ddrm/i915/bdw: Disable centroid pixel perf optimization

BDW-A workaround

BDW Bug #1899532

v2: WARN on when not using preliminary HW support
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Jesse Barnes <jbarnes@virtuosugeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_reg.h

@@ -5007,6 +5007,9 @@
 #define HSW_ROW_CHICKEN3		0xe49c
 #define  HSW_ROW_CHICKEN3_L3_GLOBAL_ATOMICS_DISABLE    (1 << 6)
 
+#define HALF_SLICE_CHICKEN3		0xe184
+#define   GEN8_CENTROID_PIXEL_OPT_DIS	(1<<8)
+
 #define G4X_AUD_VID_DID			(dev_priv->info->display_mmio_offset + 0x62020)
 #define INTEL_AUDIO_DEVCL		0x808629FB
 #define INTEL_AUDIO_DEVBLC		0x80862801

drivers/gpu/drm/i915/intel_pm.c

@@ -5286,6 +5286,10 @@ static void gen8_init_clock_gating(struct drm_device *dev)
 	/* FIXME(BDW): Check all the w/a, some might only apply to
 	 * pre-production hw. */
 
+	WARN(!i915_preliminary_hw_support,
+	     "GEN8_CENTROID_PIXEL_OPT_DIS not be needed for production\n");
+	I915_WRITE(HALF_SLICE_CHICKEN3,
+		   _MASKED_BIT_ENABLE(GEN8_CENTROID_PIXEL_OPT_DIS));
 	I915_WRITE(GAMTARBMODE, _MASKED_BIT_ENABLE(ARB_MODE_BWGTLB_DISABLE));
 
 	/* WaSwitchSolVfFArbitrationPriority */