drm/bridge: panel: Add a device link between drm device and panel device

Add the device link when panel bridge is attached and delete the link
when panel bridge is detached.  The drm device is the consumer while
the panel device is the supplier.  This makes sure that the drm device
suspends eariler and resumes later than the panel device, hence resolves
problems where the order is reversed, like the problematic case mentioned
in the below link.

Link: https://lore.kernel.org/lkml/CAPDyKFr0XjrU_udKoUKQ_q8RWaUkyqL+8fV-7s1CTMqi7u3-Rg@mail.gmail.com/T/
Suggested-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Liu Ying <victor.liu@nxp.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230807061115.3244501-1-victor.liu@nxp.com

Author: Liu Ying

drivers/gpu/drm/bridge/panel.c

@@ -4,6 +4,8 @@
  * Copyright (C) 2017 Broadcom
  */
 
+#include <linux/device.h>
+
 #include <drm/drm_atomic_helper.h>
 #include <drm/drm_bridge.h>
 #include <drm/drm_connector.h>
@@ -19,6 +21,7 @@ struct panel_bridge {
 	struct drm_bridge bridge;
 	struct drm_connector connector;
 	struct drm_panel *panel;
+	struct device_link *link;
 	u32 connector_type;
 };
 
@@ -60,6 +63,8 @@ static int panel_bridge_attach(struct drm_bridge *bridge,
 {
 	struct panel_bridge *panel_bridge = drm_bridge_to_panel_bridge(bridge);
 	struct drm_connector *connector = &panel_bridge->connector;
+	struct drm_panel *panel = panel_bridge->panel;
+	struct drm_device *drm_dev = bridge->dev;
 	int ret;
 
 	if (flags & DRM_BRIDGE_ATTACH_NO_CONNECTOR)
@@ -70,6 +75,14 @@ static int panel_bridge_attach(struct drm_bridge *bridge,
 		return -ENODEV;
 	}
 
+	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;
+	}
+
 	drm_connector_helper_add(connector,
 				 &panel_bridge_connector_helper_funcs);
 
@@ -78,6 +91,7 @@ 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);
 		return ret;
 	}
 
@@ -100,6 +114,8 @@ 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);
+
 	/*
 	 * Cleanup the connector if we know it was initialized.
 	 *