Index: head/sys/dev/uart/uart.h
===================================================================
--- head/sys/dev/uart/uart.h
+++ head/sys/dev/uart/uart.h
@@ -44,6 +44,7 @@
 	u_int	rclk;
 	u_int	regshft;
 	u_int	regiowidth;
+	u_int	busy_detect;
 };
 
 #define	uart_regofs(bas, reg)		((reg) << (bas)->regshft)
Index: head/sys/dev/uart/uart_dev_ns8250.c
===================================================================
--- head/sys/dev/uart/uart_dev_ns8250.c
+++ head/sys/dev/uart/uart_dev_ns8250.c
@@ -469,6 +469,7 @@
 
 	bas = &sc->sc_bas;
 
+	ns8250->busy_detect = bas->busy_detect;
 	ns8250->mcr = uart_getreg(bas, REG_MCR);
 	ns8250->fcr = FCR_ENABLE;
 #ifdef CPU_XBURST
Index: head/sys/dev/uart/uart_subr.c
===================================================================
--- head/sys/dev/uart/uart_subr.c
+++ head/sys/dev/uart/uart_subr.c
@@ -49,6 +49,7 @@
 #define	UART_TAG_RS	7
 #define	UART_TAG_SB	8
 #define	UART_TAG_XO	9
+#define	UART_TAG_BD	10
 
 static struct uart_class *uart_classes[] = {
 	&uart_ns8250_class,
@@ -124,6 +125,10 @@
 {
 	int tag;
 
+	if ((*p)[0] == 'b' && (*p)[1] == 'd') {
+		tag = UART_TAG_BD;
+		goto out;
+	}
 	if ((*p)[0] == 'b' && (*p)[1] == 'r') {
 		tag = UART_TAG_BR;
 		goto out;
@@ -179,6 +184,7 @@
  * separated by commas. Each attribute is a tag-value pair with the tag and
  * value separated by a colon. Supported tags are:
  *
+ *	bd = Busy Detect
  *	br = Baudrate
  *	ch = Channel
  *	db = Data bits
@@ -242,6 +248,9 @@
 	spec = cp;
 	for (;;) {
 		switch (uart_parse_tag(&spec)) {
+		case UART_TAG_BD:
+			di->bas.busy_detect = uart_parse_long(&spec);
+			break;
 		case UART_TAG_BR:
 			di->baudrate = uart_parse_long(&spec);
 			break;