drm/msm: dsi host: Use device graph parsing to parse connected panel

The dsi host looks for the connected panel node by parsing for a child
named 'panel'. This hierarchy isn't very flexible. The connected
panel is forced to be a child to the dsi host, and hence, a mipi dsi
device. This isn't suitable for dsi devices that don't use mipi dsi
as their control bus.

Follow the of_graph approach of creating ports and endpoints to
represent the connections between the dsi host and the panel connected
to it. In our case, the dsi host will only have one output port, linked
to the panel's input port.

Update DT binding documentation with device graph usage info.

v3:
- Fix return value checks of of_graph_* calls.
- Don't make port a mandatory DT property
- Fix defer check when no panel node specified
- Rename parse_dt func to align with other dsi_host funcs

Reviewed-by: Hai Li <[e-mail]>
Signed-off-by: Archit Taneja <[e-mail]>
Signed-off-by: Rob Clark <[e-mail]>

Documentation/devicetree/bindings/drm/msm/dsi.txt

···
 - pinctrl-names: the pin control state names; should contain "default"
 - pinctrl-0: the default pinctrl state (active)
 - pinctrl-n: the "sleep" pinctrl state
+- port: DSI controller output port. This contains one endpoint subnode, with its
+  remote-endpoint set to the phandle of the connected panel's endpoint.
+  See Documentation/devicetree/bindings/graph.txt for device graph info.
 
 DSI PHY:
 Required properties:
···
 
 			power-supply = <...>;
 			backlight = <...>;
+
+			port {
+				panel_in: endpoint {
+					remote-endpoint = <&dsi0_out>;
+				};
+			};
+		};
+
+		port {
+			dsi0_out: endpoint {
+				remote-endpoint = <&panel_in>;
+			};
 		};
 	};
 

drivers/gpu/drm/msm/dsi/dsi_host.c

···
 #include <linux/of_gpio.h>
 #include <linux/of_irq.h>
 #include <linux/pinctrl/consumer.h>
+#include <linux/of_graph.h>
 #include <linux/regulator/consumer.h>
 #include <linux/spinlock.h>
 #include <video/mipi_display.h>
···
 	msm_host->format = dsi->format;
 	msm_host->mode_flags = dsi->mode_flags;
 
-	msm_host->panel_node = dsi->dev.of_node;
+	WARN_ON(dsi->dev.of_node != msm_host->panel_node);
 
 	/* Some gpios defined in panel DT need to be controlled by host */
 	ret = dsi_host_init_panel_gpios(msm_host, &dsi->dev);
···
 	.transfer = dsi_host_transfer,
 };
 
+static int dsi_host_parse_dt(struct msm_dsi_host *msm_host)
+{
+	struct device *dev = &msm_host->pdev->dev;
+	struct device_node *np = dev->of_node;
+	struct device_node *endpoint, *panel_node;
+	int ret;
+
+	ret = of_property_read_u32(np, "qcom,dsi-host-index", &msm_host->id);
+	if (ret) {
+		dev_err(dev, "%s: host index not specified, ret=%d\n",
+			__func__, ret);
+		return ret;
+	}
+
+	/*
+	 * Get the first endpoint node. In our case, dsi has one output port
+	 * to which the panel is connected. Don't return an error if a port
+	 * isn't defined. It's possible that there is nothing connected to
+	 * the dsi output.
+	 */
+	endpoint = of_graph_get_next_endpoint(np, NULL);
+	if (!endpoint) {
+		dev_dbg(dev, "%s: no endpoint\n", __func__);
+		return 0;
+	}
+
+	/* Get panel node from the output port's endpoint data */
+	panel_node = of_graph_get_remote_port_parent(endpoint);
+	if (!panel_node) {
+		dev_err(dev, "%s: no valid device\n", __func__);
+		of_node_put(endpoint);
+		return -ENODEV;
+	}
+
+	of_node_put(endpoint);
+	of_node_put(panel_node);
+
+	msm_host->panel_node = panel_node;
+
+	return 0;
+}
+
 int msm_dsi_host_init(struct msm_dsi *msm_dsi)
 {
 	struct msm_dsi_host *msm_host = NULL;
···
 		goto fail;
 	}
 
-	ret = of_property_read_u32(pdev->dev.of_node,
-				"qcom,dsi-host-index", &msm_host->id);
+	msm_host->pdev = pdev;
+
+	ret = dsi_host_parse_dt(msm_host);
 	if (ret) {
-		dev_err(&pdev->dev,
-			"%s: host index not specified, ret=%d\n",
-			__func__, ret);
+		pr_err("%s: failed to parse dt\n", __func__);
 		goto fail;
 	}
-	msm_host->pdev = pdev;
 
 	ret = dsi_clk_init(msm_host);
 	if (ret) {
···
 int msm_dsi_host_register(struct mipi_dsi_host *host, bool check_defer)
 {
 	struct msm_dsi_host *msm_host = to_msm_dsi_host(host);
-	struct device_node *node;
 	int ret;
 
 	/* Register mipi dsi host */
···
 		 * It makes sure panel is connected when fbcon detects
 		 * connector status and gets the proper display mode to
 		 * create framebuffer.
+		 * Don't try to defer if there is nothing connected to the dsi
+		 * output
 		 */
-		if (check_defer) {
-			node = of_get_child_by_name(msm_host->pdev->dev.of_node,
-							"panel");
-			if (node) {
-				if (!of_drm_find_panel(node)) {
-					of_node_put(node);
-					return -EPROBE_DEFER;
-				}
-
-				of_node_put(node);
-			}
+		if (check_defer && msm_host->panel_node) {
+			if (!of_drm_find_panel(msm_host->panel_node))
+				return -EPROBE_DEFER;
 		}
 	}