drm/i915: Allow MST sinks to work even if drm_probe_ddc() fails

With HSW + Dell UP2414Q (at least) drm_probe_ddc() occasionally fails,
and then we'll assume that the entire display has been disconnected.
We don't need the EDID from the main link, so we can simply check if
the sink is MST capable, and if so treat is as connected.

v2: Skip drm_probe_ddc() entirely for MST (Daniel)

Cc: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Cc: Jim Bride <jim.bride@linux.intel.com>
Cc: Manasi D Navare <manasi.d.navare@intel.com>
Cc: Durgadoss R <durgadoss.r@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1469800276-6979-1-git-send-email-ville.syrjala@linux.intel.com

drivers/gpu/drm/i915/intel_dp.c

@@ -3545,7 +3545,7 @@ intel_dp_probe_oui(struct intel_dp *intel_dp)
 }
 
 static bool
-intel_dp_probe_mst(struct intel_dp *intel_dp)
+intel_dp_can_mst(struct intel_dp *intel_dp)
 {
 	u8 buf[1];
 
@@ -3558,18 +3558,30 @@ intel_dp_probe_mst(struct intel_dp *intel_dp)
 	if (intel_dp->dpcd[DP_DPCD_REV] < 0x12)
 		return false;
 
-	if (drm_dp_dpcd_read(&intel_dp->aux, DP_MSTM_CAP, buf, 1)) {
-		if (buf[0] & DP_MST_CAP) {
-			DRM_DEBUG_KMS("Sink is MST capable\n");
-			intel_dp->is_mst = true;
-		} else {
-			DRM_DEBUG_KMS("Sink is not MST capable\n");
-			intel_dp->is_mst = false;
-		}
-	}
+	if (drm_dp_dpcd_read(&intel_dp->aux, DP_MSTM_CAP, buf, 1) != 1)
+		return false;
 
-	drm_dp_mst_topology_mgr_set_mst(&intel_dp->mst_mgr, intel_dp->is_mst);
-	return intel_dp->is_mst;
+	return buf[0] & DP_MST_CAP;
+}
+
+static void
+intel_dp_configure_mst(struct intel_dp *intel_dp)
+{
+	if (!i915.enable_dp_mst)
+		return;
+
+	if (!intel_dp->can_mst)
+		return;
+
+	intel_dp->is_mst = intel_dp_can_mst(intel_dp);
+
+	if (intel_dp->is_mst)
+		DRM_DEBUG_KMS("Sink is MST capable\n");
+	else
+		DRM_DEBUG_KMS("Sink is not MST capable\n");
+
+	drm_dp_mst_topology_mgr_set_mst(&intel_dp->mst_mgr,
+					intel_dp->is_mst);
 }
 
 static int intel_dp_sink_crc_stop(struct intel_dp *intel_dp)
@@ -3999,6 +4011,9 @@ intel_dp_detect_dpcd(struct intel_dp *intel_dp)
 		connector_status_connected : connector_status_disconnected;
 	}
 
+	if (intel_dp_can_mst(intel_dp))
+		return connector_status_connected;
+
 	/* If no HPD, poke DDC gently */
 	if (drm_probe_ddc(&intel_dp->aux.ddc))
 		return connector_status_connected;
@@ -4236,7 +4251,6 @@ intel_dp_long_pulse(struct intel_connector *intel_connector)
 	struct drm_device *dev = connector->dev;
 	enum drm_connector_status status;
 	enum intel_display_power_domain power_domain;
-	bool ret;
 	u8 sink_irq_vector;
 
 	power_domain = intel_display_port_aux_power_domain(intel_encoder);
@@ -4279,8 +4293,9 @@ intel_dp_long_pulse(struct intel_connector *intel_connector)
 
 	intel_dp_probe_oui(intel_dp);
 
-	ret = intel_dp_probe_mst(intel_dp);
-	if (ret) {
+	intel_dp_configure_mst(intel_dp);
+
+	if (intel_dp->is_mst) {
 		/*
 		 * If we are in MST mode then this connector
 		 * won't appear connected or have anything