serial: 8250: Add IIR FIFOs enabled field properly

Don't use magic literals & comments but define a real field instead
for UART_IIR_FIFO_ENABLED and name also the values.
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20221125130509.8482-5-ilpo.jarvinen@linux.intel.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_port.c

@@ -1293,22 +1293,19 @@ static void autoconfig(struct uart_8250_port *up)
 
 	serial_out(up, UART_FCR, UART_FCR_ENABLE_FIFO);
 
-	/* Assign this as it is to truncate any bits above 7.  */
-	scratch = serial_in(up, UART_IIR);
-
-	switch (scratch >> 6) {
-	case 0:
+	switch (serial_in(up, UART_IIR) & UART_IIR_FIFO_ENABLED) {
+	case UART_IIR_FIFO_ENABLED_8250:
 		autoconfig_8250(up);
 		break;
-	case 1:
-		port->type = PORT_UNKNOWN;
-		break;
-	case 2:
+	case UART_IIR_FIFO_ENABLED_16550:
 		port->type = PORT_16550;
 		break;
-	case 3:
+	case UART_IIR_FIFO_ENABLED_16550A:
 		autoconfig_16550a(up);
 		break;
+	default:
+		port->type = PORT_UNKNOWN;
+		break;
 	}
 
 #ifdef CONFIG_SERIAL_8250_RSA

include/uapi/linux/serial_reg.h

@@ -44,6 +44,10 @@
 #define UART_IIR_RX_TIMEOUT	0x0c /* OMAP RX Timeout interrupt */
 #define UART_IIR_XOFF		0x10 /* OMAP XOFF/Special Character */
 #define UART_IIR_CTS_RTS_DSR	0x20 /* OMAP CTS/RTS/DSR Change */
+#define UART_IIR_FIFO_ENABLED	0xc0 /* FIFOs enabled / port type identification */
+#define  UART_IIR_FIFO_ENABLED_8250	0x00	/* 8250: no FIFO */
+#define  UART_IIR_FIFO_ENABLED_16550	0x80	/* 16550: (broken/unusable) FIFO */
+#define  UART_IIR_FIFO_ENABLED_16550A	0xc0	/* 16550A: FIFO enabled */
 
 #define UART_FCR	2	/* Out: FIFO Control Register */
 #define UART_FCR_ENABLE_FIFO	0x01 /* Enable the FIFO */