quoted

How does port_idx fit into this?

I think you were assuming there is only one remoteproc in the system?
In practice, the setup can look more like this:

+ remote_cm33{
+     rpmsg {
+        rpmsg-io-channel {
+          #address-cells = <1>;
+          #size-cells = <0>;
+
+          gpio@0 {
+            compatible = "rpmsg-gpio";
+            reg = <0>;
+            gpio-controller;
+            #gpio-cells = <2>;
+            #interrupt-cells = <2>;
+            interrupt-controller;
+          };
+          gpio@1 {
+            compatible = "rpmsg-gpio";
+            reg = <1>;
+            gpio-controller;
+            #gpio-cells = <2>;
+            #interrupt-cells = <2>;
+            interrupt-controller;
+          };
+  ...
+        };
+      };
+};
+
+ remote_dsp {
+     rpmsg {
+        rpmsg-io-channel {
+          #address-cells = <1>;
+          #size-cells = <0>;
+
+          gpio@0 {
+            compatible = "rpmsg-gpio";
+            reg = <0>;
+            gpio-controller;
+            #gpio-cells = <2>;
+            #interrupt-cells = <2>;
+            interrupt-controller;
+          };
+  ...
+        };
+      };
+};

And why would this require a port_idx? If they are different rpmsg
instances, the channel numbers are in different address spaces.

	Andrew