drm/i915: Use max(render, media) for Baytrail busyness calculation

Currently, we sum the render and media cycles (on different engines) to
compute a percentage - but we fail to factor in the duplication into the
threshold calculations. This makes us very eager to upclock!

If we just consider the maximum busy cycles of either counter, we should
have an accurate reflection on whether there are cycles to spare to
handle the workload at this frequency.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170309211232.28878-2-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_irq.c

@@ -1102,6 +1102,7 @@ static u32 vlv_wa_c0_ei(struct drm_i915_private *dev_priv, u32 pm_iir)
 
 	if (prev->cz_clock) {
 		u64 time, c0;
+		u32 render, media;
 		unsigned int mul;
 
 		mul = VLV_CZ_CLOCK_TO_MILLI_SEC * 100; /* scale to threshold% */
@@ -1116,8 +1117,9 @@ static u32 vlv_wa_c0_ei(struct drm_i915_private *dev_priv, u32 pm_iir)
 		 * mesa. To account for this we need to combine both engines
 		 * into our activity counter.
 		 */
-		c0 = now.render_c0 - prev->render_c0;
-		c0 += now.media_c0 - prev->media_c0;
+		render = now.render_c0 - prev->render_c0;
+		media = now.media_c0 - prev->media_c0;
+		c0 = max(render, media);
 		c0 *= mul;
 
 		if (c0 > time * dev_priv->rps.up_threshold)