drm/i915/cnl: Fix DP max rate for Cannonlake with port F.

On CNL SKUs that uses port F,  max DP rate is 8.1G for all
ports when we have the elevated voltage (higher than 0.85V).

v2: Make commit message more generic.
v3: Move conditions to a helper to get easier to read. (Ville).
v4: Add a mention to the numerical voltage on commit
    message per Manasi request.
v5: Thanks CI! "error: control reaches end of non-void function"

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180129232223.766-10-rodrigo.vivi@intel.com

drivers/gpu/drm/i915/intel_dp.c

@@ -218,15 +218,36 @@ intel_dp_downstream_max_dotclock(struct intel_dp *intel_dp)
 	return max_dotclk;
 }
 
+static int cnl_adjusted_max_rate(struct intel_dp *intel_dp, int size)
+{
+	struct intel_digital_port *dig_port = dp_to_dig_port(intel_dp);
+	struct drm_i915_private *dev_priv = to_i915(dig_port->base.base.dev);
+	enum port port = dig_port->base.port;
+
+	u32 voltage = I915_READ(CNL_PORT_COMP_DW3) & VOLTAGE_INFO_MASK;
+
+	/* Low voltage SKUs are limited to max of 5.4G */
+	if (voltage == VOLTAGE_INFO_0_85V)
+		return size - 2;
+
+	/* For this SKU 8.1G is supported in all ports */
+	if (IS_CNL_WITH_PORT_F(dev_priv))
+		return size;
+
+	/* For other SKUs, max rate on ports A and B is 5.4G */
+	if (port == PORT_A || port == PORT_D)
+		return size - 2;
+
+	return size;
+}
+
 static void
 intel_dp_set_source_rates(struct intel_dp *intel_dp)
 {
 	struct intel_digital_port *dig_port = dp_to_dig_port(intel_dp);
 	struct drm_i915_private *dev_priv = to_i915(dig_port->base.base.dev);
-	enum port port = dig_port->base.port;
 	const int *source_rates;
 	int size;
-	u32 voltage;
 
 	/* This should only be done once */
 	WARN_ON(intel_dp->source_rates || intel_dp->num_source_rates);
@@ -236,11 +257,7 @@ intel_dp_set_source_rates(struct intel_dp *intel_dp)
 		size = ARRAY_SIZE(bxt_rates);
 	} else if (IS_CANNONLAKE(dev_priv)) {
 		source_rates = cnl_rates;
-		size = ARRAY_SIZE(cnl_rates);
-		voltage = I915_READ(CNL_PORT_COMP_DW3) & VOLTAGE_INFO_MASK;
-		if (port == PORT_A || port == PORT_D ||
-		    voltage == VOLTAGE_INFO_0_85V)
-			size -= 2;
+		size = cnl_adjusted_max_rate(intel_dp, ARRAY_SIZE(cnl_rates));
 	} else if (IS_GEN9_BC(dev_priv)) {
 		source_rates = skl_rates;
 		size = ARRAY_SIZE(skl_rates);