drm/msm: mdp4 lvds: get panel node via of graph parsing

We currently get the output connected to LVDS by looking for a phandle
called 'qcom,lvds-panel' under the mdp DT node.

Use the more standard of_graph approach to create an lvds output port,
and retrieve the panel node from the port's endpoint data.

v3
- Fix return value checks of of_graph_* calls.
Tested-by: Srinivas Kandagatla <srinivas.kandagatla@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

@@ -241,22 +241,37 @@ int mdp4_enable(struct mdp4_kms *mdp4_kms)
 }
 
 #ifdef CONFIG_OF
-static struct drm_panel *detect_panel(struct drm_device *dev, const char *name)
+static struct drm_panel *detect_panel(struct drm_device *dev)
 {
-	struct device_node *n;
+	struct device_node *endpoint, *panel_node;
+	struct device_node *np = dev->dev->of_node;
 	struct drm_panel *panel = NULL;
 
-	n = of_parse_phandle(dev->dev->of_node, name, 0);
-	if (n) {
-		panel = of_drm_find_panel(n);
-		if (!panel)
-			panel = ERR_PTR(-EPROBE_DEFER);
+	endpoint = of_graph_get_next_endpoint(np, NULL);
+	if (!endpoint) {
+		dev_err(dev->dev, "no valid endpoint\n");
+		return ERR_PTR(-ENODEV);
+	}
+
+	panel_node = of_graph_get_remote_port_parent(endpoint);
+	if (!panel_node) {
+		dev_err(dev->dev, "no valid panel node\n");
+		of_node_put(endpoint);
+		return ERR_PTR(-ENODEV);
+	}
+
+	of_node_put(endpoint);
+
+	panel = of_drm_find_panel(panel_node);
+	if (!panel) {
+		of_node_put(panel_node);
+		return ERR_PTR(-EPROBE_DEFER);
 	}
 
 	return panel;
 }
 #else
-static struct drm_panel *detect_panel(struct drm_device *dev, const char *name)
+static struct drm_panel *detect_panel(struct drm_device *dev)
 {
 	// ??? maybe use a module param to specify which panel is attached?
 }
@@ -294,7 +309,7 @@ static int modeset_init(struct mdp4_kms *mdp4_kms)
 	 * Setup the LCDC/LVDS path: RGB2 -> DMA_P -> LCDC -> LVDS:
 	 */
 
-	panel = detect_panel(dev, "qcom,lvds-panel");
+	panel = detect_panel(dev);
 	if (IS_ERR(panel)) {
 		ret = PTR_ERR(panel);
 		dev_err(dev->dev, "failed to detect LVDS panel: %d\n", ret);

drivers/gpu/drm/msm/msm_drv.h

@@ -30,6 +30,7 @@
 #include <linux/list.h>
 #include <linux/iommu.h>
 #include <linux/types.h>
+#include <linux/of_graph.h>
 #include <asm/sizes.h>
 
 #ifndef CONFIG_OF