Message-ID: <4216316.IaLNu46z52@vostro.rjw.lan>
Date:	Thu, 19 Jun 2014 15:55:31 +0200
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Allen Yu <alleny@...dia.com>
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 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.

> @@ -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.

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?

Rafael

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