Re: [RFC PATCH] PM / Core: suspend_again cb for syscore_ops

["Rafael J. Wysocki" <rjw@sisk.pl>]

On Wednesday, April 20, 2011, MyungJoo Ham wrote:
> 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 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, 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 stops. 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 a system or a device to provide "suspend_again"
> callback with syscore_ops. With suspend_again callbacks registered,
> the suspend framework (kernel/power/suspend.c) tries to enter suspend
> again if conditions are met.

syscore_ops are defined to be executed on one CPU and with interrupts off.
I don't think your new callback matches this definition.

> The system enters the suspend again if and only if all of the following
> conditions are met:
> 1. None of suspend_again ops returned "I want to stop suspend"
> (suspend_again returns SUSPEND_AGAIN_STOP).
> 2. At least one of suspend_again ops returned "I want to suspend again"
> (suspend_again returns SUSPEND_AGAIN_CONTINUE)
> 
> suspend_again ops may return "I do not care. This wakeup is not related
> with me." (SUSPEND_AGAIN_NC, which is 0).
> 
> Use SUSPEND_AGAIN_STOP in order to override other devices'
> SUSPEND_AGAIN_CONTINUE and to wakeup fully. For devices that poll
> sensors during suspend may need this if any outstanding status is found.
> For conventional suspend wakeup sources, SUSPEND_AGAIN_STOP may be used
> to override SUSPEND_AGAIN devices.
> 
> Anyway, the following features may need to be added later:
> 1. An API to allow devices to express next desired wakeup-time. Then,
> the framework will combine them and setup RTC alarm accordingly and
> save/restore previously registered RTC alarms.
> 2. Create a method to declare a specific instance of delayed-work is to
> be executed in suspend by waking up the system in the middle of
> suspend. Then, let the framework handle those "critical" delayed-work
> in suspend.
> 3. If a device says SUSPEND_AGAIN_CONTINUE and there is another wakeup
> source pending (e.g., power button) without suspend_again ops, the
> system will enter suspend again. In such a case, the system should not
> suspend again. We may need to see if irqs that are enabled by
> set_irq_wake() (and not related to suspend_ops devices)
> are pending at syscore_suspend_again(). Maybe we need to add
> something like "set_irq_wake_with_suspend_again" so that IRQs with
> suspend_again ops implemented are ignored for the
> "override-suspend-again-continue" checking.
> 
> For the initial release, I have set the point of "suspend-again" after
> suspend_ops->end(). However, I'm not so sure about where to set the
> suspend-again point. Because in-suspend polling, which may require
> I/O with other devices, is supposed to be executed at suspend-again ops,
> the suspend-again point is configured to be as late as possible in
> suspend_devices_and_enter(). In order to reduce the number of devices
> waked up, we may need to set the suspend-again point ealier.
> 
> Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>
> Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>

The idea seems to be sane, but I don't like the implementation.

First off, why do you thing it's a good thing to put the callback into
struct syscore_ops?

> ---
>  drivers/base/syscore.c      |   36 +++++++++++++++++++++++++++
>  include/linux/syscore_ops.h |    7 +++++
>  kernel/power/suspend.c      |   57 +++++++++++++++++++++++-------------------
>  3 files changed, 74 insertions(+), 26 deletions(-)
> 
> diff --git a/drivers/base/syscore.c b/drivers/base/syscore.c
> index 90af294..1a7e08d 100644
> --- a/drivers/base/syscore.c
> +++ b/drivers/base/syscore.c
> @@ -95,6 +95,42 @@ void syscore_resume(void)
>  				"Interrupts enabled after %pF\n", ops->resume);
>  		}
>  }
> +
> +/**
> + * syscore_suspend_again - Execute all the registeres system core suspend_again
> + *			callbacks. If at least one returns
> + *			SUSPEND_AGAIN_CONTINUE and no one returns
> + *			SUSPEND_AGAIN_STOP, syscore_suspend_again let the system
> + *			enter suspend again.
> + */

That causes the ->suspend_again() callbacks to be quite complicated.  I'm not
sure this is necessary in general.

