[PATCH] drivers: perf: arm: implement CPU_PM notifier

[Ashwin Chaugule]

Hi Lorenzo,

On 24 February 2016 at 12:35, Lorenzo Pieralisi
<lorenzo.pieralisi at arm.com> wrote:
> On Wed, Feb 24, 2016 at 09:20:22AM -0700, Mathieu Poirier wrote:
>> On 23 February 2016 at 11:22, Lorenzo Pieralisi
>
> [...]
>
>> > +static int cpu_pm_pmu_notify(struct notifier_block *b, unsigned long cmd,
>> > +                            void *v)
>> > +{
>> > +       struct arm_pmu *armpmu = container_of(b, struct arm_pmu, cpu_pm_nb);
>> > +       struct pmu_hw_events *hw_events = this_cpu_ptr(armpmu->hw_events);
>> > +       int enabled = bitmap_weight(hw_events->used_mask, armpmu->num_events);
>> > +
>> > +       if (!cpumask_test_cpu(smp_processor_id(), &armpmu->supported_cpus))
>> > +               return NOTIFY_DONE;
>> > +
>> > +       /*
>> > +        * Always reset the PMU registers on power-up even if
>> > +        * there are no events running.
>> > +        */
>> > +       if (cmd == CPU_PM_EXIT && armpmu->reset)
>> > +               armpmu->reset(armpmu);
>>
>> I think this patch does the right thing but I can't get the above
>> reset.  Wouldn't it be better to do the reset as part of the
>> CPU_PM_EXIT case below?  At this point nothing tells us the CPU won't
>> go back down before the event is enabled, wasting the cycle needed to
>> reset the PMU.
>
> The logic goes, if the cpu is woken up and it has no events enabled,
> if we do not reset it (mind, ->reset here sets the PMU register values
> to a sane default, some of them are architecturally UNKNOWN on reset, it
> does NOT reset the PMU) _and_ we subsequently install an event on it we
> do have a problem, that's why whenever a core gets out of low-power we
> have to reset its pmu.

Dont we blow out the previous value in the counter when installing an
event? Or has that changed lately? IIRC, there was some initial value
we'd program into the counter to avoid missing the overflow event.
(sorry its been too long) ;)

Cheers,
Ashwin.