drm/sun4i: mixer: Read id from DT

Currently, TCON supports 2 ways to match TCON with engine (mixer in this
case). Old way is to just traverse of graph backwards and compare node
pointer. New way is to match TCON and engine by their respective ids.
All SoCs with DE2 enabled till now used the old way, which means mixer
id was never used and thus never implemented.

However, for R40, only the new way will be used. To prepare for that,
implement mixer id fetching from DT.
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180711112706.30222-1-jernej.skrabec@siol.net

drivers/gpu/drm/sun4i/sun8i_mixer.c

@@ -22,6 +22,7 @@
 #include <linux/component.h>
 #include <linux/dma-mapping.h>
 #include <linux/of_device.h>
+#include <linux/of_graph.h>
 #include <linux/reset.h>
 
 #include "sun4i_drv.h"
@@ -322,6 +323,42 @@ static struct regmap_config sun8i_mixer_regmap_config = {
 	.max_register	= 0xbfffc, /* guessed */
 };
 
+static int sun8i_mixer_of_get_id(struct device_node *node)
+{
+	struct device_node *port, *ep;
+	int ret = -EINVAL;
+
+	/* output is port 1 */
+	port = of_graph_get_port_by_id(node, 1);
+	if (!port)
+		return -EINVAL;
+
+	/* try to find downstream 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) {
+			of_node_put(remote);
+			of_node_put(ep);
+			of_node_put(port);
+
+			return reg;
+		}
+
+		of_node_put(remote);
+	}
+
+	of_node_put(port);
+
+	return ret;
+}
+
 static int sun8i_mixer_bind(struct device *dev, struct device *master,
 			      void *data)
 {
@@ -353,8 +390,16 @@ static int sun8i_mixer_bind(struct device *dev, struct device *master,
 	dev_set_drvdata(dev, mixer);
 	mixer->engine.ops = &sun8i_engine_ops;
 	mixer->engine.node = dev->of_node;
-	/* The ID of the mixer currently doesn't matter */
-	mixer->engine.id = -1;
+
+	/*
+	 * While this function can fail, we shouldn't do anything
+	 * if this happens. Some early DE2 DT entries don't provide
+	 * mixer id but work nevertheless because matching between
+	 * TCON and mixer is done by comparing node pointers (old
+	 * way) instead comparing ids. If this function fails and
+	 * id is needed, it will fail during id matching anyway.
+	 */
+	mixer->engine.id = sun8i_mixer_of_get_id(dev->of_node);
 
 	mixer->cfg = of_device_get_match_data(dev);
 	if (!mixer->cfg)