linux-kernel - Re: [PATCH v3 1/2] PM / sleep: Let devices force direct

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Date:	Thu, 30 Apr 2015 10:53:35 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Ulf Hansson <ulf.hansson@...aro.org>
cc:	Tomeu Vizoso <tomeu.vizoso@...labora.com>,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Pavel Machek <pavel@....cz>, Len Brown <len.brown@...el.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Kevin Hilman <khilman@...aro.org>
Subject: Re: [PATCH v3 1/2] PM / sleep: Let devices force direct_complete

On Thu, 30 Apr 2015, Ulf Hansson wrote:

> I hesitated to send this reply, since it might add confusion. If
> that's the case, please ignore it.
> 
> I have a long term vision to fully enable support for a runtime PM
> centric configuration for drivers/subsystems. The idea is, that such
> driver/subsystem should get system PM for "free".
> 
> The main goal is to simplify PM implementation for these drivers/subsystems.
> 
> They should need to implement the runtime PM callbacks only and not
> the system PM ones. During system PM suspend, the requirement is that
> the corresponding devices should be guaranteed to be "runtime PM
> suspended". Somehow that then needs to be managed by the PM core.
> 
> I am not sure it's doable, but I wanted to bring it up within the
> context of $subject patch, since it proposes yet another optimization
> path for runtime PM during system PM.

I suspect it is _not_ doable.  Consider a reasonable scenario: a driver
that does pm_runtime_get_sync() in its open routine and
pm_runtime_put() in its release routine.  If a user process holds the
device file open during a system suspend, it will be impossible for the
PM core to do a runtime suspend.

On the other hand, there's nothing to prevent drivers from setting 
their ->suspend and ->runtime_suspend structure members to point at the 
same routine.  The routine would need to handle the case where it was 
called for a system suspend while the device was already runtime 
suspended, but that doesn't seem too hard.  With the "direct-suspend" 
option, even this wouldn't be necessary.

Alan Stern

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