drm/i915/mst: use the MST mode detected previously

Drop the duplicate read of DP_MSTM_CAP DPCD register, and the duplicate
logic for choosing MST mode, and store the chosen mode in struct
intel_dp. Rename intel_dp_configure_mst() to intel_dp_mst_configure()
while at it.

v2: Rebase on drm_dp_mst_detect() returning the mode, not bool

Cc: Arun R Murthy <arun.r.murthy@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/93a48df9a77e1138bb28e645fae3f9c79b094cc7.1710839496.git.jani.nikula@intel.comSigned-off-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/display/intel_display_types.h

@@ -1804,6 +1804,7 @@ struct intel_dp {
 
 	bool is_mst;
 	int active_mst_links;
+	enum drm_dp_mst_mode mst_detect;
 
 	/* connector directly attached - won't be use for modeset in mst world */
 	struct intel_connector *attached_connector;

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

@@ -4091,19 +4091,17 @@ intel_dp_mst_detect(struct intel_dp *intel_dp)
 }
 
 static void
-intel_dp_configure_mst(struct intel_dp *intel_dp)
+intel_dp_mst_configure(struct intel_dp *intel_dp)
 {
-	struct drm_i915_private *i915 = dp_to_i915(intel_dp);
-	bool sink_can_mst = drm_dp_read_mst_cap(&intel_dp->aux, intel_dp->dpcd) == DRM_DP_MST;
-
 	if (!intel_dp_mst_source_support(intel_dp))
 		return;
 
-	intel_dp->is_mst = sink_can_mst &&
-		i915->display.params.enable_dp_mst;
+	intel_dp->is_mst = intel_dp->mst_detect != DRM_DP_SST;
 
-	drm_dp_mst_topology_mgr_set_mst(&intel_dp->mst_mgr,
-					intel_dp->is_mst);
+	drm_dp_mst_topology_mgr_set_mst(&intel_dp->mst_mgr, intel_dp->is_mst);
+
+	/* Avoid stale info on the next detect cycle. */
+	intel_dp->mst_detect = DRM_DP_SST;
 }
 
 static bool
@@ -5452,7 +5450,6 @@ intel_dp_detect_dpcd(struct intel_dp *intel_dp)
 {
 	struct drm_i915_private *i915 = dp_to_i915(intel_dp);
 	struct intel_digital_port *dig_port = dp_to_dig_port(intel_dp);
-	enum drm_dp_mst_mode mst_detect;
 	u8 *dpcd = intel_dp->dpcd;
 	u8 type;
 
@@ -5464,7 +5461,7 @@ intel_dp_detect_dpcd(struct intel_dp *intel_dp)
 	if (!intel_dp_get_dpcd(intel_dp))
 		return connector_status_disconnected;
 
-	mst_detect = intel_dp_mst_detect(intel_dp);
+	intel_dp->mst_detect = intel_dp_mst_detect(intel_dp);
 
 	/* if there's no downstream port, we're done */
 	if (!drm_dp_is_branch(dpcd))
@@ -5477,7 +5474,7 @@ intel_dp_detect_dpcd(struct intel_dp *intel_dp)
 		connector_status_connected : connector_status_disconnected;
 	}
 
-	if (mst_detect == DRM_DP_MST)
+	if (intel_dp->mst_detect == DRM_DP_MST)
 		return connector_status_connected;
 
 	/* If no HPD, poke DDC gently */
@@ -5809,7 +5806,7 @@ intel_dp_detect(struct drm_connector *connector,
 
 	intel_dp_detect_dsc_caps(intel_dp, intel_connector);
 
-	intel_dp_configure_mst(intel_dp);
+	intel_dp_mst_configure(intel_dp);
 
 	/*
 	 * TODO: Reset link params when switching to MST mode, until MST