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Date:	Thu, 19 Jun 2014 22:34:51 +0800
From:	Allen Yu <alleny@...dia.com>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
CC:	Alan Stern <stern@...land.harvard.edu>,
	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 Thursday, June 19, 2014, Rafael J. Wysocki wrote:
> On Thursday, June 19, 2014 04:23:29 PM Allen Yu wrote:
> > On Thursday, June 19, 2014, Rafael J. Wysocki wrote:
> > > On Wednesday, June 18, 2014 11:30:51 AM Alan Stern wrote:
> > > > On Tue, 17 Jun 2014, Rafael J. Wysocki wrote:
> > > >
> > > > > On Tuesday, June 17, 2014 10:37:03 PM Rafael J. Wysocki wrote:
> > > > > > On Tuesday, June 17, 2014 10:26:14 PM Rafael J. Wysocki wrote:
> > > > > > > On Tuesday, June 17, 2014 10:11:32 AM Alan Stern wrote:
> > > > > > > > On Mon, 16 Jun 2014, Rafael J. Wysocki wrote:
> > > > > > > >
> > > > > > > > > > For reasons having nothing to do with Allen's
> > > > > > > > > > suggested change, I wonder if we shouldn't replace
> > > > > > > > > > this line with
> > > something like:
> > > > > > > > > >
> > > > > > > > > > -	else if (dev->power.disable_depth == 1 && dev-
> > > >power.is_suspended
> > > > > > > > > > +	else if (dev->power.disable > 0 &&
> > > > > > > > > > +!dev->power.is_suspended
> > > > > > > > > > 	    && dev->power.runtime_status == RPM_ACTIVE)
> > > > > > > > > > 		retval = 1;
> > > > > > > > > >
> > > > > > > > > > It seems that I've been bitten by this several times in the
> past.
> > > > > > > > > > When a device is disabled for runtime PM, and more or
> > > > > > > > > > less permanently stuck in the RPM_ACTIVE state, calls
> > > > > > > > > > to
> > > > > > > > > > pm_runtime_resume() or
> > > > > > > > > > pm_runtime_get_sync() shouldn't fail.
> > > > > > > > > >
> > > > > > > > > > For example, suppose some devices of a certain type
> > > > > > > > > > support runtime power management but others don't.  We
> > > > > > > > > > naturally want to call
> > > > > > > > > > pm_runtime_disable() for the ones that don't.  But we
> > > > > > > > > > also want the same driver to work for all the devices,
> > > > > > > > > > which means that
> > > > > > > > > > pm_runtime_get_sync() should return success --
> > > > > > > > > > otherwise the driver will think that something has gone
> wrong.
> > > > > > > > > >
> > > > > > > > > > Rafael, what do you think?
> > > > > > > > >
> > > > > > > > > That condition is there specifically to take care of the
> > > > > > > > > system suspend code path.  It means that if runtime PM
> > > > > > > > > is disabled, but it only has been disabled by the system
> > > > > > > > > suspend code path, we should treat the device as "active" (ie.
> > > return 1).  That won't work after the proposed change.
> > > > > > > >
> > > > > > > > Ah, yes, quite true.  Okay, suppose we replace that line with just:
> > > > > > > >
> > > > > > > > +	else if (dev->power.disable > 0
> > > > > > > >
> > > > > > > > > I guess drivers that want to work with devices where
> > > > > > > > > runtime PM may be disabled can just check the return
> > > > > > > > > value of
> > > rpm_resume() for -EACCES?
> > > > > > > >
> > > > > > > > They could, but it's extra work and it's extremely easy to
> > > > > > > > forget about.  I'd prefer not to do things that way.
> > > > > > >
> > > > > > > In that case we need to audit all code that checks the
> > > > > > > return value of
> > > > > > > __pm_runtime_resume() to verify that it doesn't depend on
> > > > > > > the current behavior in any way.  It shouldn't, but still.
> > > > > > >
> > > > > > > Also we probably should drop the -EACCES return value from
> > > > > > > rpm_resume() in the same patch, because it specifically only
> > > > > > > covers the dev->power.disable > 0 case (which BTW is
> > > > > > > consistent with the suspend side of things, so I'm totally
> > > > > > > unsure about that being
> > > the right thing to do to be honest).
> > > >
> > > > It's still the correct action with runtime PM is disabled and the
> > > > device's runtime_status isn't RPM_ACTIVE.
> > >
> > > Well, we used to have the notion that runtime_status is not
> > > meaningful for devices with dev->power.disable_depth greater than 0
> > > (except for the special case in the suspend code path where we know
> > > why it is greater than 0).  I think it was useful. :-)
> >
> > 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.
> >
> > >
> > > > > > Perhaps it'd be better to rework __pm_runtime_resume() to
> > > > > > convert the -EACCES return value from rpm_resume() into 0 if
> > > > > > RPM_GET_PUT is
> > > set?
> > > > >
> > > > > Or do something like this?
> > > > >
> > > > > ---
> > > > >  drivers/base/power/runtime.c |    3 ++-
> > > > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > > > >
> > > > > Index: linux-pm/drivers/base/power/runtime.c
> > > > >
> > >
> ==========================================================
> > > =========
> > > > > --- linux-pm.orig/drivers/base/power/runtime.c
> > > > > +++ linux-pm/drivers/base/power/runtime.c
> > > > > @@ -608,7 +608,8 @@ static int rpm_resume(struct device *dev
> > > > >   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 && (rpmflags &
> > > RPM_GET_PUT))
> > > > > +	    || (dev->power.disable_depth == 1 && dev-
> > > >power.is_suspended))
> > > > >  	    && dev->power.runtime_status == RPM_ACTIVE)
> > > > >  		retval = 1;
> > > > >  	else if (dev->power.disable_depth > 0)
> > > >
> > > > So pm_runtime_resume() and pm_request_resume() would still fail,
> > > > but
> > > > pm_runtime_get() and pm_runtime_get_sync() would work?  I'm not
> > > > sure about the reason for this distinction.
> > >
> > > The meaning of pm_runtime_get()/pm_runtime_get_sync() is "prevent
> > > the device from being suspended from now on and resume it if
> > > necessary" while "runtime PM disabled and runtime_status ==
> > > RPM_ACTIVE" may be interpreted as "not necessary to resume", so it
> > > is reasonable to special case this particular situation for these particular
> routines IMHO.
> >
> > As Rafael mentioned above that runtime_sataus is not meaningful if
> > runtime PM is disabled, so shouldn't we avoid using the runtime_staus
> > here and instead use
> > dev->power.is_suspended only to decide the return value?
> 
> No, we shouldn't.
> 
> This is a special case. If dev->power.disable_depth == 1 and dev-
> >power.is_suspended is set at the same time, we know for a fact that
> runtime PM was only disabled by the system suspend code path and it was
> enabled otherwise, so dev->power.runtime_status equal to RPM_ACTIVE is
> actually meaningful in that particular case.

