drm/i915/pmu: Clear the previous sample value when parking

When turning off the engines, and the pmu sampling, clear the previous
value as the current measurement should be 0.

v2: Use a for-loop
v3:
 * Move clearing to timer self-dis-arm to avoid race with parking.
 * Clear frequency samples as well.
v4:
 * Init frequency to idle_freq. (Chris Wilson)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> (v3)
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20171123102654.29296-1-tvrtko.ursulin@linux.intel.com

drivers/gpu/drm/i915/i915_pmu.c

@@ -262,13 +262,31 @@ static void frequency_sample(struct drm_i915_private *dev_priv)
 	}
 }
 
+static void pmu_init_previous_samples(struct drm_i915_private *i915)
+{
+	struct intel_engine_cs *engine;
+	enum intel_engine_id id;
+	unsigned int i;
+
+	for_each_engine(engine, i915, id) {
+		for (i = 0; i < ARRAY_SIZE(engine->pmu.sample); i++)
+			engine->pmu.sample[i].prev = 0;
+	}
+
+	for (i = 0; i < ARRAY_SIZE(i915->pmu.sample); i++)
+		i915->pmu.sample[i].prev = i915->gt_pm.rps.idle_freq;
+}
+
 static enum hrtimer_restart i915_sample(struct hrtimer *hrtimer)
 {
 	struct drm_i915_private *i915 =
 		container_of(hrtimer, struct drm_i915_private, pmu.timer);
 
-	if (!READ_ONCE(i915->pmu.timer_enabled))
+	if (!READ_ONCE(i915->pmu.timer_enabled)) {
+		pmu_init_previous_samples(i915);
+
 		return HRTIMER_NORESTART;
+	}
 
 	engines_sample(i915);
 	frequency_sample(i915);
@@ -839,6 +857,8 @@ void i915_pmu_register(struct drm_i915_private *i915)
 	hrtimer_init(&i915->pmu.timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
 	i915->pmu.timer.function = i915_sample;
 
+	pmu_init_previous_samples(i915);
+
 	for_each_engine(engine, i915, id)
 		INIT_DELAYED_WORK(&engine->pmu.disable_busy_stats,
 				  __disable_busy_stats);