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

[will.deacon@arm.com (Will Deacon)]

On Thu, Feb 25, 2016 at 09:32:17AM -0700, Mathieu Poirier wrote:
> On 25 February 2016 at 02:44, Lorenzo Pieralisi
> <lorenzo.pieralisi@arm.com> wrote:
> > On Wed, Feb 24, 2016 at 05:10:20PM -0800, Kevin Hilman wrote:
> >> Lorenzo Pieralisi <lorenzo.pieralisi@arm.com> writes:
> >>
> >> > When a CPU is suspended (either through suspend-to-RAM or CPUidle),
> >> > its PMU registers content can be lost, which means that counters
> >> > registers values that were initialized on power down entry have to be
> >> > reprogrammed on power-up to make sure the counters set-up is preserved
> >>
> >> This assumes that the PMUs are always sharing a power rail with a
> >> specific CPU, not the cluster.  Is that a reliable assumption?
> >
> > As reliable as assuming the GIC HW state needs save/restore on idle-state
> > entry, if we have no power domains information linked to CPU PM
> > notifiers saving/restoring everything every time an idle state deeper
> > than wfi is entered is the best we can do.
> >
> > As far as this patch is concerned, if the PMU is on a different power
> > domain than the CPU, I agree that stopping/resetting/restoring the
> > PMU registers is a waste of cycles (I will test it also by triggering
> > the notifiers on wfi, to make sure the reset is not disruptive on
> > a cpu that is not powered off), I see no alternative other than disabling
> > idle to carry out profiling sanely (or implement CPU PM notifiers with
> > runtime PM).
> 
> I totally understand - I'm faced with the same problem on Coresight.
> To me the ultimate solution is still to integrate CPU power domain
> management with runtime PM (like we talked about many times before).
> I know Lina is working on something that will get us close to that but
> it's not finished yet.

I've been waiting for that code for *years* now...

Ultimately, Juno-r2 loses its PMU state over idle if you run mainline
on it and, worse still, you end up getting junk numbers back to userspace,
so it's not even obvious what happened.

I'm not aware of any flags that indicate loss of state.

Will