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[RFC] perf/x86/intel/uncore: pmu->type->single_fixed question

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hi,
I'm trying to find out some documentation background for this part of uncore code:

---
static int uncore_pmu_event_init(struct perf_event *event)
{
        ...
        if (event->attr.config == UNCORE_FIXED_EVENT) {
                /* no fixed counter */
                if (!pmu->type->fixed_ctl)
                        return -EINVAL;
                /*
                 * if there is only one fixed counter, only the first pmu
                 * can access the fixed counter
                 */
                if (pmu->type->single_fixed && pmu->pmu_idx > 0)
                        return -EINVAL;
        ...
---

that for some uncore types (those with pmu->type->single_fixed) only
the first pmu (code_id == 0) will allow to touch the clocktick event

other cores boxes will not allow to open clocktick event, eventhough
it's announced via /sys/../events/.. 

I'm probably missing some HW logic of specific boxes that would explain
that, but I can't find it.

thanks for info,
jirka

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[RFC] perf/x86/intel/uncore: pmu->type->single_fixed question Jiri Olsa <jolsa@redhat.com> - 2016-11-30 12:00 +0100
  RE: [RFC] perf/x86/intel/uncore: pmu->type->single_fixed question "Liang, Kan" <kan.liang@intel.com> - 2016-11-30 15:50 +0100
    Re: [RFC] perf/x86/intel/uncore: pmu->type->single_fixed question Andi Kleen <andi@firstfloor.org> - 2016-11-30 19:00 +0100
      Re: [RFC] perf/x86/intel/uncore: pmu->type->single_fixed question Peter Zijlstra <peterz@infradead.org> - 2016-12-01 17:40 +0100
      RE: [RFC] perf/x86/intel/uncore: pmu->type->single_fixed question "Liang, Kan" <kan.liang@intel.com> - 2016-12-01 17:40 +0100
  RE: [RFC] perf/x86/intel/uncore: pmu->type->single_fixed question "Liang, Kan" <kan.liang@intel.com> - 2016-11-30 16:30 +0100
    Re: [RFC] perf/x86/intel/uncore: pmu->type->single_fixed question Jiri Olsa <jolsa@redhat.com> - 2016-11-30 16:40 +0100

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