drm/bridge: panel: Check device dependency before managing device link

Some panel devices already depend on DRM device, like the panel in
arch/arm/boot/dts/st/ste-ux500-samsung-skomer.dts, because DRM device is
the ancestor of those panel devices.  device_link_add() would fail by
returning a NULL pointer for those panel devices because of the existing
dependency.  So, check the dependency by calling device_is_dependent()
before adding or deleting device link between panel device and DRM device
so that the link is managed only for independent panel devices.

Fixes: 8878780 ("drm/bridge: panel: Fix device link for DRM_BRIDGE_ATTACH_NO_CONNECTOR")
Fixes: 199cf07 ("drm/bridge: panel: Add a device link between drm device and panel device")
Reported-by: Linus Walleij <linus.walleij@linaro.org>
Closes: https://lore.kernel.org/lkml/CACRpkdaGzXD6HbiX7mVUNJAJtMEPG00Pp6+nJ1P0JrfJ-ArMvQ@mail.gmail.com/T/
Tested-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Liu Ying <victor.liu@nxp.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20231123032615.3760488-1-victor.liu@nxp.com

Author: Liu Ying

drivers/gpu/drm/bridge/panel.c

@@ -23,6 +23,7 @@ struct panel_bridge {
 	struct drm_panel *panel;
 	struct device_link *link;
 	u32 connector_type;
+	bool is_independent;
 };
 
 static inline struct panel_bridge *
@@ -67,20 +68,26 @@ static int panel_bridge_attach(struct drm_bridge *bridge,
 	struct drm_device *drm_dev = bridge->dev;
 	int ret;
 
-	panel_bridge->link = device_link_add(drm_dev->dev, panel->dev,
-					     DL_FLAG_STATELESS);
-	if (!panel_bridge->link) {
-		DRM_ERROR("Failed to add device link between %s and %s\n",
-			  dev_name(drm_dev->dev), dev_name(panel->dev));
-		return -EINVAL;
+	panel_bridge->is_independent = !device_is_dependent(drm_dev->dev,
+							    panel->dev);
+
+	if (panel_bridge->is_independent) {
+		panel_bridge->link = device_link_add(drm_dev->dev, panel->dev,
+						     DL_FLAG_STATELESS);
+		if (!panel_bridge->link) {
+			DRM_ERROR("Failed to add device link between %s and %s\n",
+				  dev_name(drm_dev->dev), dev_name(panel->dev));
+			return -EINVAL;
+		}
 	}
 
 	if (flags & DRM_BRIDGE_ATTACH_NO_CONNECTOR)
 		return 0;
 
 	if (!bridge->encoder) {
 		DRM_ERROR("Missing encoder\n");
-		device_link_del(panel_bridge->link);
+		if (panel_bridge->is_independent)
+			device_link_del(panel_bridge->link);
 		return -ENODEV;
 	}
 
@@ -92,7 +99,8 @@ static int panel_bridge_attach(struct drm_bridge *bridge,
 				 panel_bridge->connector_type);
 	if (ret) {
 		DRM_ERROR("Failed to initialize connector\n");
-		device_link_del(panel_bridge->link);
+		if (panel_bridge->is_independent)
+			device_link_del(panel_bridge->link);
 		return ret;
 	}
 
@@ -115,7 +123,8 @@ static void panel_bridge_detach(struct drm_bridge *bridge)
 	struct panel_bridge *panel_bridge = drm_bridge_to_panel_bridge(bridge);
 	struct drm_connector *connector = &panel_bridge->connector;
 
-	device_link_del(panel_bridge->link);
+	if (panel_bridge->is_independent)
+		device_link_del(panel_bridge->link);
 
 	/*
 	 * Cleanup the connector if we know it was initialized.