Re: [PATCH v5] PM / Core: suspend_again callback for suspend_ops.

[Pavel Machek]

> 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.
> 
> Signed-off-by: MyungJoo Ham <myungjoo.ham@xxxxxxxxxxx>
> Signed-off-by: Kyungmin Park <kyungmin.park@xxxxxxxxxxx>

ACK.
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