drm/i915: Kick the rps worker when changing the boost frequency

The boost frequency is only applied from the RPS worker while someone is
waiting on a request and requested a boost. As such, when the user
wishes to change the frequency, we have to kick the worker in order to
re-evaluate whether to apply the boost frequency.

v2: Check num_waiters to decide if we should kick the worker to handle
boosting.

Fixes: 29ecd78d ("drm/i915: Define a separate variable and control for RPS waitboost frequency")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180308142648.4016-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_sysfs.c

@@ -304,8 +304,9 @@ static ssize_t gt_boost_freq_mhz_store(struct device *kdev,
 {
 	struct drm_i915_private *dev_priv = kdev_minor_to_i915(kdev);
 	struct intel_rps *rps = &dev_priv->gt_pm.rps;
-	u32 val;
+	bool boost = false;
 	ssize_t ret;
+	u32 val;
 
 	ret = kstrtou32(buf, 0, &val);
 	if (ret)
@@ -317,8 +318,13 @@ static ssize_t gt_boost_freq_mhz_store(struct device *kdev,
 		return -EINVAL;
 
 	mutex_lock(&dev_priv->pcu_lock);
+	if (val != rps->boost_freq) {
 		rps->boost_freq = val;
+		boost = atomic_read(&rps->num_waiters);
+	}
 	mutex_unlock(&dev_priv->pcu_lock);
+	if (boost)
+		schedule_work(&rps->work);
 
 	return count;
 }