diff --git a/core.c b/core.c
index c6896d1..c11564a 100644
--- a/core.c
+++ b/core.c
@@ -229,20 +229,22 @@
 	return &mctp->busses[0];
 }
 
-unsigned long mctp_register_bus(struct mctp *mctp,
+int mctp_register_bus(struct mctp *mctp,
 		struct mctp_binding *binding,
 		mctp_eid_t eid)
 {
+	/* todo: multiple busses */
 	assert(!mctp->busses[0].binding);
 	mctp->busses[0].binding = binding;
 	mctp->busses[0].eid = eid;
+	binding->bus = &mctp->busses[0];
 	return 0;
 }
 
-void mctp_bus_rx(struct mctp *mctp, unsigned long bus_id,
+void mctp_bus_rx(struct mctp *mctp, struct mctp_binding *binding,
 		struct mctp_pktbuf *pkt)
 {
-	struct mctp_bus *bus = &mctp->busses[bus_id];
+	struct mctp_bus *bus = binding->bus;
 	struct mctp_msg_ctx *ctx;
 	struct mctp_hdr *hdr;
 	uint8_t flags, seq, tag;
@@ -250,6 +252,8 @@
 	void *p;
 	int rc;
 
+	assert(bus);
+
 	hdr = mctp_pktbuf_hdr(pkt);
 
 	if (hdr->dest != bus->eid)
diff --git a/libmctp.h b/libmctp.h
index 28903d7..c477a2a 100644
--- a/libmctp.h
+++ b/libmctp.h
@@ -57,11 +57,15 @@
 
 /* MCTP core */
 struct mctp;
+struct mctp_bus;
 struct mctp_binding;
 
 struct mctp *mctp_init(void);
 
-unsigned long mctp_register_bus(struct mctp *mctp,
+/* Register a binding to the MCTP core, and creates a bus (populating
+ * binding->bus).
+ */
+int mctp_register_bus(struct mctp *mctp,
 		struct mctp_binding *binding,
 		mctp_eid_t eid);
 
@@ -77,11 +81,12 @@
 struct mctp_binding {
 	const char	*name;
 	uint8_t		version;
+	struct mctp_bus	*bus;
 	int		(*tx)(struct mctp_binding *binding,
 				struct mctp_pktbuf *pkt);
 };
 
-void mctp_bus_rx(struct mctp *mctp, unsigned long bus_id,
+void mctp_bus_rx(struct mctp *mctp, struct mctp_binding *binding,
 		struct mctp_pktbuf *pkt);
 
 /* environment-specific allocation */
diff --git a/serial.c b/serial.c
index 87524f6..482cedb 100644
--- a/serial.c
+++ b/serial.c
@@ -138,7 +138,7 @@
 	assert(pkt);
 
 	if (valid)
-		mctp_bus_rx(serial->mctp, serial->bus_id, pkt);
+		mctp_bus_rx(serial->mctp, &serial->binding, pkt);
 
 	mctp_pktbuf_free(pkt);
 	serial->rx_pkt = NULL;
@@ -295,7 +295,7 @@
 {
 	assert(serial->fd >= 0);
 	serial->mctp = mctp;
-	serial->bus_id = mctp_register_bus(mctp, &serial->binding, eid);
+	mctp_register_bus(mctp, &serial->binding, eid);
 }
 
 struct mctp_binding_serial *mctp_serial_init(void)