serial 8250: tighten test for using backup timer

Thomas Koeller had reported an issue where a device that had been making use
of the UART_BUG_TXEN code in the 8250 driver was mistakenly being caught by
the backup timer test, causing the device to work improperly.

To fix this, tighten the test requirements to enable the backup timer
workaround.

The backup timer is really meant to catch UARTs that don't re-assert the THRE
interrupt.  The expectation is that they do initially assert THRE.  This patch
clarifies the test.
Signed-off-by: NAlex Williamson <alex.williamson@hp.com>
Cc: Thomas Koeller <thomas@koeller.dyndns.org>
Cc: Russell King <rmk@arm.linux.org.uk>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

drivers/serial/8250.c

@@ -1868,6 +1868,7 @@ static int serial8250_startup(struct uart_port *port)
 	}
 
 	if (is_real_interrupt(up->port.irq)) {
+		unsigned char iir1;
 		/*
 		 * Test for UARTs that do not reassert THRE when the
 		 * transmitter is idle and the interrupt has already
@@ -1881,7 +1882,7 @@ static int serial8250_startup(struct uart_port *port)
 		wait_for_xmitr(up, UART_LSR_THRE);
 		serial_out_sync(up, UART_IER, UART_IER_THRI);
 		udelay(1); /* allow THRE to set */
-		serial_in(up, UART_IIR);
+		iir1 = serial_in(up, UART_IIR);
 		serial_out(up, UART_IER, 0);
 		serial_out_sync(up, UART_IER, UART_IER_THRI);
 		udelay(1); /* allow a working UART time to re-assert THRE */
@@ -1894,7 +1895,7 @@ static int serial8250_startup(struct uart_port *port)
 		 * If the interrupt is not reasserted, setup a timer to
 		 * kick the UART on a regular basis.
 		 */
-		if (iir & UART_IIR_NO_INT) {
+		if (!(iir1 & UART_IIR_NO_INT) && (iir & UART_IIR_NO_INT)) {
 			pr_debug("ttyS%d - using backup timer\n", port->line);
 			up->timer.function = serial8250_backup_timeout;
 			up->timer.data = (unsigned long)up;