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

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Date:	Wed, 9 Sep 2015 00:25:52 +0200
From:	Ulf Hansson <ulf.hansson@...aro.org>
To:	"Rafael J. Wysocki" <rafael@...nel.org>,
	"Tirdea, Irina" <irina.tirdea@...el.com>
Cc:	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

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()...

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.

>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!?

Kind regards
Uffe
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