[PATCH] serial 8250: tighten test for using backup timer

[Alex Williamson <alex.williamson@hp.com>]

Hi,

   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, the patch below tightens 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.
Thanks,

	Alex

Signed-off-by: Alex Williamson <alex.williamson@hp.com>
---

diff -r 2202c155b8c3 drivers/serial/8250.c
--- a/drivers/serial/8250.c	Tue Mar 18 21:34:48 2008 -0700
+++ b/drivers/serial/8250.c	Wed Mar 19 10:32:40 2008 -0600
@@ -1814,6 +1814,7 @@ static int serial8250_startup(struct uar
 	}
 
 	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
@@ -1827,7 +1828,7 @@ static int serial8250_startup(struct uar
 		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 */
@@ -1840,7 +1841,7 @@ static int serial8250_startup(struct uar
 		 * 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;