drm/i915/pmu: Correct the rc6 offset upon enabling

The rc6 residency starts ticking from 0 from BIOS POST, but the kernel
starts measuring the time from its boot. If we start measuruing
I915_PMU_RC6_RESIDENCY while the GT is idle, we start our sampling from
0 and then upon first activity (park/unpark) add in all the rc6
residency since boot. After the first park with the sampler engaged, the
sleep/active counters are aligned.

v2: With a wakeref to be sure

Closes: https://gitlab.freedesktop.org/drm/intel/issues/973
Fixes: df6a4205 ("drm/i915/pmu: Ensure monotonic rc6")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200114105648.2172026-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_pmu.c

@@ -637,8 +637,10 @@ static void i915_pmu_enable(struct perf_event *event)
 		container_of(event->pmu, typeof(*i915), pmu.base);
 	unsigned int bit = event_enabled_bit(event);
 	struct i915_pmu *pmu = &i915->pmu;
+	intel_wakeref_t wakeref;
 	unsigned long flags;
 
+	wakeref = intel_runtime_pm_get(&i915->runtime_pm);
 	spin_lock_irqsave(&pmu->lock, flags);
 
 	/*
@@ -648,6 +650,14 @@ static void i915_pmu_enable(struct perf_event *event)
 	BUILD_BUG_ON(ARRAY_SIZE(pmu->enable_count) != I915_PMU_MASK_BITS);
 	GEM_BUG_ON(bit >= ARRAY_SIZE(pmu->enable_count));
 	GEM_BUG_ON(pmu->enable_count[bit] == ~0);
+
+	if (pmu->enable_count[bit] == 0 &&
+	    config_enabled_mask(I915_PMU_RC6_RESIDENCY) & BIT_ULL(bit)) {
+		pmu->sample[__I915_SAMPLE_RC6_LAST_REPORTED].cur = 0;
+		pmu->sample[__I915_SAMPLE_RC6].cur = __get_rc6(&i915->gt);
+		pmu->sleep_last = ktime_get();
+	}
+
 	pmu->enable |= BIT_ULL(bit);
 	pmu->enable_count[bit]++;
 
@@ -688,6 +698,8 @@ static void i915_pmu_enable(struct perf_event *event)
 	 * an existing non-zero value.
 	 */
 	local64_set(&event->hw.prev_count, __i915_pmu_event_read(event));
+
+	intel_runtime_pm_put(&i915->runtime_pm, wakeref);
 }
 
 static void i915_pmu_disable(struct perf_event *event)