USB: serial: ti_usb_3410_5052: drop drain delay for 3410

Unlike the TUSB5052, the TUSB3410 has an LSR TEMT bit to tell if both
the transmitter data and shift registers are empty.

Make sure to check also the shift register on TUSB3410 when waiting for
the transmit buffer to drain during close and drop the time-based
one-char delay which is otherwise needed (e.g. 90 ms at 110 bps).
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/ti_usb_3410_5052.c

@@ -121,6 +121,7 @@
 #define TI_LSR_ERROR			0x0F
 #define TI_LSR_RX_FULL			0x10
 #define TI_LSR_TX_EMPTY			0x20
+#define TI_LSR_TX_EMPTY_BOTH		0x40
 
 /* Line control */
 #define TI_LCR_BREAK			0x40
@@ -614,7 +615,8 @@ static int ti_port_probe(struct usb_serial_port *port)
 	 * The TUSB5052 LSR does not tell when the transmitter shift register
 	 * has emptied so add a one-character drain delay.
 	 */
-	port->port.drain_delay = 1;
+	if (!tport->tp_tdev->td_is_3410)
+		port->port.drain_delay = 1;
 
 	return 0;
 }
@@ -851,11 +853,20 @@ static int ti_chars_in_buffer(struct tty_struct *tty)
 static bool ti_tx_empty(struct usb_serial_port *port)
 {
 	struct ti_port *tport = usb_get_serial_port_data(port);
+	u8 lsr, mask;
 	int ret;
-	u8 lsr;
+
+	/*
+	 * TUSB5052 does not have the TEMT bit to tell if the shift register
+	 * is empty.
+	 */
+	if (tport->tp_tdev->td_is_3410)
+		mask = TI_LSR_TX_EMPTY_BOTH;
+	else
+		mask = TI_LSR_TX_EMPTY;
 
 	ret = ti_get_lsr(tport, &lsr);
-	if (!ret && !(lsr & TI_LSR_TX_EMPTY))
+	if (!ret && !(lsr & mask))
 		return false;
 
 	return true;