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

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Date:	Tue, 17 Jun 2014 22:26:14 +0200
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Allen Yu <alleny@...dia.com>, Pavel Machek <pavel@....cz>,
	Len Brown <len.brown@...el.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-pm@...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 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).

Rafael

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