I'm still confused about the 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"?

> 
> > @@ -608,11 +608,13 @@ static int rpm_resume(struct device *dev, int
> rpmflags)
> >   repeat:
> >         if (dev->power.runtime_error)
> >                 retval = -EINVAL;
> > -       else if (dev->power.disable_depth == 1 && dev->power.is_suspended
> > -           && dev->power.runtime_status == RPM_ACTIVE)
> > -               retval = 1;
> > -       else if (dev->power.disable_depth > 0)
> > -               retval = -EACCES;
> > +       else if (dev->power.disable_depth > 0) {
> > +               if (!dev->power.is_suspended)
> > +                       retval = 1;
> > +               else
> > +                       retval = -EACCES;
> > +       }
> > +
> >         if (retval)
> >                 goto out;
> >
> > However, this requires us to make sure device is in full functional
> > state if it's not suspended before disabling runtime PM, just like the
> > case runtime PM is not configured at all.
> 
> If runtime PM is not configured at all, the device has to be in full functional
> state (from the PM perspective) outside of the system suspend-resume
> sequence.

So before disabling runtime PM of a device, caller need to make it full functional as long as  it's outside of system suspend-resume sequence (or to be more specific, is_suspended flag is not set)?  - This is in fact what my comment above meant.
If this is true, can't we just return 1 in case dev->power.disable_depth > 0 && dev->power.is_suspended == false? - This is in fact what the code above meant to do.
This should work for both pm_runtime_resume() and pm_runtime_get() then.

> 
> The only problematic case I can see is when runtime PM is disabled,
> runtime_status is RPM_ACTIVE, but the device is actually suspended for
> some reason.  I'd say that avoiding it is the caller's problem.
> 
> > And also requires device suspend routine to check
> > dev->power.usage_count before suspending device.
> 
> Why?  And which routine exactly are you talking about?

I meant the device suspend callback like dev->bus->pm->suspend. 
Suspend callback need to check dev->power.usage_count because if it's greater than 1 (device_prepare already called pm_runtime_get_noresume() on device so the usage_count is at least 1), it means somewhere else has called pm_runtime_get() on the device thus it should not be suspended. 

> 
> Rafael

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