drm/i915: Enable CB tuning of the Display PLL

Magic numbers from the specs. This is supposed to allow the PLL some
variance to improve jitter performance and VCO headroom across
manufacturing and environmental variations.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/i915_reg.h

@@ -2712,6 +2712,7 @@
 #define PCH_DPLL(pipe) _PIPE(pipe, PCH_DPLL_A, PCH_DPLL_B)
 
 #define PCH_FPA0                0xc6040
+#define  FP_CB_TUNE		(0x3<<22)
 #define PCH_FPA1                0xc6044
 #define PCH_FPB0                0xc6048
 #define PCH_FPB1                0xc604c

drivers/gpu/drm/i915/intel_display.c

@@ -3857,6 +3857,22 @@ static int intel_crtc_mode_set(struct drm_crtc *crtc,
 				reduced_clock.m2;
 	}
 
+	/* Enable autotuning of the PLL clock (if permissible) */
+	if (HAS_PCH_SPLIT(dev)) {
+		int factor = 21;
+
+		if (is_lvds) {
+			if ((dev_priv->lvds_use_ssc &&
+			     dev_priv->lvds_ssc_freq == 100) ||
+			    (I915_READ(PCH_LVDS) & LVDS_CLKB_POWER_MASK) == LVDS_CLKB_POWER_UP)
+				factor = 25;
+		} else if (is_sdvo && is_tv)
+			factor = 20;
+
+		if (clock.m1 < factor * clock.n)
+			fp |= FP_CB_TUNE;
+	}
+
 	dpll = 0;
 	if (!HAS_PCH_SPLIT(dev))
 		dpll = DPLL_VGA_MODE_DIS;
@@ -4071,7 +4087,6 @@ static int intel_crtc_mode_set(struct drm_crtc *crtc,
 	}
 
 	if (!has_edp_encoder || intel_encoder_is_pch_edp(&has_edp_encoder->base)) {
-		I915_WRITE(fp_reg, fp);
 		I915_WRITE(dpll_reg, dpll);
 
 		/* Wait for the clocks to stabilize. */