> +bool syscore_suspend_again(void)
> +{
> +	struct syscore_ops *ops;
> +	enum suspend_again_cond condition = SUSPEND_AGAIN_NC;
> +
> +	list_for_each_entry(ops, &syscore_ops_list, node)
> +		if (ops->suspend_again) {
> +			switch (ops->suspend_again()) {
> +			case SUSPEND_AGAIN_NC:
> +				break;
> +			case SUSPEND_AGAIN_CONTINUE:
> +				if (condition == SUSPEND_AGAIN_NC)
> +					condition = SUSPEND_AGAIN_CONTINUE;
> +				break;
> +			case SUSPEND_AGAIN_STOP:
> +				condition = SUSPEND_AGAIN_STOP;
> +				break;
> +			default:
> +				pr_warn("PM: incorrect return from %pF\n",
> +					ops->suspend_again);
> +			}
> +		}
> +
> +	if (condition == SUSPEND_AGAIN_CONTINUE)
> +		return true;
> +
> +	return false;
> +}
>  #endif /* CONFIG_PM_SLEEP */
>  
>  /**
> diff --git a/include/linux/syscore_ops.h b/include/linux/syscore_ops.h
> index 27b3b0b..bf9bc4e 100644
> --- a/include/linux/syscore_ops.h
> +++ b/include/linux/syscore_ops.h
> @@ -11,10 +11,16 @@
>  
>  #include <linux/list.h>
>  
> +enum suspend_again_cond {
> +	SUSPEND_AGAIN_NC = 0, /* Do Not Care */
> +	SUSPEND_AGAIN_CONTINUE, /* Start or keep the again */
> +	SUSPEND_AGAIN_STOP, /* Stop or do not start. Override CONTINUE */
> +};
>  struct syscore_ops {
>  	struct list_head node;
>  	int (*suspend)(void);
>  	void (*resume)(void);
> +	enum suspend_again_cond (*suspend_again)(void);
>  	void (*shutdown)(void);
>  };
>  
> @@ -23,6 +29,7 @@ extern void unregister_syscore_ops(struct syscore_ops *ops);
>  #ifdef CONFIG_PM_SLEEP
>  extern int syscore_suspend(void);
>  extern void syscore_resume(void);
> +extern bool syscore_suspend_again(void);
>  #endif
>  extern void syscore_shutdown(void);
>  
> diff --git a/kernel/power/suspend.c b/kernel/power/suspend.c
> index 2814c32..aa6a3d1 100644
> --- a/kernel/power/suspend.c
> +++ b/kernel/power/suspend.c
> @@ -202,43 +202,48 @@ static int suspend_enter(suspend_state_t state)
>  int suspend_devices_and_enter(suspend_state_t state)
>  {
>  	int error;
> +	bool recover = false;
>  
>  	if (!suspend_ops)
>  		return -ENOSYS;
>  
> -	trace_machine_suspend(state);
> -	if (suspend_ops->begin) {
> -		error = suspend_ops->begin(state);
> -		if (error)
> -			goto Close;
> -	}
> -	suspend_console();
> -	pm_restrict_gfp_mask();
> -	suspend_test_start();
> -	error = dpm_suspend_start(PMSG_SUSPEND);
> -	if (error) {
> -		printk(KERN_ERR "PM: Some devices failed to suspend\n");
> -		goto Recover_platform;
> -	}
> -	suspend_test_finish("suspend devices");
> -	if (suspend_test(TEST_DEVICES))
> -		goto Recover_platform;
> +	do {
> +		trace_machine_suspend(state);
> +		if (suspend_ops->begin) {
> +			error = suspend_ops->begin(state);
> +			if (error)
> +				goto Close;

All of those goto statements from the inside of the loop don't really look
good.  Isn't there any way to avoid them?

> +		}
> +		suspend_console();
> +		pm_restrict_gfp_mask();
> +		suspend_test_start();
> +		error = dpm_suspend_start(PMSG_SUSPEND);
> +		if (error) {
> +			printk(KERN_ERR "PM: Some devices failed to suspend\n");
> +			goto Recover_platform;
> +		}
> +		suspend_test_finish("suspend devices");
> +		if (suspend_test(TEST_DEVICES))
> +			goto Recover_platform;
>  
> -	suspend_enter(state);
> +		error = suspend_enter(state);
>  
>   Resume_devices:
> -	suspend_test_start();
> -	dpm_resume_end(PMSG_RESUME);
> -	suspend_test_finish("resume devices");
> -	pm_restore_gfp_mask();
> -	resume_console();
> +		suspend_test_start();
> +		dpm_resume_end(PMSG_RESUME);
> +		suspend_test_finish("resume devices");
> +		pm_restore_gfp_mask();
> +		resume_console();
>   Close:
> -	if (suspend_ops->end)
> -		suspend_ops->end();
> -	trace_machine_suspend(PWR_EVENT_EXIT);
> +		if (suspend_ops->end)
> +			suspend_ops->end();
> +		trace_machine_suspend(PWR_EVENT_EXIT);
> +	} while (syscore_suspend_again() && !error && !recover);

Why exactly do you think that the next automatic suspend should occur at
this particular point?

> +
>  	return error;
>  
>   Recover_platform:
> +	recover = true;
>  	if (suspend_ops->recover)
>  		suspend_ops->recover();
>  	goto Resume_devices;
> 

To summarize:
* I don't think struct syscore_ops is the right place for the new callback.
* The necessity to take care of the possibly many different return values
  with different meanings have a potential to introduce unnecessary
  complications into the subsystems implementing the new callback.
* It isn't particularly clear to me why the new suspend should occur at the
  point you want it to.

Moreover, what's wrong with thawing user space processes and _then_
automatically suspending again?  Why do you want to do that from the inside
of the kernel?

Rafael