linux-kernel - RE: [PATCH v2] Added perf functionality to mmdc driver

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Date:	Tue, 16 Aug 2016 20:40:08 +0000
From:	Zhengyu Shen <zhengyu.shen@....com>
To:	Mark Rutland <mark.rutland@....com>
CC:	"shawnguo@...nel.org" <shawnguo@...nel.org>,
	"peterz@...radead.org" <peterz@...radead.org>,
	Frank Li <frank.li@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"acme@...nel.org" <acme@...nel.org>,
	"alexander.shishkin@...ux.intel.com" 
	<alexander.shishkin@...ux.intel.com>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"lznuaa@...il.com" <lznuaa@...il.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: RE: [PATCH v2] Added perf functionality to mmdc driver

> > > > +	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.