kernel/power/main.c中：

/**
 *	state - control system power state.
 *
 *	show() returns what states are supported, which is hard-coded to
 *	‘standby‘ (Power-On Suspend), ‘mem‘ (Suspend-to-RAM), and
 *	‘disk‘ (Suspend-to-Disk).
 *
 *	store() accepts one of those strings, translates it into the 
 *	proper enumerated value, and initiates a suspend transition.
 */
static ssize_t state_show(struct kobject *kobj, struct kobj_attribute *attr,
			  char *buf)
{
	char *s = buf;
#ifdef CONFIG_SUSPEND
	int i;

	for (i = 0; i < PM_SUSPEND_MAX; i++) {
		if (pm_states[i] && valid_state(i))
			s += sprintf(s,"%s ", pm_states[i]);
	}
#endif
#ifdef CONFIG_HIBERNATION
	s += sprintf(s, "%s\n", "disk");
#else
	if (s != buf)
		/* convert the last space to a newline */
		*(s-1) = ‘\n‘;
#endif
	return (s - buf);
}

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);
		if (error) {
			suspend_stats.fail++;
			dpm_save_failed_errno(error);
		} else
			suspend_stats.success++;
#endif
	}
#endif

 Exit:
	return error ? error : n;
}

power_attr(state);

state_show()和state_store()即是用戶層/sys/power/state的讀寫操作，power_attr屬性的分析見博文：點擊打開鏈接
state_show()顯示了支持寫入到/sys/power/state的字符串，它能夠是：on, mem, standby, disk。

相應代碼中的：

typedef int __bitwise suspend_state_t;

#define PM_SUSPEND_ON		((__force suspend_state_t) 0)
#define PM_SUSPEND_STANDBY	((__force suspend_state_t) 1)
#define PM_SUSPEND_MEM		((__force suspend_state_t) 3)
#define PM_SUSPEND_MAX		((__force suspend_state_t) 4)

state_store()即是寫入操作：

它先推斷是否是“disk“, 再比較是否是pm_states數組中的一個，pm_states定義在kernel/power/Suspend.c中：

const char *const pm_states[PM_SUSPEND_MAX] = {
#ifdef CONFIG_EARLYSUSPEND
	[PM_SUSPEND_ON]		= "on",
#endif
	[PM_SUSPEND_STANDBY]	= "standby",
	[PM_SUSPEND_MEM]	= "mem",
};

在android中定義了CONFIG_EARLYSUSPEND，所以進入了kernel/power/Earlysuspend.c中的request_suspend_state函數：

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);
}

這段代碼的意思是當進入suspend前運行early_suspend_work，當從suspend狀態恢復後運行late_resume_work。

這個文件裏定義了它們：

static DECLARE_WORK(early_suspend_work, early_suspend);
static DECLARE_WORK(late_resume_work, late_resume);

early_suspend()將運行驅動程序中使用register_early_suspend()註冊了的suspend()函數。

late_resume()將運行驅動程序中使用register_early_suspend()註冊了的resume()函數。

/sys/power/state