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Date:	Thu, 19 Jun 2014 16:13:07 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Kevin Hilman <khilman@...aro.org>
cc:	Allen Yu <alleny@...dia.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Pavel Machek <pavel@....cz>, Len Brown <len.brown@...el.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] PM / Runtime: let rpm_resume fail if rpm disabled
 and device suspended.

On Thu, 19 Jun 2014, Kevin Hilman wrote:

> Alan Stern <stern@...land.harvard.edu> writes:
> 
> > On Thu, 19 Jun 2014, Allen Yu wrote:
> >
> >> So what's the exact state of device if dev->power.is_suspended flag
> >> is set and runtime_status is RPM_ACTIVE? Is it a state like
> >> "suspended but still can be accessed"?
> >> 
> >> I'm just afraid the existing code would cause a device hang if we
> >> allow it to be accessed even though it's suspended (at this point
> >> RPM_ACTIVE could be meaningless). I don't understand the original
> >> motivation of these code. If it's a valid case, most likely it should
> >> be handled in the specific device driver instead of the PM core.
> >
> > You should read the Changelog for commit 6f3c77b040f (PM / Runtime:  
> > let rpm_resume() succeed if RPM_ACTIVE, even when disabled, v2).  It
> > explains why the code looks the way it does.
> >
> > However, I'm starting to think the reasoning in that commit may not be
> > valid.  While perhaps it is okay for some I2C devices (mentioned in the
> > commit log), it probably isn't okay in general.
> 
> Why not?

See below.

> > Kevin, do have any comments on this matter?  What do you think about 
> > making the following change to rpm_resume():
> >
> >  repeat:
> > 	if (dev->power.runtime_error)
> > 		retval = -EINVAL;
> > -	else if (dev->power.disable_depth == 1 && dev->power.is_suspended
> > +	else if (dev->power.disable_depth > 0
> > 	    && dev->power.runtime_status == RPM_ACTIVE)
> > 		retval = 1;
> > 	else if (dev->power.disable_depth > 0)
> >
> > Or even:
> >
> > +	else if (dev->power.disable_depth > 0 && !dev->power.is_suspended
> >
> > although this would require the I2C driver you mentioned in your commit 
> > to change.
> 
> My change was introduced to catch a very specific case.  Namely, when we
> know that the core has successfully asked the device to do a system suspend
> (dev->power.is_suspended == true) *and* we know that runtime PM was
> disabled *only* by the PM core (disable_depth == 0) while the device was
> still active (runtime_status == RPM_ACTIVE.)

For a general device, the fact that dev->power.is_suspended is set
means the device _has_ been powered down.  Even though the
runtime_status may not have changed, the PM core has to assume the
device is not available for use.

While your I2C devices may be useable even after the ->suspend callback
returns, for most devices this isn't true.  So we shouldn't allow
rpm_resume() to return imediately when is_suspended is set.

> In your first idea above, it would allow a _get() to succeed even if
> someone other than the core (including the driver itself) had called
> pm_runtime_disable().  I don't think we want that.

Why not?  The fact that the device is disabled for runtime PM means
that the PM core mustn't try to change its power state.  But if the
runtime status is RPM_ACTIVE then the device should already be powered
up, so there's no harm in letting runtime_resume() succeed.

To put it another way, disabled_depth > 0 means that the PM core isn't
allowed to invoke any of the runtime PM callbacks.  But when
runtime_status == RPM_ACTIVE, runtime_resume() can run successfully
without invoking any callbacks.

> In the second idea above, we'd completely miss the case where runtime PM
> has been disabled by the core (because the core would have set
> dev->power.is_suspended)

That's the intention.  When is_suspended is set, the PM core assumes
that the device has been powered down in preparation for system
suspend.  We don't want to mess that up by performing a runtime resume.

> In both cases, we're no longer just checking for that specific condition
> I was after, so I'd have to spend more time thinking about any other
> possible consequences as well.

Indeed.  Hopefully the fallout won't affect more than a few drivers.

> I think part of the confusion here is coming from what
> dev->power.is_suspended means.  From the core's perspective, it just
> means that the core has called the ->suspend callback, and didn't detect
> any errors.  

Yes.  But the core also has to assume that the ->runtime_resume 
callback will undo the effect of ->suspend.  Therefore the core should 
not call ->runtime_resume if is_suspended is set.

> Depending on the driver though, it doesn't have to mean that the device
> is actually fully suspended.  For example, there are several cases of
> "runtime PM centric" drivers are may be needed by other devices during
> the system suspend/resume process and so are runtime PM resumed during
> system suspend.

At what stage do these devices get powered down?  During suspend_late
or suspend_irq?  At such times the PM core won't invoke the runtime PM
callbacks anyway.

It sounds like what you really want for these devices is to have
dev->power.is_suspended get set at the start of
__device_suspend_late() rather than at the end of __device_suspend().  
Or maybe even not to get set at all.

Alan Stern

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