drm/i915: Fix platform default aux ch for skl

SKL/derivatives have DDI E but no AUX E, so we need to pick
another aux ch as the platform default. DDI E is more or less
the other half of DDI A, so we pick AUX A.

In all other cases we should have a corresponding aux ch for
each DDI.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230216231312.32664-3-ville.syrjala@linux.intel.comReviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>

drivers/gpu/drm/i915/display/intel_dp_aux.c

@@ -739,10 +739,20 @@ void intel_dp_aux_init(struct intel_dp *intel_dp)
 	cpu_latency_qos_add_request(&intel_dp->pm_qos, PM_QOS_DEFAULT_VALUE);
 }
 
+static enum aux_ch default_aux_ch(struct intel_encoder *encoder)
+{
+	struct drm_i915_private *i915 = to_i915(encoder->base.dev);
+
+	/* SKL has DDI E but no AUX E */
+	if (DISPLAY_VER(i915) == 9 && encoder->port == PORT_E)
+		return AUX_CH_A;
+
+	return (enum aux_ch)encoder->port;
+}
+
 enum aux_ch intel_dp_aux_ch(struct intel_encoder *encoder)
 {
 	struct drm_i915_private *i915 = to_i915(encoder->base.dev);
-	enum port port = encoder->port;
 	enum aux_ch aux_ch;
 
 	aux_ch = intel_bios_dp_aux_ch(encoder->devdata);
@@ -753,7 +763,7 @@ enum aux_ch intel_dp_aux_ch(struct intel_encoder *encoder)
 		return aux_ch;
 	}
 
-	aux_ch = (enum aux_ch)port;
+	aux_ch = default_aux_ch(encoder);
 
 	drm_dbg_kms(&i915->drm,
 		    "[ENCODER:%d:%s] using AUX %c (platform default)\n",