linux-kernel - Re: [PATCH] PM / sleep: Let devices force direct

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Message-ID: <Pine.LNX.4.44L0.1505011321280.1365-100000@iolanthe.rowland.org>
Date:	Fri, 1 May 2015 13:24:30 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Tomeu Vizoso <tomeu.vizoso@...labora.com>
cc:	linux-kernel@...r.kernel.org,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	<linux-pm@...r.kernel.org>
Subject: Re: [PATCH] PM / sleep: Let devices force direct_complete

On Fri, 1 May 2015, Tomeu Vizoso wrote:

> Introduce a new per-device flag power.force_direct_complete that will
> instruct the PM core to let that device remain in runtime suspend when
> the system goes into a sleep power state, regardless of the PM state of
> any of its descendants.
> 
> This is needed because otherwise it would be needed to get dozens of
> drivers to implement the prepare() callback and be runtime PM active
> even if they don't have a 1-to-1 relationship with a piece of HW.
> 
> This only applies to devices that aren't wakeup-capable, as those would
> need to setup their IRQs as wakeup-capable in their prepare() callbacks.
> 
> Signed-off-by: Tomeu Vizoso <tomeu.vizoso@...labora.com>
> ---
> 
> Hi,
> 
> I'm sending this as a standalone patch as suggested by Alan.
> 
> Thanks,
> 
> Tomeu
> 
> ---
>  Documentation/power/runtime_pm.txt | 10 ++++++++++
>  drivers/base/power/main.c          | 14 ++++++++++----
>  include/linux/pm.h                 |  1 +
>  3 files changed, 21 insertions(+), 4 deletions(-)
> 
> diff --git a/Documentation/power/runtime_pm.txt b/Documentation/power/runtime_pm.txt
> index 44fe1d2..3b0c68d 100644
> --- a/Documentation/power/runtime_pm.txt
> +++ b/Documentation/power/runtime_pm.txt
> @@ -665,6 +665,16 @@ as appropriate.  This only applies to system suspend transitions that are not
>  related to hibernation (see Documentation/power/devices.txt for more
>  information).
>  
> +For devices that know that can remain runtime suspended when the system
> +transitions to a sleep state regardless of the PM state of their descendants,
> +the flag power.force_direct_complete can be set on their device structures.
> +This can be useful when a real device has several virtual devices as
> +descendants and it would be very cumbersome to make sure that they return a
> +positive value in their .prepare() callback and have runtime PM enabled. Usage
> +of power.force_direct_complete is only allowed to devices that aren't
> +wakeup-capable, as they would need to set their IRQs as wakeups in their
> +.prepare() callbacks before the system transitions to a sleep state.
> +

This seems okay to me.  It specifically states that 
force_direct_complete should be used only when it is known that the PM 
states of the descendant devices don't matter.

>  The PM core does its best to reduce the probability of race conditions between
>  the runtime PM and system suspend/resume (and hibernation) callbacks by carrying
>  out the following operations:
> diff --git a/drivers/base/power/main.c b/drivers/base/power/main.c
> index 3d874ec..7b962f5 100644
> --- a/drivers/base/power/main.c
> +++ b/drivers/base/power/main.c
> @@ -1438,7 +1438,9 @@ static int __device_suspend(struct device *dev, pm_message_t state, bool async)
>  		if (parent) {
>  			spin_lock_irq(&parent->power.lock);
>  
> -			dev->parent->power.direct_complete = false;
> +			if (!dev->parent->power.force_direct_complete)
> +				dev->parent->power.direct_complete = false;
> +
>  			if (dev->power.wakeup_path
>  			    && !dev->parent->power.ignore_children)
>  				dev->parent->power.wakeup_path = true;
> @@ -1605,9 +1607,13 @@ static int device_prepare(struct device *dev, pm_message_t state)
>  	 * will do the same thing with all of its descendants".  This only
>  	 * applies to suspend transitions, however.
>  	 */
> -	spin_lock_irq(&dev->power.lock);
> -	dev->power.direct_complete = ret > 0 && state.event == PM_EVENT_SUSPEND;
> -	spin_unlock_irq(&dev->power.lock);
> +	if (state.event == PM_EVENT_SUSPEND) {
> +		spin_lock_irq(&dev->power.lock);
> +		dev->power.direct_complete = ret > 0 ||
> +			(dev->power.force_direct_complete &&
> +			 !device_can_wakeup(dev));
> +		spin_unlock_irq(&dev->power.lock);
> +	}
>  	return 0;
>  }
>  
> diff --git a/include/linux/pm.h b/include/linux/pm.h
> index 2d29c64..2e41cfd 100644
> --- a/include/linux/pm.h
> +++ b/include/linux/pm.h
> @@ -553,6 +553,7 @@ struct dev_pm_info {
>  	bool			ignore_children:1;
>  	bool			early_init:1;	/* Owned by the PM core */
>  	bool			direct_complete:1;	/* Owned by the PM core */
> +	bool			force_direct_complete:1;
>  	spinlock_t		lock;
>  #ifdef CONFIG_PM_SLEEP
>  	struct list_head	entry;

Rafael, do you think we need a setter routine that acquires the
spinlock before turning on this bit?

Alan Stern

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