Re: [PATCH 01/32] perf: Allow a PMU to have a parent

[Robin Murphy]

On 2023-04-06 17:44, Jonathan Cameron wrote:
> On Thu, 6 Apr 2023 14:40:40 +0200
> Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> > On Thu, Apr 06, 2023 at 11:16:07AM +0100, Jonathan Cameron wrote:
> >
> > > In the long run I agree it would be good.  Short term there are more instances of
> > > struct pmu that don't have parents than those that do (even after this series).
> > > We need to figure out what to do about those before adding checks on it being
> > > set.
> >
> > Right, I don't think you've touched *any* of the x86 PMUs for example,
> > and getting everybody that boots an x86 kernel a warning isn't going to
> > go over well :-)
>
> It was tempting :) "Warning: Parentless PMU: try a different architecture."
>
> I'd love some inputs on what the x86 PMU devices parents should be?
> CPU counters in general tend to just spin out of deep in the architecture code.
>
> My overall favorite is an l2 cache related PMU that is spun up in
> arch/arm/kernel/irq.c init_IRQ()
>
> I'm just not going to try and figure out why...

I think that's simply because the PMU support was hung off the existing 
PL310 configuration code, which still supports non-DT boardfiles. The 
PMU shouldn't strictly need to be registered that early, it would just 
be a bunch more work to ensure that a platform device is available for 
it to bind to as a regular driver model driver, which wasn't justifiable 
at the time.

Thanks,
Robin.