Index: sys/dev/uart/uart_dev_mu.c
===================================================================
--- sys/dev/uart/uart_dev_mu.c
+++ sys/dev/uart/uart_dev_mu.c
@@ -140,6 +140,7 @@
 static void uart_mu_putc(struct uart_bas *bas, int);
 static int uart_mu_rxready(struct uart_bas *bas);
 static int uart_mu_getc(struct uart_bas *bas, struct mtx *);
+static int cpu_clock(void);
 
 static struct uart_ops uart_mu_ops = {
 	.probe = uart_mu_probe,
@@ -157,12 +158,6 @@
 	return (0);
 }
 
-/* 
- * According to the docs, the cpu clock is locked to 250Mhz when
- * the micro-uart is used 
- */
-#define CPU_CLOCK	250000000
-
 static void
 uart_mu_param(struct uart_bas *bas, int baudrate, int databits, int stopbits,
     int parity)
@@ -188,15 +183,11 @@
 		line |= LCR_WLEN8;
 		break;
 	}
-
 	__uart_setreg(bas, AUX_MU_LCR_REG, line);
 
 	/* See 2.2.1 BCM2835-ARM-Peripherals baudrate */
 	if (baudrate != 0) {
-		baud = CPU_CLOCK / (8 * baudrate);
-		/* XXX	
-		 *  baud = cpu_clock() / (8 * baudrate);
-		 */
+		baud = cpu_clock() / (8 * baudrate);
 		__uart_setreg(bas, AUX_MU_BAUD_REG, ((uint32_t)(baud & 0xFFFF)));
 	}
 	
@@ -252,7 +243,7 @@
  */
 struct uart_mu_softc {
 	struct uart_softc	bas;
-  	uint16_t		aux_ier; /* Interrupt mask */
+  	uint16_t		aux_ier;	/* Interrupt mask */
 };
 
 static int uart_mu_bus_attach(struct uart_softc *);
@@ -518,4 +509,40 @@
 	__uart_setreg(bas, AUX_MU_CNTL_REG, CNTL_RXENAB|CNTL_TXENAB);
 	__uart_setreg(bas, AUX_MU_IER_REG, psc->aux_ier);
 	uart_unlock(sc->sc_hwmtx);
+}
+
+/* 
+ * From www.raspberrypi.org/documentation/configuration/uart.md
+ *
+ * "The baud rate of the mini UART is linked to the core frequency of
+ *  the VPU on the VC4 GPU. This means that, as the VPU frequency governor
+ *  varies the core frequency, the baud rate of the mini UART also changes"
+ * 
+ * Since this is rather unusable for a uart the core frequency is locked
+ * if this uart is enabled in the config. Unfortantely it is locked
+ * to 250Mhz on Rpi2 and Rpi3 but 500Mhz on Rpi4 so this driver needs
+ * to determine which is needed. This is one approach, another approach
+ * might be to query bcm2835_cpufreq.c directly.
+ */
+
+#define RPI2_AND_3_CORE_CLOCK	250000000
+#define RPI4_CORE_CLOCK		500000000
+
+static int
+cpu_clock(void)
+{
+	char compat[255];
+	ssize_t compatlen;
+	char *curstr;
+	phandle_t root;
+
+	root = OF_peer(0);
+	if (root == 0)
+		return (RPI2_AND_3_CORE_CLOCK);
+	compatlen = OF_getprop(root, "compatible", compat, sizeof(compat));
+	for (curstr = compat; curstr < compat + compatlen; curstr++) {
+		if (strncmp(curstr, "4-model", 7) == 0)
+			return (RPI4_CORE_CLOCK);
+	}
+	return (RPI2_AND_3_CORE_CLOCK);
 }