linux-kernel - Re: [PATCH v2 8/8] drivers/perf: Add Apple icestorm/firestorm CPU PMU driver

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Message-ID: <871r2tmze3.wl-maz@kernel.org>
Date:   Fri, 03 Dec 2021 16:22:44 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, Will Deacon <will@...nel.org>,
        Hector Martin <marcan@...can.st>,
        Sven Peter <sven@...npeter.dev>,
        Alyssa Rosenzweig <alyssa@...enzweig.io>,
        Rob Herring <robh+dt@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dougall <dougallj@...il.com>, kernel-team@...roid.com
Subject: Re: [PATCH v2 8/8] drivers/perf: Add Apple icestorm/firestorm CPU PMU driver

On Fri, 03 Dec 2021 12:04:35 +0000,
Mark Rutland <mark.rutland@....com> wrote:
> 
> On Fri, Dec 03, 2021 at 11:22:53AM +0000, Marc Zyngier wrote:
> > On Thu, 02 Dec 2021 16:14:01 +0000, Mark Rutland <mark.rutland@....com> wrote:
> > > On Thu, Dec 02, 2021 at 03:39:46PM +0000, Marc Zyngier wrote:
> > > > On Wed, 01 Dec 2021 16:58:10 +0000, Mark Rutland <mark.rutland@....com> wrote:
> > > > > On Wed, Dec 01, 2021 at 01:49:09PM +0000, Marc Zyngier wrote:
> > > > > > +	state = read_sysreg_s(SYS_IMP_APL_PMCR0_EL1);
> > > > > > +	overflow = read_sysreg_s(SYS_IMP_APL_PMSR_EL1);
> > > > > 
> > > > > I assume the overflow behaviour is free-running rather than stopping?
> > > > 
> > > > Configurable, apparently. At the moment, I set it to stop on overflow.
> > > > Happy to change the behaviour though.
> > > 
> > > The architected PMU continues counting upon overflow (which prevents
> > > losing counts around the overlflow occurring), so I'd prefer that.
> > > 
> > > Is that behaviour per-counter, or for the PMU as a whole?
> > 
> > It is global. This will probably require some additional rework to
> > clear bit 47 in overflowing counters, which we can't do atomically.
> 
> Ah; I see.
> 
> To calrify my comment above, the reason for wanting the counter to keep
> counting is to count during the window between the IRQ being asserted and the
> PMU IRQ handler being invoked, and it's fine for there to be a blackout period
> *within* the PMU IRQ handler.
> 
> So for example it would be fine to have:
> 
> 	irq_handler() 
> 	{
> 		if (!any_counter_overflowed())
> 			return IRQ_NONE;
> 
> 		stop_all_counters();
> 
> 		for_each_counter(c) {
> 			handle_counter(c);
> 		}
> 		
> 		start_all_counters();
> 
> 		return IRQ_HANDLED;
> 
> 	}
> 
> ... and I think with that the regular per-counter period
> reprogramming would do the right thing?

Yup. It looks like this works just fine.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.