PM / Suspend: Add .suspend_again() callback to suspend_ops

A system or a device may need to control suspend/wakeup events. It may
want to wakeup the system after a predefined amount of time or at a
predefined event decided while entering suspend for polling or delayed
work. Then, it may want to enter suspend again if its predefined wakeup
condition is the only wakeup reason and there is no outstanding events;
thus, it does not wakeup the userspace unnecessary or unnecessary
devices and keeps suspended as long as possible (saving the power).

Enabling a system to wakeup after a specified time can be easily
achieved by using RTC. However, to enter suspend again immediately
without invoking userland and unrelated devices, we need additional
features in the suspend framework.

Such need comes from:

 1. Monitoring a critical device status without interrupts that can
wakeup the system. (in-suspend polling)
 An example is ambient temperature monitoring that needs to shut down
the system or a specific device function if it is too hot or cold. The
temperature of a specific device may be needed to be monitored as well;
e.g., a charger monitors battery temperature in order to stop charging
if overheated.

 2. Execute critical "delayed work" at suspend.
 A driver or a system/board may have a delayed work (or any similar
things) that it wants to execute at the requested time.
 For example, some chargers want to check the battery voltage some
time (e.g., 30 seconds) after the battery is fully charged and the
charger has stopped. Then, the charger restarts charging if the voltage
has dropped more than a threshold, which is smaller than "restart-charger"
voltage, which is a threshold to restart charging regardless of the
time passed.

This patch allows to add "suspend_again" callback at struct
platform_suspend_ops and let the "suspend_again" callback return true if
the system is required to enter suspend again after the current instance
of wakeup. Device-wise suspend_again implemented at dev_pm_ops or
syscore is not done because: a) suspend_again feature is usually under
platform-wise decision and controls the behavior of the whole platform
and b) There are very limited devices related to the usage cases of
suspend_again; chargers and temperature sensors are mentioned so far.

With suspend_again callback registered at struct platform_suspend_ops
suspend_ops in kernel/power/suspend.c with suspend_set_ops by the
platform, the suspend framework tries to enter suspend again by
looping suspend_enter() if suspend_again has returned true and there has
been no errors in the suspending sequence or pending wakeups (by
pm_wakeup_pending).

Tested at Exynos4-NURI.

[rjw: Fixed up kerneldoc comment for suspend_enter().]
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>

include/linux/suspend.h

@@ -92,6 +92,13 @@ typedef int __bitwise suspend_state_t;
  *	@enter() and @wake(), even if any of them fails.  It is executed after
  *	a failing @prepare.
  *
+ * @suspend_again: Returns whether the system should suspend again (true) or
+ *	not (false). If the platform wants to poll sensors or execute some
+ *	code during suspended without invoking userspace and most of devices,
+ *	suspend_again callback is the place assuming that periodic-wakeup or
+ *	alarm-wakeup is already setup. This allows to execute some codes while
+ *	being kept suspended in the view of userland and devices.
+ *
  * @end: Called by the PM core right after resuming devices, to indicate to
  *	the platform that the system has returned to the working state or
  *	the transition to the sleep state has been aborted.
@@ -113,6 +120,7 @@ struct platform_suspend_ops {
 	int (*enter)(suspend_state_t state);
 	void (*wake)(void);
 	void (*finish)(void);
+	bool (*suspend_again)(void);
 	void (*end)(void);
 	void (*recover)(void);
 };

kernel/power/suspend.c

@@ -127,11 +127,12 @@ void __attribute__ ((weak)) arch_suspend_enable_irqs(void)
 
 /**
  * suspend_enter - enter the desired system sleep state.
- *	@state:		state to enter
+ * @state: State to enter
+ * @wakeup: Returns information that suspend should not be entered again.
  *
  * This function should be called after devices have been suspended.
  */
-static int suspend_enter(suspend_state_t state)
+static int suspend_enter(suspend_state_t state, bool *wakeup)
 {
 	int error;
 
@@ -165,7 +166,8 @@ static int suspend_enter(suspend_state_t state)
 
 	error = syscore_suspend();
 	if (!error) {
-		if (!(suspend_test(TEST_CORE) || pm_wakeup_pending())) {
+		*wakeup = pm_wakeup_pending();
+		if (!(suspend_test(TEST_CORE) || *wakeup)) {
 			error = suspend_ops->enter(state);
 			events_check_enabled = false;
 		}
@@ -199,6 +201,7 @@ static int suspend_enter(suspend_state_t state)
 int suspend_devices_and_enter(suspend_state_t state)
 {
 	int error;
+	bool wakeup = false;
 
 	if (!suspend_ops)
 		return -ENOSYS;
@@ -220,7 +223,10 @@ int suspend_devices_and_enter(suspend_state_t state)
 	if (suspend_test(TEST_DEVICES))
 		goto Recover_platform;
 
-	error = suspend_enter(state);
+	do {
+		error = suspend_enter(state, &wakeup);
+	} while (!error && !wakeup
+		&& suspend_ops->suspend_again && suspend_ops->suspend_again());
 
  Resume_devices:
 	suspend_test_start();