drm/i915: Use CK505 as non-SSC source where available

When trying to use SSC on Ibex Peak without CK505, any non-SSC outputs
(like VGA or TV) get broken. So, do not use SSC on Ibex Peak unless
there is a CK505 available (as specified by the VBT).

On Cougar Point, all clocking is internal, so SSC can always be used,
and there will never be a CK505 available.

This eliminates VGA shimmer on some Ironlake machines which have a
CK505 clock source.

References: https://bugzilla.kernel.org/show_bug.cgi?id=21742
References: https://bugs.freedesktop.org/show_bug.cgi?id=38750
Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/intel_display.c

@@ -5119,6 +5119,8 @@ static void ironlake_update_pch_refclk(struct drm_device *dev)
 	bool has_cpu_edp = false;
 	bool has_pch_edp = false;
 	bool has_panel = false;
+	bool has_ck505 = false;
+	bool can_ssc = false;
 
 	/* We need to take the global config into account */
 	list_for_each_entry(encoder, &mode_config->encoder_list,
@@ -5137,8 +5139,18 @@ static void ironlake_update_pch_refclk(struct drm_device *dev)
 			break;
 		}
 	}
-	DRM_DEBUG_KMS("has_panel %d has_lvds %d has_pch_edp %d has_cpu_edp %d\n",
-		      has_panel, has_lvds, has_pch_edp, has_cpu_edp);
+
+	if (HAS_PCH_IBX(dev)) {
+		has_ck505 = dev_priv->display_clock_mode;
+		can_ssc = has_ck505;
+	} else {
+		has_ck505 = false;
+		can_ssc = true;
+	}
+
+	DRM_DEBUG_KMS("has_panel %d has_lvds %d has_pch_edp %d has_cpu_edp %d has_ck505 %d\n",
+		      has_panel, has_lvds, has_pch_edp, has_cpu_edp,
+		      has_ck505);
 
 	/* Ironlake: try to setup display ref clock before DPLL
 	 * enabling. This is only under driver's control after
@@ -5148,14 +5160,18 @@ static void ironlake_update_pch_refclk(struct drm_device *dev)
 	temp = I915_READ(PCH_DREF_CONTROL);
 	/* Always enable nonspread source */
 	temp &= ~DREF_NONSPREAD_SOURCE_MASK;
-	temp |= DREF_NONSPREAD_SOURCE_ENABLE;
+
+	if (has_ck505)
+		temp |= DREF_NONSPREAD_CK505_ENABLE;
+	else
+		temp |= DREF_NONSPREAD_SOURCE_ENABLE;
 
 	if (has_panel) {
 		temp &= ~DREF_SSC_SOURCE_MASK;
 		temp |= DREF_SSC_SOURCE_ENABLE;
 
 		/* SSC must be turned on before enabling the CPU output  */
-		if (intel_panel_use_ssc(dev_priv)) {
+		if (intel_panel_use_ssc(dev_priv) && can_ssc) {
 			DRM_DEBUG_KMS("Using SSC on panel\n");
 			temp |= DREF_SSC1_ENABLE;
 		}
@@ -5169,7 +5185,7 @@ static void ironlake_update_pch_refclk(struct drm_device *dev)
 
 		/* Enable CPU source on CPU attached eDP */
 		if (has_cpu_edp) {
-			if (intel_panel_use_ssc(dev_priv)) {
+			if (intel_panel_use_ssc(dev_priv) && can_ssc) {
 				DRM_DEBUG_KMS("Using SSC on eDP\n");
 				temp |= DREF_CPU_SOURCE_OUTPUT_DOWNSPREAD;
 			}