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

[Ashwin Chaugule <ashwin.chaugule@linaro.org>]

On 24 February 2015 at 12:23, Ashwin Chaugule
<ashwin.chaugule@linaro.org> wrote:
> On 20 February 2015 at 15:16, Stephen Boyd <sboyd@codeaurora.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?

Cheers,
Ashwin.