bluetooth: host: Add API to get a connection's UATT MTU

Similar to bt_att_get_mtu but only considering the Unenhanced ATT
channel.

Signed-off-by: Luis Ubieda <luisf@croxel.com>

include/zephyr/bluetooth/gatt.h

@@ -1488,6 +1488,23 @@ bool bt_gatt_is_subscribed(struct bt_conn *conn,
  */
 uint16_t bt_gatt_get_mtu(struct bt_conn *conn);
 
+/** @brief Get Unenhanced ATT (UATT) MTU for a connection
+ *
+ *  Get UATT connection MTU.
+ *
+ *  The ATT_MTU defines the largest size of an ATT PDU, encompassing the ATT
+ *  opcode, additional fields, and any attribute value payload. Consequently,
+ *  the maximum size of a value payload is less and varies based on the type
+ *  of ATT PDU. For example, in an ATT_HANDLE_VALUE_NTF PDU, the Attribute Value
+ *  field can contain up to ATT_MTU - 3 octets (size of opcode and handle).
+ *
+ *  @param conn Connection object.
+ *
+ *  @return 0 if @p conn does not have an UATT ATT_MTU (e.g: disconnected).
+ *  @return Current UATT ATT_MTU.
+ */
+uint16_t bt_gatt_get_uatt_mtu(struct bt_conn *conn);
+
 /** @} */
 
 /**

subsys/bluetooth/host/att.c

@@ -3832,6 +3832,27 @@ uint16_t bt_att_get_mtu(struct bt_conn *conn)
 	return mtu;
 }
 
+uint16_t bt_att_get_uatt_mtu(struct bt_conn *conn)
+{
+	struct bt_att_chan *chan, *tmp;
+	struct bt_att *att;
+
+	att = att_get(conn);
+	if (!att) {
+		return 0;
+	}
+
+	SYS_SLIST_FOR_EACH_CONTAINER_SAFE(&att->chans, chan, tmp, node) {
+		if (!bt_att_is_enhanced(chan)) {
+			return bt_att_mtu(chan);
+		}
+	}
+
+	LOG_WRN("No UATT channel found in %p", conn);
+
+	return 0;
+}
+
 static void att_chan_mtu_updated(struct bt_att_chan *updated_chan)
 {
 	struct bt_att *att = updated_chan->att;

subsys/bluetooth/host/att_internal.h

@@ -293,6 +293,7 @@ struct bt_att_req {
 
 void bt_att_init(void);
 uint16_t bt_att_get_mtu(struct bt_conn *conn);
+uint16_t bt_att_get_uatt_mtu(struct bt_conn *conn);
 struct net_buf *bt_att_create_pdu(struct bt_conn *conn, uint8_t op,
 				  size_t len);
 

subsys/bluetooth/host/gatt.c

@@ -3164,6 +3164,11 @@ uint16_t bt_gatt_get_mtu(struct bt_conn *conn)
 	return bt_att_get_mtu(conn);
 }
 
+uint16_t bt_gatt_get_uatt_mtu(struct bt_conn *conn)
+{
+	return bt_att_get_uatt_mtu(conn);
+}
+
 uint8_t bt_gatt_check_perm(struct bt_conn *conn, const struct bt_gatt_attr *attr,
 			uint16_t mask)
 {