8250: improve workaround for UARTs that don't re-assert THRE correctly

Recent changes to tighten the check for UARTs that don't correctly
re-assert THRE (01c194d9278efc15d4785ff205643e9c0bdcef53: "serial 8250:
tighten test for using backup timer") caused problems when such a UART was
opened for the second time - the bug could only successfully be detected
at first initialization.  For users of this version of this particular
UART IP it is fatal.

This patch stores the information about the bug in the bugs field of the
port structure when the port is first started up so subsequent opens can
check this bit even if the test for the bug fails.

David Brownell: "My own exposure to this is that the UART on DaVinci
hardware, which TI allegedly derived from its original 16550 logic, has
periodically gone from working to unusable with the mainline 8250.c ...
and back and forth a bunch.  Currently it's "unusable", a regression from
some previous versions.  With this patch from Will, it's usable."

Signed-off-by: Will Newton <will.newton@gmail.com>
Acked-by: Alex Williamson <alex.williamson@hp.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: David Brownell <david-b@pacbell.net>
Cc: <stable@kernel.org>		[2.6.26.x]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 12 insertions, 4 deletions

diff --git a/drivers/serial/8250.c b/drivers/serial/8250.c
index 342e12f..9ccc563 100644
--- a/drivers/serial/8250.c
+++ b/drivers/serial/8250.c
@@ -1908,15 +1908,23 @@ static int serial8250_startup(struct uart_port *port)
 		 * kick the UART on a regular basis.
 		 */
 		if (!(iir1 & UART_IIR_NO_INT) && (iir & UART_IIR_NO_INT)) {
+			up->bugs |= UART_BUG_THRE;
 			pr_debug("ttyS%d - using backup timer\n", port->line);
-			up->timer.function = serial8250_backup_timeout;
-			up->timer.data = (unsigned long)up;
-			mod_timer(&up->timer, jiffies +
-				poll_timeout(up->port.timeout) + HZ / 5);
 		}
 	}
 
 	/*
+	 * The above check will only give an accurate result the first time
+	 * the port is opened so this value needs to be preserved.
+	 */
+	if (up->bugs & UART_BUG_THRE) {
+		up->timer.function = serial8250_backup_timeout;
+		up->timer.data = (unsigned long)up;
+		mod_timer(&up->timer, jiffies +
+			  poll_timeout(up->port.timeout) + HZ / 5);
+	}
+
+	/*
 	 * If the "interrupt" for this port doesn't correspond with any
 	 * hardware interrupt, we use a timer-based system.  The original
 	 * driver used to do this with IRQ0.