drm/i915: Get correct display clock on 945gm

This is according to Mobile Intel® 945 Express Chipset
Family datasheet.

Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Link: http://patchwork.freedesktop.org/patch/msgid/20170131235026.26003-1-arthur@aheymans.xyz
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

drivers/gpu/drm/i915/i915_reg.h

@@ -119,7 +119,7 @@ static inline bool i915_mmio_reg_valid(i915_reg_t reg)
 #define GCFGC	0xf0 /* 915+ only */
 #define   GC_LOW_FREQUENCY_ENABLE	(1 << 7)
 #define   GC_DISPLAY_CLOCK_190_200_MHZ	(0 << 4)
-#define   GC_DISPLAY_CLOCK_333_MHZ	(4 << 4)
+#define   GC_DISPLAY_CLOCK_333_320_MHZ	(4 << 4)
 #define   GC_DISPLAY_CLOCK_267_MHZ_PNV	(0 << 4)
 #define   GC_DISPLAY_CLOCK_333_MHZ_PNV	(1 << 4)
 #define   GC_DISPLAY_CLOCK_444_MHZ_PNV	(2 << 4)

drivers/gpu/drm/i915/intel_display.c

@@ -7411,6 +7411,26 @@ static int i945_get_display_clock_speed(struct drm_i915_private *dev_priv)
 	return 400000;
 }
 
+static int i945gm_get_display_clock_speed(struct drm_i915_private *dev_priv)
+{
+	struct pci_dev *pdev = dev_priv->drm.pdev;
+	u16 gcfgc = 0;
+
+	pci_read_config_word(pdev, GCFGC, &gcfgc);
+
+	if (gcfgc & GC_LOW_FREQUENCY_ENABLE)
+		return 133333;
+	else {
+		switch (gcfgc & GC_DISPLAY_CLOCK_MASK) {
+		case GC_DISPLAY_CLOCK_333_320_MHZ:
+			return 320000;
+		default:
+		case GC_DISPLAY_CLOCK_190_200_MHZ:
+			return 200000;
+		}
+	}
+}
+
 static int i915_get_display_clock_speed(struct drm_i915_private *dev_priv)
 {
 	return 333333;
@@ -7457,7 +7477,7 @@ static int i915gm_get_display_clock_speed(struct drm_i915_private *dev_priv)
 		return 133333;
 	else {
 		switch (gcfgc & GC_DISPLAY_CLOCK_MASK) {
-		case GC_DISPLAY_CLOCK_333_MHZ:
+		case GC_DISPLAY_CLOCK_333_320_MHZ:
 			return 333333;
 		default:
 		case GC_DISPLAY_CLOCK_190_200_MHZ:
@@ -16268,9 +16288,12 @@ void intel_init_display_hooks(struct drm_i915_private *dev_priv)
 	else if (IS_I915G(dev_priv))
 		dev_priv->display.get_display_clock_speed =
 			i915_get_display_clock_speed;
-	else if (IS_I945GM(dev_priv) || IS_I845G(dev_priv))
+	else if (IS_I845G(dev_priv))
 		dev_priv->display.get_display_clock_speed =
 			i9xx_misc_get_display_clock_speed;
+	else if (IS_I945GM(dev_priv))
+		dev_priv->display.get_display_clock_speed =
+			i945gm_get_display_clock_speed;
 	else if (IS_I915GM(dev_priv))
 		dev_priv->display.get_display_clock_speed =
 			i915gm_get_display_clock_speed;