drm/i915: Don't unset intel_connector->mst_port

Currently we set intel_connector->mst_port to NULL to signify that the
MST port has been removed from the system so that we can prevent further
action on the port such as connector probes, mode probing, etc.
However, we're going to need access to intel_connector->mst_port in
order to fixup ->best_encoder() so that it can always return the correct
encoder for an MST port to prevent legacy DPMS prop changes from
failing. This should be safe, so instead keep intel_connector->mst_port
always set and instead just check the status of
drm_connector->regustered to signify whether or not the connector has
disappeared from the system.

Changes since v2:
- Add a comment to mst_port_gone (Jani Nikula)
- Change mst_port_gone to a u8 instead of a bool, per the kernel bot.
  Apparently bool is discouraged in structs these days
Changes since v4:
- Don't use mst_port_gone at all! Just check if the connector is
  registered or not - Daniel Vetter
Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: stable@vger.kernel.org
Link: https://patchwork.freedesktop.org/patch/msgid/20181008232437.5571-4-lyude@redhat.com

drivers/gpu/drm/i915/intel_dp_mst.c

@@ -307,9 +307,8 @@ static int intel_dp_mst_get_ddc_modes(struct drm_connector *connector)
 	struct edid *edid;
 	int ret;
 
-	if (!intel_dp) {
+	if (!READ_ONCE(connector->registered))
 		return intel_connector_update_modes(connector, NULL);
-	}
 
 	edid = drm_dp_mst_get_edid(connector, &intel_dp->mst_mgr, intel_connector->port);
 	ret = intel_connector_update_modes(connector, edid);
@@ -324,9 +323,10 @@ intel_dp_mst_detect(struct drm_connector *connector, bool force)
 	struct intel_connector *intel_connector = to_intel_connector(connector);
 	struct intel_dp *intel_dp = intel_connector->mst_port;
 
-	if (!intel_dp)
+	if (!READ_ONCE(connector->registered))
 		return connector_status_disconnected;
-	return drm_dp_mst_detect_port(connector, &intel_dp->mst_mgr, intel_connector->port);
+	return drm_dp_mst_detect_port(connector, &intel_dp->mst_mgr,
+				      intel_connector->port);
 }
 
 static void
@@ -366,7 +366,7 @@ intel_dp_mst_mode_valid(struct drm_connector *connector,
 	int bpp = 24; /* MST uses fixed bpp */
 	int max_rate, mode_rate, max_lanes, max_link_clock;
 
-	if (!intel_dp)
+	if (!READ_ONCE(connector->registered))
 		return MODE_ERROR;
 
 	if (mode->flags & DRM_MODE_FLAG_DBLSCAN)
@@ -398,7 +398,7 @@ static struct drm_encoder *intel_mst_atomic_best_encoder(struct drm_connector *c
 	struct intel_dp *intel_dp = intel_connector->mst_port;
 	struct intel_crtc *crtc = to_intel_crtc(state->crtc);
 
-	if (!intel_dp)
+	if (!READ_ONCE(connector->registered))
 		return NULL;
 	return &intel_dp->mst_encoders[crtc->pipe]->base.base;
 }
@@ -499,7 +499,6 @@ static void intel_dp_register_mst_connector(struct drm_connector *connector)
 static void intel_dp_destroy_mst_connector(struct drm_dp_mst_topology_mgr *mgr,
 					   struct drm_connector *connector)
 {
-	struct intel_connector *intel_connector = to_intel_connector(connector);
 	struct drm_i915_private *dev_priv = to_i915(connector->dev);
 
 	DRM_DEBUG_KMS("[CONNECTOR:%d:%s]\n", connector->base.id, connector->name);
@@ -508,10 +507,6 @@ static void intel_dp_destroy_mst_connector(struct drm_dp_mst_topology_mgr *mgr,
 	if (dev_priv->fbdev)
 		drm_fb_helper_remove_one_connector(&dev_priv->fbdev->helper,
 						   connector);
-	/* prevent race with the check in ->detect */
-	drm_modeset_lock(&connector->dev->mode_config.connection_mutex, NULL);
-	intel_connector->mst_port = NULL;
-	drm_modeset_unlock(&connector->dev->mode_config.connection_mutex);
 
 	drm_connector_put(connector);
 }