
23: }
该接口有两个参数，一个是保存返回的count值得指针，另一个指示是否block，具体请参考代码逻辑：
a）如果block为false，直接读取registered wakeup events和wakeup events in progress两个counter值，将registered wakeup events交给第一个参数，并返回wakeup events in progress的状态（若返回false，说明当前有wakeup events正在处理，不适合suspend）。
b）如果block为true，定义一个等待队列，等待wakeup events in progress为0，再返回counter。
注1：由4.2小节可知，sysfs发起的read动作，block为true，所以如果有正在处理的wakeup events，read进程会阻塞。其它模块（如auto sleep）发起的read，则可能不需要阻塞。
pm_save_wakeup_count的实现如下：
1: bool pm_save_wakeup_count(unsigned int count)
2: { 3: unsigned int cnt, inpr;
4: unsigned long flags;
5:
6: events_check_enabled = false;
7: spin_lock_irqsave(&events_lock, flags);
8: split_counters(&cnt, &inpr);
9: if (cnt == count && inpr == 0) { 10: saved_count = count;
11: events_check_enabled = true;
12: }
13: spin_unlock_irqrestore(&events_lock, flags);
14: return events_check_enabled;
15: }
1）注意这个变量，events_check_enabled，如果它不为真，pm_wakeup_pending接口直接返回false，意味着如果不利用wakeup count功能，suspend过程中不会做任何wakeup events检查，也就不会进行任何的同步。
2）解除当前的registered wakeup events、wakeup events in progress，保存在变量cnt和inpr中。
3）如果写入的值和cnt不同（说明读、写的过程中产生events），或者inpr不为零（说明有events正在被处理），返回false（说明此时不宜suspend）。
4）否则，events_check_enabled置位（后续的pm_wakeup_pending才会干活），返回true（可以suspend），并将当前的wakeup count保存在saved count变量中。
再回忆一下“”中suspend的流程，在suspend_enter接口中，suspend前的最后一刻，会调用pm_wakeup_pending接口，代码如下：
1: static int suspend_enter(suspend_state_t state, bool *wakeup)
2: { 3: ...
4: error = syscore_suspend();
5: if (!error) { 6: *wakeup = pm_wakeup_pending();
7: if (!(suspend_test(TEST_CORE) || *wakeup)) { 8: error = suspend_ops->enter(state);
9: events_check_enabled = false;
10: }
11: syscore_resume();
12: }
13: ...
14: }
在write wakeup_count到调用pm_wakeup_pending这一段时间内，wakeup events framework会照常产生wakeup events，因此如果pending返回true，则不能“enter”，终止suspend吧！
注2：wakeup后，会清除events_check_enabled标记。
“”中已经介绍过pm_wakeup_pending了，让我们再看一遍吧：
1: bool pm_wakeup_pending(void)
2: { 3: unsigned long flags;
4: bool ret = false;
5:
6: spin_lock_irqsave(&events_lock, flags);
7: if (events_check_enabled) { 8: unsigned int cnt, inpr;
9:
10: split_counters(&cnt, &inpr);
11: ret = (cnt != saved_count || inpr > 0);
12: events_check_enabled = !ret;
13: }
14: spin_unlock_irqrestore(&events_lock, flags);
15:
16: if (ret)
17: print_active_wakeup_sources();
18:
19: return ret;
20: }
a）首先会判断events_check_enabled是否有效，无效直接返回false。有效的话：
b）获得cnt和inpr，如果cnt不等于saved_count（说明这段时间内有events产生），或者inpr不为0（说明有events正在被处理），返回true（告诉调用者，放弃吧，时机不到）。同时清除events_check_enabled的状态。
c）否则，返回false（放心睡吧），同时保持events_check_enabled的置位状态（以免pm_wakeup_pending再次调用）。
Okay，结束了，等待wakeup吧~~~~
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