Re: [PATCH v2] Added perf functionality to mmdc driver

From	Mark Rutland <mark.rutland@arm.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH v2] Added perf functionality to mmdc driver
Date	2016-08-17 12:10 +0200
Message-ID	<s75Xj-81g-11@gated-at.bofh.it> (permalink)
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On Tue, Aug 16, 2016 at 08:40:08PM +0000, Zhengyu Shen wrote:
> > > > > +	hrtimer_start(&pmu_mmdc->hrtimer, mmdc_timer_period(),
> > > > > +			HRTIMER_MODE_REL_PINNED);
> > > >
> > > > Why is a hrtimer necessary? Is this just copy-pasted from CCN, or do
> > > > you have similar HW issues?
> > > >
> > > > Is there no overflow interrupt?
> > >
> > > When overflow occurs, a register bit is set to one. There is no
> > > overflow interrupt which is why the timer is needed.
> > 
> > I see. Please have add comment in the driver explaining this, so that this is
> > obvious.
> > 
> > Does the counter itself wrap and continue counting, or does it saturate?
> > 
> > How have you tuned your polling period so as to avoid missing events in the
> > case of an overflow?
> > 
> > Thanks,
> > Mark.
> The counter wraps around once every ten seconds for total-cycles (which is the 
> Fastest increasing counter). Polling is done every one second just to be safe.

Ok. It would be worth noting this with a comment next to either the
hrtimer handler or registration thereof.

I assume that on overflow the counter wraps rather than saturating?

Thanks,
Mark.

Thread

Re: [PATCH v2] Added perf functionality to mmdc driver Mark Rutland <mark.rutland@arm.com> - 2016-08-16 12:50 +0200
  Re: [PATCH v2] Added perf functionality to mmdc driver Peter Zijlstra <peterz@infradead.org> - 2016-08-16 14:10 +0200
  Re: [PATCH v2] Added perf functionality to mmdc driver Mark Rutland <mark.rutland@arm.com> - 2016-08-16 16:50 +0200
    RE: [PATCH v2] Added perf functionality to mmdc driver Zhengyu Shen <zhengyu.shen@nxp.com> - 2016-08-17 07:20 +0200
      Re: [PATCH v2] Added perf functionality to mmdc driver Mark Rutland <mark.rutland@arm.com> - 2016-08-17 12:10 +0200
  RE: [PATCH v2] Added perf functionality to mmdc driver Zhengyu Shen <zhengyu.shen@nxp.com> - 2016-08-16 19:20 +0200

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