linux-kernel - Re: [PATCH 1/2] PM / QoS: Create device constraints objects on notifier registration

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Message-Id: <201204282310.37811.rjw@sisk.pl>
Date:	Sat, 28 Apr 2012 23:10:37 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	markgross@...gnar.org
Cc:	Linux PM list <linux-pm@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Kevin Hilman <khilman@...com>,
	Magnus Damm <magnus.damm@...il.com>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	Jean Pihet <j-pihet@...com>
Subject: Re: [PATCH 1/2] PM / QoS: Create device constraints objects on notifier registration

On Saturday, April 28, 2012, mark gross wrote:
> On Fri, Apr 27, 2012 at 11:48:18PM +0200, Rafael J. Wysocki wrote:
> > From: Rafael J. Wysocki <rjw@...k.pl>
> > 
> > The current behavior of dev_pm_qos_add_notifier() makes device PM QoS
> > notifiers less than useful.  Namely, it silently returns success when
> > called before any PM QoS constraints are added for the device, so the
> > caller will assume that the notifier has been registered, but when
> > someone actually adds some nontrivial constraints for the device
> > eventually, the previous callers of dev_pm_qos_add_notifier()
> > will not know about that and their notifier routines will not be
> > executed (contrary to their expectations).
> > 
> > To address this problem make dev_pm_qos_add_notifier() create the
> > constraints object for the device if it is not present when the
> > routine is called.
> > 
> > Signed-off-by: Rafael J. Wysocki <rjw@...k.pl>
> > ---
> >  drivers/base/power/qos.c |   19 ++++++++++++-------
> >  1 file changed, 12 insertions(+), 7 deletions(-)
> > 
> > Index: linux/drivers/base/power/qos.c
> > ===================================================================
> > --- linux.orig/drivers/base/power/qos.c
> > +++ linux/drivers/base/power/qos.c
> > @@ -352,21 +352,26 @@ EXPORT_SYMBOL_GPL(dev_pm_qos_remove_requ
> >   *
> >   * Will register the notifier into a notification chain that gets called
> >   * upon changes to the target value for the device.
> > + *
> > + * If the device's constraints object doesn't exist when this routine is called,
> > + * it will be created (or error code will be returned if that fails).
> >   */
> >  int dev_pm_qos_add_notifier(struct device *dev, struct notifier_block *notifier)
> >  {
> > -	int retval = 0;
> > +	int ret = 0;
> >  
> >  	mutex_lock(&dev_pm_qos_mtx);
> >  
> > -	/* Silently return if the constraints object is not present. */
> > -	if (dev->power.constraints)
> > -		retval = blocking_notifier_chain_register(
> > -				dev->power.constraints->notifiers,
> > -				notifier);
> FWIW (and IIRC) I did this because some audio use of pm_qos had a series
> of re-allocation of constraints and clean up that would fall over so I
> had this silent hack to deal with that "complexity".
> 
> > +	if (!dev->power.constraints)
> > +		ret = dev->power.power_state.event != PM_EVENT_INVALID ?
> > +			dev_pm_qos_constraints_allocate(dev) : -ENODEV;
> > +
> > +	if (!ret)
> > +		ret = blocking_notifier_chain_register(
> > +				dev->power.constraints->notifiers, notifier);
> >  
> >  	mutex_unlock(&dev_pm_qos_mtx);
> > -	return retval;
> > +	return ret;
> >  }
> >  EXPORT_SYMBOL_GPL(dev_pm_qos_add_notifier);
> >  
> Definitely a change for the better (but we should check that the audio
> stuff still works before committing this.  I'll see if I can check that
> this weekend.)

Well, I don't see why not.  Obviously, we won't set the dev->power.constraints
pointer more than once. :-)

> Acked-by : markgross <markgross@...gnar.org>

Thanks!

Rafael
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