drm/sun4i: tcon: get TCON ID and matching engine with remote endpoint ID

The device tree binding for sun4i-drm says:

    For all connections between components up to the TCONs in the display
    pipeline, when there are multiple components of the same type at the
    same depth, the local endpoint ID must be the same as the remote
    component's index. For example, if the remote endpoint is Frontend 1,
    then the local endpoint ID must be 1.

We should be able to get the TCON's ID directly from any of the remote
endpoints from its input port. With the ID, we can then go through the
list of registered engines and find a matching one by ID.

However the A31 device tree is incorrect. We assumed that there were no
cross pipeline connections between the backends and TCONs. As a result,
in all the endpoints between the backends and TCONs of the second
display pipeline, the endpoint IDs were incorrectly set to 0, when in
fact they should've been set to 1.

To maintain compatibility with this incorrect device tree, we first
check if the TCON's input port has 1 or many endpoints. If there are
more than 1, then it is likely a fixed version, and we can proceed
with the new method. If there is only 1 endpoint, then it is possibly
an incorrect version, or it could be the SoC only has one pipeline.
In either case we fall back to using the old method of traversing
the input connections to find a matching engine, and then get its
ID.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170908075016.18657-5-wens@csie.org

drivers/gpu/drm/sun4i/sun4i_tcon.c

@@ -463,8 +463,9 @@ static int sun4i_tcon_init_regmap(struct device *dev,
  * function in fact searches the corresponding engine, and the ID is
  * requested via the get_id function of the engine.
  */
-static struct sunxi_engine *sun4i_tcon_find_engine(struct sun4i_drv *drv,
-						   struct device_node *node)
+static struct sunxi_engine *
+sun4i_tcon_find_engine_traverse(struct sun4i_drv *drv,
+				struct device_node *node)
 {
 	struct device_node *port, *ep, *remote;
 	struct sunxi_engine *engine;
@@ -502,7 +503,7 @@ static struct sunxi_engine *sun4i_tcon_find_engine(struct sun4i_drv *drv,
 		}
 
 		/* keep looking through upstream ports */
-		engine = sun4i_tcon_find_engine(drv, remote);
+		engine = sun4i_tcon_find_engine_traverse(drv, remote);
 		if (!IS_ERR(engine)) {
 			of_node_put(remote);
 			of_node_put(port);
@@ -513,6 +514,120 @@ static struct sunxi_engine *sun4i_tcon_find_engine(struct sun4i_drv *drv,
 	return ERR_PTR(-EINVAL);
 }
 
+/*
+ * The device tree binding says that the remote endpoint ID of any
+ * connection between components, up to and including the TCON, of
+ * the display pipeline should be equal to the actual ID of the local
+ * component. Thus we can look at any one of the input connections of
+ * the TCONs, and use that connection's remote endpoint ID as our own.
+ *
+ * Since the user of this function already finds the input port,
+ * the port is passed in directly without further checks.
+ */
+static int sun4i_tcon_of_get_id_from_port(struct device_node *port)
+{
+	struct device_node *ep;
+	int ret = -EINVAL;
+
+	/* try finding an upstream endpoint */
+	for_each_available_child_of_node(port, ep) {
+		struct device_node *remote;
+		u32 reg;
+
+		remote = of_graph_get_remote_endpoint(ep);
+		if (!remote)
+			continue;
+
+		ret = of_property_read_u32(remote, "reg", &reg);
+		if (ret)
+			continue;
+
+		ret = reg;
+	}
+
+	return ret;
+}
+
+/*
+ * Once we know the TCON's id, we can look through the list of
+ * engines to find a matching one. We assume all engines have
+ * been probed and added to the list.
+ */
+static struct sunxi_engine *sun4i_tcon_get_engine_by_id(struct sun4i_drv *drv,
+							int id)
+{
+	struct sunxi_engine *engine;
+
+	list_for_each_entry(engine, &drv->engine_list, list)
+		if (engine->id == id)
+			return engine;
+
+	return ERR_PTR(-EINVAL);
+}
+
+/*
+ * On SoCs with the old display pipeline design (Display Engine 1.0),
+ * we assumed the TCON was always tied to just one backend. However
+ * this proved not to be the case. On the A31, the TCON can select
+ * either backend as its source. On the A20 (and likely on the A10),
+ * the backend can choose which TCON to output to.
+ *
+ * The device tree binding says that the remote endpoint ID of any
+ * connection between components, up to and including the TCON, of
+ * the display pipeline should be equal to the actual ID of the local
+ * component. Thus we should be able to look at any one of the input
+ * connections of the TCONs, and use that connection's remote endpoint
+ * ID as our own.
+ *
+ * However  the connections between the backend and TCON were assumed
+ * to be always singular, and their endpoit IDs were all incorrectly
+ * set to 0. This means for these old device trees, we cannot just look
+ * up the remote endpoint ID of a TCON input endpoint. TCON1 would be
+ * incorrectly identified as TCON0.
+ *
+ * This function first checks if the TCON node has 2 input endpoints.
+ * If so, then the device tree is a corrected version, and it will use
+ * sun4i_tcon_of_get_id() and sun4i_tcon_get_engine_by_id() from above
+ * to fetch the ID and engine directly. If not, then it is likely an
+ * old device trees, where the endpoint IDs were incorrect, but did not
+ * have endpoint connections between the backend and TCON across
+ * different display pipelines. It will fall back to the old method of
+ * traversing the  of_graph to try and find a matching engine by device
+ * node.
+ *
+ * In the case of single display pipeline device trees, either method
+ * works.
+ */
+static struct sunxi_engine *sun4i_tcon_find_engine(struct sun4i_drv *drv,
+						   struct device_node *node)
+{
+	struct device_node *port;
+	struct sunxi_engine *engine;
+
+	port = of_graph_get_port_by_id(node, 0);
+	if (!port)
+		return ERR_PTR(-EINVAL);
+
+	/*
+	 * Is this a corrected device tree with cross pipeline
+	 * connections between the backend and TCON?
+	 */
+	if (of_get_child_count(port) > 1) {
+		/* Get our ID directly from an upstream endpoint */
+		int id = sun4i_tcon_of_get_id_from_port(port);
+
+		/* Get our engine by matching our ID */
+		engine = sun4i_tcon_get_engine_by_id(drv, id);
+
+		of_node_put(port);
+		return engine;
+	}
+
+	/* Fallback to old method by traversing input endpoints */
+	of_node_put(port);
+	return sun4i_tcon_find_engine_traverse(drv, node);
+}
+
 static int sun4i_tcon_bind(struct device *dev, struct device *master,
 			   void *data)
 {