diff --git a/sys/dev/uart/uart.h b/sys/dev/uart/uart.h
--- a/sys/dev/uart/uart.h
+++ b/sys/dev/uart/uart.h
@@ -45,6 +45,7 @@
 	u_int	regshft;
 	u_int	regiowidth;
 	u_int	busy_detect;
+	u_int	rclk_guess;/* if rclk == 0, use baud + divisor to compute rclk */
 };
 
 #define	uart_regofs(bas, reg)		((reg) << (bas)->regshft)
diff --git a/sys/dev/uart/uart_cpu_acpi.c b/sys/dev/uart/uart_cpu_acpi.c
--- a/sys/dev/uart/uart_cpu_acpi.c
+++ b/sys/dev/uart/uart_cpu_acpi.c
@@ -189,6 +189,14 @@
 		    (int)spcr->BaudRate);
 		goto out;
 	}
+	/*
+	 * If no rclk is set, then we will assume the BIOS has configured the
+	 * hardware at the stated baudrate, so we can use it to guess the rclk
+	 * relatively accurately, so make a note for later.
+	 */
+	if (di->bas.rclk == 0)
+		di->bas.rclk_guess = 1;
+
 	if (spcr->PciVendorId != PCIV_INVALID &&
 	    spcr->PciDeviceId != PCIV_INVALID) {
 		di->pci_info.vendor = spcr->PciVendorId;
diff --git a/sys/dev/uart/uart_subr.c b/sys/dev/uart/uart_subr.c
--- a/sys/dev/uart/uart_subr.c
+++ b/sys/dev/uart/uart_subr.c
@@ -279,6 +279,8 @@
 			break;
 		case UART_TAG_XO:
 			di->bas.rclk = uart_parse_long(&spec);
+			if (di->bas.rclk == 0)
+				di->bas.rclk_guess = 1;
 			break;
 		default:
 			goto inval;