USB: serial: mos7840: clean up register handling

In the read/write function, set port 2 independently in the 2-port case.

When setting the offset of port registers, the offset between port 1 and
other ports is different, so port 1 is set independently.
Then in the rest of ports, the port 2 between 2-ports case and 4-ports case
is different, so port 2 in 2-ports case is set independently.

Specifically, port 2 in the 2-port case maps to the registers used by
port 3 in the 4-port case.

Signed-off-by: JackyChou <[e-mail]>
[ johan: simplify register-offset handling at port probe, add a comment
and amend commit message ]
Signed-off-by: Johan Hovold <[e-mail]>

drivers/usb/serial/mos7840.c

···
 	val = val & 0x00ff;
 	/* For the UART control registers, the application number need
 	   to be Or'ed */
-	if (port->serial->num_ports == 4) {
+	if (port->serial->num_ports == 2 && port->port_number != 0)
+		val |= ((__u16)port->port_number + 2) << 8;
+	else
 		val |= ((__u16)port->port_number + 1) << 8;
-	} else {
-		if (port->port_number == 0) {
-			val |= ((__u16)port->port_number + 1) << 8;
-		} else {
-			val |= ((__u16)port->port_number + 2) << 8;
-		}
-	}
 	dev_dbg(&port->dev, "%s application number is %x\n", __func__, val);
 	return usb_control_msg(dev, usb_sndctrlpipe(dev, 0), MCS_WRREQ,
 			       MCS_WR_RTYPE, val, reg, NULL, 0,
···
 		return -ENOMEM;
 
 	/* Wval  is same as application number */
-	if (port->serial->num_ports == 4) {
+	if (port->serial->num_ports == 2 && port->port_number != 0)
+		Wval = ((__u16)port->port_number + 2) << 8;
+	else
 		Wval = ((__u16)port->port_number + 1) << 8;
-	} else {
-		if (port->port_number == 0) {
-			Wval = ((__u16)port->port_number + 1) << 8;
-		} else {
-			Wval = ((__u16)port->port_number + 2) << 8;
-		}
-	}
 	dev_dbg(&port->dev, "%s application number is %x\n", __func__, Wval);
 	ret = usb_control_msg(dev, usb_rcvctrlpipe(dev, 0), MCS_RDREQ,
 			      MCS_RD_RTYPE, Wval, reg, buf, VENDOR_READ_LENGTH,
···
 		mos7840_port->SpRegOffset = 0x0;
 		mos7840_port->ControlRegOffset = 0x1;
 		mos7840_port->DcrRegOffset = 0x4;
-	} else if ((mos7840_port->port_num == 2) && (serial->num_ports == 4)) {
-		mos7840_port->SpRegOffset = 0x8;
-		mos7840_port->ControlRegOffset = 0x9;
-		mos7840_port->DcrRegOffset = 0x16;
-	} else if ((mos7840_port->port_num == 2) && (serial->num_ports == 2)) {
-		mos7840_port->SpRegOffset = 0xa;
-		mos7840_port->ControlRegOffset = 0xb;
-		mos7840_port->DcrRegOffset = 0x19;
-	} else if ((mos7840_port->port_num == 3) && (serial->num_ports == 4)) {
-		mos7840_port->SpRegOffset = 0xa;
-		mos7840_port->ControlRegOffset = 0xb;
-		mos7840_port->DcrRegOffset = 0x19;
-	} else if ((mos7840_port->port_num == 4) && (serial->num_ports == 4)) {
-		mos7840_port->SpRegOffset = 0xc;
-		mos7840_port->ControlRegOffset = 0xd;
-		mos7840_port->DcrRegOffset = 0x1c;
+	} else {
+		u8 phy_num = mos7840_port->port_num;
+
+		/* Port 2 in the 2-port case uses registers of port 3 */
+		if (serial->num_ports == 2)
+			phy_num = 3;
+
+		mos7840_port->SpRegOffset = 0x8 + 2 * (phy_num - 2);
+		mos7840_port->ControlRegOffset = 0x9 + 2 * (phy_num - 2);
+		mos7840_port->DcrRegOffset = 0x16 + 3 * (phy_num - 2);
 	}
 	mos7840_dump_serial_port(port, mos7840_port);
 	mos7840_set_port_private(port, mos7840_port);