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

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Date:	Fri, 27 Feb 2015 11:36:11 -0800
From:	Stephen Boyd <sboyd@...eaurora.org>
To:	Ashwin Chaugule <ashwin.chaugule@...aro.org>
CC:	Will Deacon <will.deacon@....com>,
	"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>,
	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 02/25/15 08:58, Ashwin Chaugule wrote:
> On 24 February 2015 at 12:23, Ashwin Chaugule
> <ashwin.chaugule@...aro.org> wrote:
>> On 20 February 2015 at 15:16, Stephen Boyd <sboyd@...eaurora.org> wrote:
>>> 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?
> Its a count control register (PMxEVCNTCR). Theres various conditions
> on which you can select when to start/stop counting. e.g. start when
> another counter register overflows. Setting it to 0 was the
> recommended default value on Scorpions and Kraits. Reset value is
> unpredictable. So, just need to make sure this is set every time a
> counter is setup. Will that still work if this is moved to the reset
> path?
>
>

I don't think anything in this register changes unless we reset the CPU
or software changes it, right? We already have a hotplug notifier to
handle the case where the CPU is hotplugged out (and may get reset) and
we need to reset the PMU. The other case is deep idle, which we don't
have yet but we'll need to do a save/reset/restore across idle when we
get there. So it seems possible to move it to the PMU reset path.

-- 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
a Linux Foundation Collaborative Project

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