drm/i915/icl: Refine PG_HYSTERESIS

After doing some measuring, Icelake behaves on a par with Broadwell, and
without having to compromise for low power cores with long latencies, we
can reduce the powergating hysteresis so that the powersaving is enabled
faster. No impact observed on client side throughput measures (so
negligible increase in extra switching), and inspection from high
frequency polling using igt/gem_exec_nop/sequential, provided an estimate
for the upper bound before we can measure a substantial impact on
latency.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191110185806.17413-9-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gt/intel_rc6.c

@@ -88,15 +88,12 @@ static void gen11_rc6_enable(struct intel_rc6 *rc6)
 	 * do not want the enable hysteresis to less than the wakeup latency.
 	 *
 	 * igt/gem_exec_nop/sequential provides a rough estimate for the
-	 * service latency, and puts it around 10us for Broadwell (and other
-	 * big core) and around 40us for Broxton (and other low power cores).
-	 * [Note that for legacy ringbuffer submission, this is less than 1us!]
-	 * However, the wakeup latency on Broxton is closer to 100us. To be
-	 * conservative, we have to factor in a context switch on top (due
-	 * to ksoftirqd).
+	 * service latency, and puts it under 10us for Icelake, similar to
+	 * Broadwell+, To be conservative, we want to factor in a context
+	 * switch on top (due to ksoftirqd).
 	 */
-	set(uncore, GEN9_MEDIA_PG_IDLE_HYSTERESIS, 250);
-	set(uncore, GEN9_RENDER_PG_IDLE_HYSTERESIS, 250);
+	set(uncore, GEN9_MEDIA_PG_IDLE_HYSTERESIS, 60);
+	set(uncore, GEN9_RENDER_PG_IDLE_HYSTERESIS, 60);
 
 	/* 3a: Enable RC6 */
 	set(uncore, GEN6_RC_CONTROL,