drm/i915: enable RC6 workaround on Haswell

For Haswell, on some of the early hardware revisions, it is possible to
run into issues when RC6 state is enabled and when pipes change state.

v2: add comment saying that this is for early revisions only.

v3: beautify as suggested by Daniel Vetter.
Signed-off-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Acked-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_reg.h

@@ -4453,4 +4453,9 @@
 #define  SFUSE_STRAP_DDIC_DETECTED	(1<<1)
 #define  SFUSE_STRAP_DDID_DETECTED	(1<<0)
 
+#define WM_DBG				0x45280
+#define  WM_DBG_DISALLOW_MULTIPLE_LP	(1<<0)
+#define  WM_DBG_DISALLOW_MAXFIFO	(1<<1)
+#define  WM_DBG_DISALLOW_SPRITE		(1<<2)
+
 #endif /* _I915_REG_H_ */

drivers/gpu/drm/i915/intel_pm.c

@@ -3467,6 +3467,16 @@ static void haswell_init_clock_gating(struct drm_device *dev)
 	/* WaDisable4x2SubspanOptimization */
 	I915_WRITE(CACHE_MODE_1,
 		   _MASKED_BIT_ENABLE(PIXEL_SUBSPAN_COLLECT_OPT_DISABLE));
+
+	/* XXX: This is a workaround for early silicon revisions and should be
+	 * removed later.
+	 */
+	I915_WRITE(WM_DBG,
+			I915_READ(WM_DBG) |
+			WM_DBG_DISALLOW_MULTIPLE_LP |
+			WM_DBG_DISALLOW_SPRITE |
+			WM_DBG_DISALLOW_MAXFIFO);
+
 }
 
 static void ivybridge_init_clock_gating(struct drm_device *dev)