sc16is7xx: remove RS-485 delay RTS handling

Users of RS-485 can request via ioctl that RTS signals should
be activated selected number of milliseconds before the actual
data transmission or delay reception certain number of milli-
seconds after the transmission is finished.  In sc16is7xx,
however, RTS signalling is handled by the hardware and driver
has no way of providing this feature.

We still try to provide .delay_rts_before_send by delaying
transmission but without actual effect on the RTS line.

Note: this change will make the driver return -EINVAL when the
      feature is requested (.delay_rts_after_send is set).
Signed-off-by: Jakub Kicinski <kubakici@wp.pl>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/sc16is7xx.c

@@ -652,21 +652,6 @@ static void sc16is7xx_wq_proc(struct work_struct *ws)
 
 static void sc16is7xx_stop_tx(struct uart_port* port)
 {
-	struct sc16is7xx_one *one = to_sc16is7xx_one(port, port);
-	struct circ_buf *xmit = &one->port.state->xmit;
-
-	/* handle rs485 */
-	if (port->rs485.flags & SER_RS485_ENABLED) {
-		/* do nothing if current tx not yet completed */
-		int lsr = sc16is7xx_port_read(port, SC16IS7XX_LSR_REG);
-		if (!(lsr & SC16IS7XX_LSR_TEMT_BIT))
-			return;
-
-		if (uart_circ_empty(xmit) &&
-		    (port->rs485.delay_rts_after_send > 0))
-			mdelay(port->rs485.delay_rts_after_send);
-	}
-
 	sc16is7xx_port_update(port, SC16IS7XX_IER_REG,
 			      SC16IS7XX_IER_THRI_BIT,
 			      0);
@@ -852,6 +837,14 @@ static int sc16is7xx_config_rs485(struct uart_port *port,
 			dev_err(port->dev,
 				"unsupported RTS signalling on_send:%d after_send:%d - exactly one of RS485 RTS flags should be set\n",
 				rts_during_tx, rts_during_rx);
+
+		/*
+		 * RTS signal is handled by HW, it's timing can't be influenced.
+		 * However, it's sometimes useful to delay TX even without RTS
+		 * control therefore we try to handle .delay_rts_before_send.
+		 */
+		if (rs485->delay_rts_after_send)
+			return -EINVAL;
 	}
 
 	sc16is7xx_port_update(port, SC16IS7XX_EFCR_REG, mask, efcr);