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

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Date:	Wed, 6 May 2015 10:43:34 -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 Wed, 6 May 2015, Ulf Hansson wrote:

> On 30 April 2015 at 16:53, Alan Stern <stern@...land.harvard.edu> wrote:
> > 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.
> 
> Alan, thanks for your reply.
> 
> There are certainly drivers/subsystems that can't full-fill the
> requirements to have the PM core to deal with what I propose. Somehow
> drivers/subsystems would have to announce its capability for this.
> 
> Those drivers/subsystems I have been looking at, is dealing with I/O.
> Typically platform/amba devices, which drivers has registered
> subsystem specific callbacks at ->probe(). One of these callbacks are
> invoked when there is an I/O request to serve from the subsystem's
> core layer.
> 
> In the beginning of that callback, pm_runtime_get_sync() is invoked.
> When the request has been served and the controller can be runtime PM
> suspended, the driver call pm_runtime_put() or possibly
> pm_runtime_put_autosuspend().
> 
> These drivers/subsystem may be considered as being "runtime PM
> centric", since during system PM suspend they don't have any system PM
> specific things to deal with. They only want to make sure their
> devices becomes "runtime PM suspended".
> 
> There's no doubt that they can do that by implementing the system PM
> ->suspend() callbacks, in one way or the other.
> 
> To simplify PM implementation for these drivers/subsystems, it would
> have been nice if the PM core could handle this "automagically", thus
> drivers/subsystems wouldn't have to implement the system PM callbacks
> at all. Reaching that point, would likely make it easier to understand
> how to implement a "runtime PM centric" driver/subsystem.

The drivers/subsystems don't have to implement these things, because
you have _already_ implemented them: pm_runtime_force_suspend() and 
pm_runtime_force_resume().  A driver/subsystem merely has to store 
pointers to these routines in its dev_pm_ops structure.

> > 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.
> 
> That would likely work, but again it would require drivers/subsystems
> to assign system PM callbacks.

You said just above that the driver/subsystem would have to announce
its capability for this somehow.  Using suitable callback pointers
would be a good way to make that announcement.

Alan Stern

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