drm/i915/dp_mst: Make HBLANK expansion quirk work for logical ports

The DPCD OUI of the logical port on a Dell UHBR monitor - on which the
AUX device is used to enable DSC - is all 0. To detect if the HBLANK
expansion quirk is required for this monitor use the OUI of the port's
parent instead.

Since in the above case the DPCD of both the logical port and the parent
port reports being a sink device (vs. branch device) type, read the
proper sink/branch OUI based on the DPCD device type.

This is required by a follow-up patch enabling the quirk for the above
Dell monitor.
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240416221010.376865-11-imre.deak@intel.com

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

@@ -1577,21 +1577,31 @@ intel_dp_mst_read_decompression_port_dsc_caps(struct intel_dp *intel_dp,
 static bool detect_dsc_hblank_expansion_quirk(const struct intel_connector *connector)
 {
 	struct drm_i915_private *i915 = to_i915(connector->base.dev);
+	struct drm_dp_aux *aux = connector->dp.dsc_decompression_aux;
 	struct drm_dp_desc desc;
 	u8 dpcd[DP_RECEIVER_CAP_SIZE];
 
-	if (!connector->dp.dsc_decompression_aux)
+	if (!aux)
 		return false;
 
-	if (drm_dp_read_desc(connector->dp.dsc_decompression_aux,
-			     &desc, true) < 0)
+	/*
+	 * A logical port's OUI (at least for affected sinks) is all 0, so
+	 * instead of that the parent port's OUI is used for identification.
+	 */
+	if (drm_dp_mst_port_is_logical(connector->port)) {
+		aux = drm_dp_mst_aux_for_parent(connector->port);
+		if (!aux)
+			aux = &connector->mst_port->aux;
+	}
+
+	if (drm_dp_read_dpcd_caps(aux, dpcd) < 0)
 		return false;
 
-	if (!drm_dp_has_quirk(&desc,
-			      DP_DPCD_QUIRK_HBLANK_EXPANSION_REQUIRES_DSC))
+	if (drm_dp_read_desc(aux, &desc, drm_dp_is_branch(dpcd)) < 0)
 		return false;
 
-	if (drm_dp_read_dpcd_caps(connector->dp.dsc_decompression_aux, dpcd) < 0)
+	if (!drm_dp_has_quirk(&desc,
+			      DP_DPCD_QUIRK_HBLANK_EXPANSION_REQUIRES_DSC))
 		return false;
 
 	if (!(dpcd[DP_RECEIVE_PORT_0_CAP_0] & DP_HBLANK_EXPANSION_CAPABLE))