drm/i915/bxt: add GEN8_HDCUNIT_CLOCK_GATE_DISABLE_HDCREQ workaround

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Nick Hoath <nicholas.hoath@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_reg.h

@@ -6104,6 +6104,7 @@ enum skl_disp_power_wells {
 #define GEN8_UCGCTL6				0x9430
 #define   GEN8_GAPSUNIT_CLOCK_GATE_DISABLE	(1<<24)
 #define   GEN8_SDEUNIT_CLOCK_GATE_DISABLE	(1<<14)
+#define   GEN8_HDCUNIT_CLOCK_GATE_DISABLE_HDCREQ (1<<28)
 
 #define GEN6_GFXPAUSE				0xA000
 #define GEN6_RPNSWREQ				0xA008

drivers/gpu/drm/i915/intel_pm.c

@@ -103,10 +103,12 @@ static void bxt_init_clock_gating(struct drm_device *dev)
 	/*
 	 * FIXME:
 	 * GEN8_SDEUNIT_CLOCK_GATE_DISABLE applies on A0 only.
+	 * GEN8_HDCUNIT_CLOCK_GATE_DISABLE_HDCREQ applies on 3x6 GT SKUs only.
 	 */
 	 /* WaDisableSDEUnitClockGating:bxt */
 	I915_WRITE(GEN8_UCGCTL6, I915_READ(GEN8_UCGCTL6) |
-		   GEN8_SDEUNIT_CLOCK_GATE_DISABLE);
+		   GEN8_SDEUNIT_CLOCK_GATE_DISABLE |
+		   GEN8_HDCUNIT_CLOCK_GATE_DISABLE_HDCREQ);
 
 }