serial: 8250: Define IIR 64 byte bit & cleanup related code

16750 indicates 64 bytes FIFO with a IIR bit. Add define for it and
make related code more obvious.
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20221125130509.8482-6-ilpo.jarvinen@linux.intel.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

@@ -1050,11 +1050,12 @@ static void autoconfig_16550a(struct uart_8250_port *up)
 			serial_out(up, UART_LCR, 0);
 			serial_out(up, UART_FCR, UART_FCR_ENABLE_FIFO |
 				   UART_FCR7_64BYTE);
-			status1 = serial_in(up, UART_IIR) >> 5;
+			status1 = serial_in(up, UART_IIR) & (UART_IIR_64BYTE_FIFO |
+							     UART_IIR_FIFO_ENABLED);
 			serial_out(up, UART_FCR, 0);
 			serial_out(up, UART_LCR, 0);
 
-			if (status1 == 7)
+			if (status1 == (UART_IIR_64BYTE_FIFO | UART_IIR_FIFO_ENABLED))
 				up->port.type = PORT_16550A_FSL64;
 			else
 				DEBUG_AUTOCONF("Motorola 8xxx DUART ");
@@ -1122,17 +1123,20 @@ static void autoconfig_16550a(struct uart_8250_port *up)
 	 */
 	serial_out(up, UART_LCR, 0);
 	serial_out(up, UART_FCR, UART_FCR_ENABLE_FIFO | UART_FCR7_64BYTE);
-	status1 = serial_in(up, UART_IIR) >> 5;
+	status1 = serial_in(up, UART_IIR) & (UART_IIR_64BYTE_FIFO | UART_IIR_FIFO_ENABLED);
 	serial_out(up, UART_FCR, UART_FCR_ENABLE_FIFO);
+
 	serial_out(up, UART_LCR, UART_LCR_CONF_MODE_A);
 	serial_out(up, UART_FCR, UART_FCR_ENABLE_FIFO | UART_FCR7_64BYTE);
-	status2 = serial_in(up, UART_IIR) >> 5;
+	status2 = serial_in(up, UART_IIR) & (UART_IIR_64BYTE_FIFO | UART_IIR_FIFO_ENABLED);
 	serial_out(up, UART_FCR, UART_FCR_ENABLE_FIFO);
+
 	serial_out(up, UART_LCR, 0);
 
 	DEBUG_AUTOCONF("iir1=%d iir2=%d ", status1, status2);
 
-	if (status1 == 6 && status2 == 7) {
+	if (status1 == UART_IIR_FIFO_ENABLED_16550A &&
+	    status2 == (UART_IIR_64BYTE_FIFO | UART_IIR_FIFO_ENABLED_16550A)) {
 		up->port.type = PORT_16750;
 		up->capabilities |= UART_CAP_AFE | UART_CAP_SLEEP;
 		return;

include/uapi/linux/serial_reg.h

@@ -44,6 +44,7 @@
 #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_64BYTE_FIFO	0x20 /* 16750 64 bytes FIFO */
 #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 */