linux-kernel - Re: [PATCH v3 2/2] PM / Runtime: Add pm_runtime_enable

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Message-ID: <1714292.iHVMDeHUzi@vostro.rjw.lan>
Date:	Mon, 06 Jul 2015 01:36:46 +0200
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Tomeu Vizoso <tomeu.vizoso@...labora.com>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Ulf Hansson <ulf.hansson@...aro.org>,
	Kevin Hilman <khilman@...aro.org>,
	Russell King <rmk+kernel@....linux.org.uk>,
	Krzysztof Kozlowski <k.kozlowski@...sung.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 2/2] PM / Runtime: Add pm_runtime_enable_recursive

On Saturday, July 04, 2015 10:37:55 AM Alan Stern wrote:
> On Sat, 4 Jul 2015, Rafael J. Wysocki wrote:
> 
> > > >> > Perhaps the pm_runtime_suspended_if_enabled() test should be changed to
> > > >> > pm_runtime_status_suspended().  Then it won't matter whether the
> > > >> > descendant devices are enabled for runtime PM.
> > > >>
> > > >> Yeah, that would remove the need for messing with the runtime PM
> > > >> enable status of descendant devices, but I wonder why Rafael went that
> > > >> way initially.
> > > >
> > > > I forget the details.  Probably it was just to be safe.  We probably
> > > > thought that if a device was disabled for runtime PM then its runtime
> > > > PM status might not be accurate.  But if direct_complete is set then it
> > > > may be reasonable to assume that the runtime PM status _is_ accurate.
> > > 
> > > Cool.
> > 
> > We're walking a grey area here.  What exactly does power.direct_complete mean
> > for devices whose runtime PM is disabled?
> 
> > > Let's see what Rafael thinks about these two issues.  It seems to me
> > > that the hardest part is dealing with drivers/subsystems that have no
> > > runtime PM support.  In such cases, we have to be very careful not to
> > > use direct_complete unless we know that the device does no power
> > > management at all.
> > 
> > Precisely.
> 
> All right, we can make a decision and document it.  The following seems
> reasonable to me:
> 
> 	If dev->power.direct_complete is set then the PM core will
> 	assume that dev->power.rpm_status is accurate even when
> 	dev->power.disable_depth > 0.  The core will obey the
> 	.direct_complete setting regardless of .disable_depth.
> 
> 	As a consequence, devices that support system sleep but don't 
> 	support runtime PM must _never_ have .direct_complete set.
> 
> 	On the other hand, if a device (such as a "virtual" device)
> 	requires no callbacks for either system sleep or runtime PM, 
> 	then there is no harm in setting .direct_complete.  Indeed,
> 	doing so may help speed up an ancestor device's sleep
> 	transition.
> 
> How does that sound?

It would be workable I think, but I'd prefer the core to be told directly
about devices whose runtime PM status doesn't matter (because nothing changes
between "suspended" and "active"), so they may be treated in a special way
safely.

If we had that information, no special rules other than "that is a device
whose runtime PM status doesn't matter, so treat it accordingly" would be
necessary.

Thanks,
Rafael

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