drm/i915: Reduce CHV DDL multiplier to 16/8

Apparently we must yet halve the DDL drain latency from what we're
using currently. This little nugget is not in any spec, but came
down through the grapevine.

This makes the displays a bit more stable. Not quite fully stable but at
least they don't fall over immediately on driver load.

v2: Update high_precision in valleyview_update_sprite_wm() too (Jesse)
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_reg.h

@@ -4205,6 +4205,7 @@ enum skl_disp_power_wells {
 #define   DSPFW_PLANEA_WM1_HI_MASK	(1<<0)
 
 /* drain latency register values*/
+#define DRAIN_LATENCY_PRECISION_8	8
 #define DRAIN_LATENCY_PRECISION_16	16
 #define DRAIN_LATENCY_PRECISION_32	32
 #define DRAIN_LATENCY_PRECISION_64	64

drivers/gpu/drm/i915/intel_pm.c

@@ -756,8 +756,8 @@ static bool vlv_compute_drain_latency(struct drm_crtc *crtc,
 
 	entries = DIV_ROUND_UP(clock, 1000) * pixel_size;
 	if (IS_CHERRYVIEW(dev))
-		*prec_mult = (entries > 128) ? DRAIN_LATENCY_PRECISION_32 :
-					       DRAIN_LATENCY_PRECISION_16;
+		*prec_mult = (entries > 32) ? DRAIN_LATENCY_PRECISION_16 :
+					      DRAIN_LATENCY_PRECISION_8;
 	else
 		*prec_mult = (entries > 128) ? DRAIN_LATENCY_PRECISION_64 :
 					       DRAIN_LATENCY_PRECISION_32;
@@ -787,7 +787,7 @@ static void vlv_update_drain_latency(struct drm_crtc *crtc)
 	enum pipe pipe = intel_crtc->pipe;
 	int plane_prec, prec_mult, plane_dl;
 	const int high_precision = IS_CHERRYVIEW(dev) ?
-		DRAIN_LATENCY_PRECISION_32 : DRAIN_LATENCY_PRECISION_64;
+		DRAIN_LATENCY_PRECISION_16 : DRAIN_LATENCY_PRECISION_64;
 
 	plane_dl = I915_READ(VLV_DDL(pipe)) & ~(DDL_PLANE_PRECISION_HIGH |
 		   DRAIN_LATENCY_MASK | DDL_CURSOR_PRECISION_HIGH |
@@ -986,7 +986,7 @@ static void valleyview_update_sprite_wm(struct drm_plane *plane,
 	int sprite_dl;
 	int prec_mult;
 	const int high_precision = IS_CHERRYVIEW(dev) ?
-		DRAIN_LATENCY_PRECISION_32 : DRAIN_LATENCY_PRECISION_64;
+		DRAIN_LATENCY_PRECISION_16 : DRAIN_LATENCY_PRECISION_64;
 
 	sprite_dl = I915_READ(VLV_DDL(pipe)) & ~(DDL_SPRITE_PRECISION_HIGH(sprite) |
 		    (DRAIN_LATENCY_MASK << DDL_SPRITE_SHIFT(sprite)));