[Intel-gfx] [PATCH 1/2] drm/i915: Increase render/media power gating hysteresis for gen9+

[Chris Wilson]

On gen9+, after an idle period the HW will disable the entire power well
to conserve power (by preventing current leakage). It takes around a 100
microseconds to bring the power well back online afterwards. With the
current hysteresis value of 25us, we do not have sufficient time to
respond to an interrupt and schedule the next execution before the HW
powers itself down. (At present, we prevent this by grabbing the
forcewake for prolonged periods of time, but that is overkill fixed in
the next patch.) The minimum we want to set the power gating hysteresis
to is the length of time it takes us to service the GPU, which across a
broad spectrum of machines is about 250us.

(Note this also brings guc latency into the same ballpark as execlists.)

Testcase: igt/gem_exec_nop/sequential
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko at intel.com>
Cc: Sagar Arun Kamble <sagar.a.kamble at intel.com>
Cc: Michel Thierry <michel.thierry at intel.com>
Cc: Michal Winiarski <michal.winiarski at intel.com>
---
 drivers/gpu/drm/i915/intel_pm.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_pm.c b/drivers/gpu/drm/i915/intel_pm.c
index 1db79a860b96..6748d3efb537 100644
--- a/drivers/gpu/drm/i915/intel_pm.c
+++ b/drivers/gpu/drm/i915/intel_pm.c
@@ -6627,8 +6627,8 @@ static void gen9_enable_rc6(struct drm_i915_private *dev_priv)
 	I915_WRITE(GEN6_RC_SLEEP, 0);
 
 	/* 2c: Program Coarse Power Gating Policies. */
-	I915_WRITE(GEN9_MEDIA_PG_IDLE_HYSTERESIS, 25);
-	I915_WRITE(GEN9_RENDER_PG_IDLE_HYSTERESIS, 25);
+	I915_WRITE(GEN9_MEDIA_PG_IDLE_HYSTERESIS, 250);
+	I915_WRITE(GEN9_RENDER_PG_IDLE_HYSTERESIS, 250);
 
 	/* 3a: Enable RC6 */
 	I915_WRITE(GEN6_RC6_THRESHOLD, 37500); /* 37.5/125ms per EI */
-- 
2.15.1