drm: xlnx: zynqmp_dpsub: Attach to the next bridge

The next component in the display chain, after the DP encoder, is most
likely a DP connector. The display connector driver registers a bridge
for it. That bridge doesn't need to be controlled, but is needed in
order to use the DRM connector bridge helper. Retrieve it at init time,
and attach to it in the DP bridge attach handler.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

drivers/gpu/drm/xlnx/zynqmp_dp.c

@@ -283,6 +283,7 @@ struct zynqmp_dp_config {
  * @reset: reset controller
  * @irq: irq
  * @bridge: DRM bridge for the DP encoder
+ * @next_bridge: The downstream bridge
  * @config: IP core configuration from DTS
  * @aux: aux channel
  * @phy: PHY handles for DP lanes
@@ -305,6 +306,7 @@ struct zynqmp_dp {
 	int irq;
 
 	struct drm_bridge bridge;
+	struct drm_bridge *next_bridge;
 
 	struct zynqmp_dp_config config;
 	struct drm_dp_aux aux;
@@ -1307,6 +1309,13 @@ static int zynqmp_dp_bridge_attach(struct drm_bridge *bridge,
 	drm_connector_register(connector);
 	drm_connector_attach_encoder(connector, bridge->encoder);
 
+	if (dp->next_bridge) {
+		ret = drm_bridge_attach(bridge->encoder, dp->next_bridge,
+					bridge, DRM_BRIDGE_ATTACH_NO_CONNECTOR);
+		if (ret < 0)
+			return ret;
+	}
+
 	return 0;
 }
 
@@ -1744,6 +1753,15 @@ int zynqmp_dp_probe(struct zynqmp_dpsub *dpsub, struct drm_device *drm)
 	if (ret)
 		goto err_reset;
 
+	/*
+	 * Acquire the next bridge in the chain. Ignore errors caused by port@5
+	 * not being connected for backward-compatibility with older DTs.
+	 */
+	ret = drm_of_find_panel_or_bridge(dp->dev->of_node, 5, 0, NULL,
+					  &dp->next_bridge);
+	if (ret < 0 && ret != -ENODEV)
+		goto err_reset;
+
 	/* Initialize the hardware. */
 	zynqmp_dp_write(dp, ZYNQMP_DP_TX_PHY_POWER_DOWN,
 			ZYNQMP_DP_TX_PHY_POWER_DOWN_ALL);