[nrf fromtree] Bluetooth: rfcomm: Support dynamic channel allocation

In the function `bt_rfcomm_server_register()`, the channel cannot be
dynamic allocated. And only pre-set channel is supported.

Improve the function `bt_rfcomm_server_register()` to support the
dynamic channel allocation if the passed channel is zero.

Signed-off-by: Lyle Zhu <[email protected]>
(cherry picked from commit 87b2a30)

Author: lylezhu2012

include/zephyr/bluetooth/classic/rfcomm.h

@@ -47,6 +47,7 @@ enum {
 	BT_RFCOMM_CHAN_HSP_AG,
 	BT_RFCOMM_CHAN_HSP_HS,
 	BT_RFCOMM_CHAN_SPP,
+	BT_RFCOMM_CHAN_DYNAMIC_START,
 };
 
 struct bt_rfcomm_dlc;
@@ -125,7 +126,16 @@ struct bt_rfcomm_dlc {
 };
 
 struct bt_rfcomm_server {
-	/** Server Channel */
+	/** Server Channel
+	 *
+	 *  Possible values:
+	 *  0           A dynamic value will be auto-allocated when bt_rfcomm_server_register() is
+	 *              called.
+	 *
+	 *  0x01 - 0x1e Dynamically allocated. May be pre-set by the application before server
+	 *              registration (not recommended however), or auto-allocated by the stack
+	 *              if the 0 is passed.
+	 */
 	uint8_t channel;
 
 	/** Server accept callback

subsys/bluetooth/host/classic/rfcomm.c

@@ -199,15 +199,32 @@ rfcomm_sessions_lookup_bt_conn(struct bt_conn *conn)
 
 int bt_rfcomm_server_register(struct bt_rfcomm_server *server)
 {
-	if (server->channel < RFCOMM_CHANNEL_START ||
-	    server->channel > RFCOMM_CHANNEL_END || !server->accept) {
+	if (server->channel > RFCOMM_CHANNEL_END || !server->accept) {
 		return -EINVAL;
 	}
 
-	/* Check if given channel is already in use */
-	if (rfcomm_server_lookup_channel(server->channel)) {
-		LOG_DBG("Channel already registered");
-		return -EADDRINUSE;
+	if (!server->channel) {
+		uint8_t chan = (uint8_t)BT_RFCOMM_CHAN_DYNAMIC_START;
+
+		for (; chan <= RFCOMM_CHANNEL_END; chan++) {
+			/* Check if given channel is already in use */
+			if (!rfcomm_server_lookup_channel(chan)) {
+				server->channel = chan;
+				LOG_DBG("Allocated channel 0x%02x for new server", chan);
+				break;
+			}
+		}
+
+		if (!server->channel) {
+			LOG_WRN("No free dynamic rfcomm channels available");
+			return -EADDRNOTAVAIL;
+		}
+	} else {
+		/* Check if given channel is already in use */
+		if (rfcomm_server_lookup_channel(server->channel)) {
+			LOG_WRN("Channel already registered");
+			return -EADDRINUSE;
+		}
 	}
 
 	LOG_DBG("Channel 0x%02x", server->channel);