On Sat, 9 Aug 2014, Rafael J. Wysocki wrote:
> > > > Well it works currently. So where do you see the problem?
> > >
> > > Sampling registers from an timer - in particular, we really do not want
> > > to disable runtime pm whilst trying to monitor the impact of runtime pm.
> >
> > In that case you can grab a runtime pm reference iff the device is powered
> > on already. Which won't call anything scary, just amounts to an
> > atomic_add_unless or so, and then drop it again.
> >
> > Unfortunately there doesn't seem to be such a thing around already, so
> > need to add it first. Greg, how much would you freak out if we add
> > something like
> >
> > /**
> > * pm_runtime_get_unless_suspended - grab a rpm ref if the device is on
> > *
> > * Returns true if an rpm ref has been acquire, false otherwise. Can be
> > * called from atomic context to e.g. sample perfomance counters (where we
> > * obviously don't want to disturb system state if everything is off atm).
> > */
> > static inline bool pm_runtime_get_unless_suspended(struct device *dev)
> > {
> > return atomic_add_unless(&dev->power.usage_count, 1, 0);
> > }
>
> I don't think it'll work universally.
>
> That'd need to be synchronized with other stuff done under the spinlock
> and in fact, what you're interested in is runtime_status (and that being
> RPM_ACTIVE) and not just the usage count.
That's right. You'd need to acquire the spinlock, test runtime_status,
do the register sampling if the status is RPM_ACTIVE, and then drop the
spinlock.
I suppose wrapper routines for acquiring and releasing the spinlock
could be added to the runtime-PM API. Something like this:
#define pm_runtime_lock(dev, flags) \
spin_lock_irqsave(&(dev)->power.lock, flags)
#define pm_runtime_unlock(dev, flags) \
spin_unlock_irqrestore(&(dev)->power.lock, flags)
It looks a little silly but it would work.
Alan Stern
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