drm/msm/mdp4: Fix issue with LCDC/LVDS port parsing

The LVDS port is the first in the list of the output ports in MDP4.
The driver assumed that if the port and its corresponding endpoint
is defined, then there should be a panel node too. This isn't
necessary since boards may not really use a LVDS panel. Don't fail
if there isn't a panel node available.

While we're at it, use of_graph_get_endpoint_by_regs instead of
of_graph_get_next_endpoint to make it more explicit that the LVDS
output is at port 0.

Tested-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>

drivers/gpu/drm/msm/mdp/mdp4/mdp4_kms.c

@@ -228,18 +228,21 @@ static struct device_node *mdp4_detect_lcdc_panel(struct drm_device *dev)
 	struct device_node *endpoint, *panel_node;
 	struct device_node *np = dev->dev->of_node;
 
-	endpoint = of_graph_get_next_endpoint(np, NULL);
+	/*
+	 * LVDS/LCDC is the first port described in the list of ports in the
+	 * MDP4 DT node.
+	 */
+	endpoint = of_graph_get_endpoint_by_regs(np, 0, -1);
 	if (!endpoint) {
-		DBG("no endpoint in MDP4 to fetch LVDS panel\n");
+		DBG("no LVDS remote endpoint\n");
 		return NULL;
 	}
 
-	/* don't proceed if we have an endpoint but no panel_node tied to it */
 	panel_node = of_graph_get_remote_port_parent(endpoint);
 	if (!panel_node) {
-		dev_err(dev->dev, "no valid panel node\n");
+		DBG("no valid panel node in LVDS endpoint\n");
 		of_node_put(endpoint);
-		return ERR_PTR(-ENODEV);
+		return NULL;
 	}
 
 	of_node_put(endpoint);
@@ -262,14 +265,12 @@ static int mdp4_modeset_init_intf(struct mdp4_kms *mdp4_kms,
 	switch (intf_type) {
 	case DRM_MODE_ENCODER_LVDS:
 		/*
-		 * bail out early if:
-		 * - there is no panel node (no need to initialize lcdc
-		 *   encoder and lvds connector), or
-		 * - panel node is a bad pointer
+		 * bail out early if there is no panel node (no need to
+		 * initialize LCDC encoder and LVDS connector)
 		 */
 		panel_node = mdp4_detect_lcdc_panel(dev);
-		if (IS_ERR_OR_NULL(panel_node))
-			return PTR_ERR(panel_node);
+		if (!panel_node)
+			return 0;
 
 		encoder = mdp4_lcdc_encoder_init(dev, panel_node);
 		if (IS_ERR(encoder)) {