[Intel-gfx] [PATCH v2] PM / Runtime: Introduce pm_runtime_get_noidle

[Rafael J. Wysocki]

On Friday, December 11, 2015 04:59:45 PM Ulf Hansson wrote:
> On 11 December 2015 at 16:13, Rafael J. Wysocki <rjw at rjwysocki.net> wrote:
> > On Friday, December 11, 2015 01:03:50 PM Ulf Hansson wrote:
> >> [...]
> >>
> >> >> >
> >> >> > Which basically means you can call pm_runtime_resume() just fine,
> >> >> > because it will do nothing if the status is RPM_ACTIVE already.
> >> >> >
> >> >> > So really, why don't you use pm_runtime_get_sync()?
> >> >>
> >> >> The difference would be that if the status is not RPM_ACTIVE already we
> >> >> would drop the reference and report error. The caller would in this
> >> >> case forego of doing something, since we the device is suspended or on
> >> >> the way to being suspended. One example of such a scenario is a
> >> >> watchdog like functionality: the watchdog work would
> >> >> call pm_runtime_get_noidle() and check if the device is ok by doing
> >> >> some HW access, but only if the device is powered. Otherwise the work
> >> >> item would do nothing (meaning it also won't reschedule itself). The
> >> >> watchdog work would get rescheduled next time the device is woken up
> >> >> and some work is submitted to the device.
> >> >
> >> > So first of all the name "pm_runtime_get_noidle" doesn't make sense.
> >> >
> >> > I guess what you need is something like
> >> >
> >> > bool pm_runtime_get_if_active(struct device *dev)
> >> > {
> >> >         unsigned log flags;
> >> >         bool ret;
> >> >
> >> >         spin_lock_irqsave(&dev->power.lock, flags);
> >> >
> >> >         if (dev->power.runtime_status == RPM_ACTIVE) {
> >> >                 atomic_inc(&dev->power.usage_count);
> >> >                 ret = true;
> >> >         } else {
> >> >                 ret = false;
> >> >         }
> >> >
> >> >         spin_unlock_irqrestore(&dev->power.lock, flags);
> >> > }
> >> >
> >> > and the caller will simply bail out if "false" is returned, but if "true"
> >> > is returned, it will have to drop the usage count, right?
> >> >
> >> > Thanks,
> >> > Rafael
> >> >
> >>
> >> Why not just:
> >>
> >> pm_runtime_get_noresume():
> >> if (RPM_ACTIVE)
> >>   "do some actions"
> >> pm_runtime_put();
> >
> > Because that's racy?
> 
> Right, that was too easy. :-)
> 
> >
> > What if the rpm_suspend() is running for the device, but it hasn't changed
> > the status yet?
> 
> So if we can add a pm_runtime_barrier() or even simplifier, just hold
> the spin_lock when checking if the rpm status is RPM_ACTIVE.

That would work too, but then we'd need to carry out one extra atomic operation.

Thanks,
Rafael