linux-kernel - Re: [PATCH v2 2/2] ARM: perf: Add support for Scorpion PMUs

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Date:	Fri, 20 Feb 2015 12:16:27 -0800
From:	Stephen Boyd <sboyd@...eaurora.org>
To:	Will Deacon <will.deacon@....com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-msm@...r.kernel.org" <linux-arm-msm@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Ashwin Chaugule <ashwin.chaugule@...aro.org>,
	Mark Rutland <Mark.Rutland@....com>,
	Neil Leeder <nleeder@...eaurora.org>,
	Ashwin Chaugule <ashwinc@...eaurora.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>
Subject: Re: [PATCH v2 2/2] ARM: perf: Add support for Scorpion PMUs

On 02/20, Will Deacon wrote:
> On Fri, Feb 13, 2015 at 06:24:09PM +0000, Stephen Boyd wrote:
> 
> > +static void scorpion_evt_setup(int idx, u32 config_base)
> > +{
> > +       u32 val;
> > +       u32 mask;
> > +       u32 vval, fval;
> > +       unsigned int region;
> > +       unsigned int group;
> > +       unsigned int code;
> > +       unsigned int group_shift;
> > +       bool venum_event;
> > +
> > +       krait_decode_event(config_base, &region, &group, &code, &venum_event,
> > +                          NULL);
> > +
> > +       group_shift = group * 8;
> > +       mask = 0xff << group_shift;
> > +
> > +       /* Configure evtsel for the region and group */
> > +       if (venum_event)
> > +               val = SCORPION_VLPM_GROUP0;
> > +       else
> > +               val = scorpion_get_pmresrn_event(region);
> > +       val += group;
> > +       /* Mix in mode-exclusion bits */
> > +       val |= config_base & (ARMV7_EXCLUDE_USER | ARMV7_EXCLUDE_PL1);
> > +       armv7_pmnc_write_evtsel(idx, val);
> > +
> > +       asm volatile("mcr p15, 0, %0, c9, c15, 0" : : "r" (0));
> 
> What's this guy doing?

This is the same as Krait. It's clearing some implementation
defined register. From what I can tell it's a per-event register
(i.e. PMSELR decides which event this register write actually
affects) and we do this here to reset this register to some
defined value, zero. Otherwise the reset value of this register
is UNPREDICTABLE and that would be bad. I think we might be able
to move it to the pmu reset path, but I don't know. Ashwin?

> 
> > +static int scorpion_pmu_get_event_idx(struct pmu_hw_events *cpuc,
> > +                                  struct perf_event *event)
> > +{
> > +       int idx;
> > +       int bit = -1;
> > +       unsigned int region;
> > +       unsigned int code;
> > +       unsigned int group;
> > +       bool venum_event, scorpion_event;
> > +       struct hw_perf_event *hwc = &event->hw;
> > +
> > +       krait_decode_event(hwc->config_base, &region, &group, &code,
> > +                          &venum_event, &scorpion_event);
> > +
> > +       if (venum_event || scorpion_event) {
> > +               /* Ignore invalid events */
> > +               if (group > 3 || region > 3)
> 
> Where does the 3 come from?
> 

There are four "R"s and "G"s in the ascii art. This checks to
make sure we're within the range of possible pmresr and groups.
Krait has 3 "R"s and 4 "G"s.

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