Re: [PATCH] arm64: dts: allwinner: a64: Drop PMU node

Maxime Ripard
2019-08-08T16:26:50Z

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On Wed, Aug 07, 2019 at 10:36:08AM -0700, Vasily Khoruzhick wrote:
> On Wed, Aug 7, 2019 at 4:56 AM Maxime Ripard <maxime.ripard@bootlin.com> wrote:
> >
> > On Tue, Aug 06, 2019 at 07:39:26PM -0700, Vasily Khoruzhick wrote:
> > > On Tue, Aug 6, 2019 at 2:14 PM Robin Murphy <robin.murphy@arm.com> wrote:
> > > >
> > > > On 2019-08-06 9:52 pm, Vasily Khoruzhick wrote:
> > > > > On Tue, Aug 6, 2019 at 1:19 PM Harald Geyer <harald@ccbib.org> wrote:
> > > > >>
> > > > >> Vasily Khoruzhick writes:
> > > > >>> On Tue, Aug 6, 2019 at 7:35 AM Robin Murphy <robin.murphy@arm.com> wrote:
> > > > >>>>
> > > > >>>> On 06/08/2019 15:01, Vasily Khoruzhick wrote:
> > > > >>>>> Looks like PMU in A64 is broken, it generates no interrupts at all and
> > > > >>>>> as result 'perf top' shows no events.
> > > > >>>>
> > > > >>>> Does something like 'perf stat sleep 1' at least count cycles correctly?
> > > > >>>> It could well just be that the interrupt numbers are wrong...
> > > > >>>
> > > > >>> Looks like it does, at least result looks plausible:
> > > > >>
> > > > >> I'm using perf stat regularly (cache benchmarks) and it works fine.
> > > > >>
> > > > >> Unfortunately I wasn't aware that perf stat is a poor test for
> > > > >> the interrupts part of the node, when I added it. So I'm not too
> > > > >> surprised I got it wrong.
> > > > >>
> > > > >> However, it would be unfortunate if the node got removed completely,
> > > > >> because perf stat would not work anymore. Maybe we can only remove
> > > > >> the interrupts or just fix them even if the HW doesn't work?
> > > > >
> > > > > I'm not familiar with PMU driver. Is it possible to get it working
> > > > > without interrupts?
> > > >
> > > > Yup - you get a grumpy message from the driver, it will refuse sampling
> > > > events (the ones which weren't working anyway), and if you measure
> > > > anything for long enough that a counter overflows you'll get wonky
> > > > results. But for counting hardware events over relatively short periods
> > > > it'll still do the job.
> > >
> > > I tried to drop interrupts completely from the node but 'perf top' is
> > > still broken. Though now in different way: it complains "cycles: PMU
> > > Hardware doesn't support sampling/overflow-interrupts. Try 'perf
> > > stat'"
> >
> > I have no idea if that's the culprit, but what is the state of the
> > 0x09010000 register?
>
> What register is that and how do I check it?

It's in the CPUX Configuration block, and the bits are labelled as CPU
Debug Reset.

And if you have busybox, you can use devmem.

Maxime

--
Maxime Ripard, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com

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