linux-kernel - Re: [RFC PATCH] PM / Runtime: runtime: Add sysfs option for forcing runtime suspend

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Message-ID: <CAJZ5v0iCRUJoDJ4oy8TKDvngjZ7SbBUngK2+a7ccUEtVFhW0gg@mail.gmail.com>
Date:	Wed, 9 Sep 2015 01:50:19 +0200
From:	"Rafael J. Wysocki" <rafael@...nel.org>
To:	Ulf Hansson <ulf.hansson@...aro.org>
Cc:	"Rafael J. Wysocki" <rafael@...nel.org>,
	"Tirdea, Irina" <irina.tirdea@...el.com>,
	Oliver Neukum <oneukum@...e.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Alan Stern <stern@...land.harvard.edu>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"linux-input@...r.kernel.org" <linux-input@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Brown, Len" <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
	"Purdila, Octavian" <octavian.purdila@...el.com>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>
Subject: Re: [RFC PATCH] PM / Runtime: runtime: Add sysfs option for forcing
 runtime suspend

Hi,

On Wed, Sep 9, 2015 at 12:25 AM, Ulf Hansson <ulf.hansson@...aro.org> wrote:
> On 8 September 2015 at 22:56, Rafael J. Wysocki <rafael@...nel.org> wrote:
>> On Tue, Sep 8, 2015 at 9:35 AM, Oliver Neukum <oneukum@...e.com> wrote:
>>> On Tue, 2015-09-08 at 01:10 +0000, Tirdea, Irina wrote:
>>
>> [cut]
>>
>>>> this would work except for adding a sysfs attribute that would trigger
>>>> a runtime suspend while ignoring usage count. Would that be a
>>>> better direction?
>>>
>>> No. If we want this at all, we need a new callback to notify drivers
>>> that user space is temporarily uninterested in a device. And the reverse
>>> of course.
>>> The power model is good. We must not assume that devices can be
>>> suspended at will. If we do this at all, we ought to see it as giving
>>> strong hints to drivers when a device can be considered idle.
>>
>> This is a good summary in my view.
>>
>> The only thing we can add, realistically, is an interface for user
>> space to "kick" drivers to check if the devices they handle may be
>> suspended at this point (or to run their ->runtime_idle callbacks
>> IOW).
>>
>> That would be quite similar to autosuspend except that the "kick" will
>> come from user space rather than from a timer function in the kernel.
>
> Apologize for interrupting the discussion!
>
> Unless I miss the point, I assumes the above is somewhat already
> achievable via sysfs when changing the value of the auto-suspend
> timeout, since it triggers a call to
> pm_runtime_set_autosuspend_delay()...

Well, from the initial comment in drivers/base/power/sysfs.c:

 *
 *    NOTE: The autosuspend_delay_ms attribute and the autosuspend_delay
 *    value are used only if the driver calls pm_runtime_use_autosuspend().
 *

Some drivers don't do that and they would be the primary target
audience for the new interface (if we agreed that it was useful after
all).

> Also, according to the discussion so far, it seems like we are on
> agreement that we should really think twice when considering to extend
> the sysfs interface for runtime PM.

That certainly is correct and not limited to runtime PM. :-)

> From the change-log/description to $subject patch, I fail to
> understand *why* the regular runtime PM *autosuspend* feature isn't
> sufficient. Perhaps Irina can elaborate more on the use case, to help
> me get a better understanding of the issue!?

My understanding is that the idea would be to trigger an attempt to
suspend via a specific event (eg. lid closes) rather then via an
inactivity timer.

Thanks,
Rafael
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