diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index 3a8747ec672b..1311b00f8194 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -336,27 +336,29 @@ static void default_serial_dl_write(struct uart_8250_port *up, int value)
 
 #ifdef CONFIG_SERIAL_8250_RT288X
 
+#define UART_REG_UNMAPPED	-1
+
 /* Au1x00/RT288x UART hardware has a weird register layout */
 static const s8 au_io_in_map[8] = {
-	 0,	/* UART_RX  */
-	 2,	/* UART_IER */
-	 3,	/* UART_IIR */
-	 5,	/* UART_LCR */
-	 6,	/* UART_MCR */
-	 7,	/* UART_LSR */
-	 8,	/* UART_MSR */
-	-1,	/* UART_SCR (unmapped) */
+	[UART_RX]	= 0,
+	[UART_IER]	= 2,
+	[UART_IIR]	= 3,
+	[UART_LCR]	= 5,
+	[UART_MCR]	= 6,
+	[UART_LSR]	= 7,
+	[UART_MSR]	= 8,
+	[UART_SCR]	= UART_REG_UNMAPPED,
 };
 
 static const s8 au_io_out_map[8] = {
-	 1,	/* UART_TX  */
-	 2,	/* UART_IER */
-	 4,	/* UART_FCR */
-	 5,	/* UART_LCR */
-	 6,	/* UART_MCR */
-	-1,	/* UART_LSR (unmapped) */
-	-1,	/* UART_MSR (unmapped) */
-	-1,	/* UART_SCR (unmapped) */
+	[UART_TX]	= 1,
+	[UART_IER]	= 2,
+	[UART_FCR]	= 4,
+	[UART_LCR]	= 5,
+	[UART_MCR]	= 6,
+	[UART_LSR]	= UART_REG_UNMAPPED,
+	[UART_MSR]	= UART_REG_UNMAPPED,
+	[UART_SCR]	= UART_REG_UNMAPPED,
 };
 
 unsigned int au_serial_in(struct uart_port *p, int offset)
@@ -364,7 +366,7 @@ unsigned int au_serial_in(struct uart_port *p, int offset)
 	if (offset >= ARRAY_SIZE(au_io_in_map))
 		return UINT_MAX;
 	offset = au_io_in_map[offset];
-	if (offset < 0)
+	if (offset == UART_REG_UNMAPPED)
 		return UINT_MAX;
 	return __raw_readl(p->membase + (offset << p->regshift));
 }
@@ -374,7 +376,7 @@ void au_serial_out(struct uart_port *p, int offset, int value)
 	if (offset >= ARRAY_SIZE(au_io_out_map))
 		return;
 	offset = au_io_out_map[offset];
-	if (offset < 0)
+	if (offset == UART_REG_UNMAPPED)
 		return;
 	__raw_writel(value, p->membase + (offset << p->regshift));
 }