Re: [PATCH 1/1] PM / Runtime: let rpm_resume fail if rpm disabled and device suspended.

["Rafael J. Wysocki" <rjw@rjwysocki.net>]

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. :-)

> > > 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.

Rafael