serial/mxs-auart: fix race condition in interrupt handler

The handler needs to ack the pending events before actually handling them.
Otherwise a new event might come in after it it considered non-pending or
handled and is acked then without being handled. So this event is only
noticed when the next interrupt happens.

Without this patch an i.MX28 based machine running an rt-patched kernel
regularly hangs during boot.

Cc: stable@vger.kernel.org # v2.6.39+
Signed-off-by: NUwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/mxs-auart.c

@@ -678,11 +678,18 @@ static void mxs_auart_settermios(struct uart_port *u,
 
 static irqreturn_t mxs_auart_irq_handle(int irq, void *context)
 {
-	u32 istatus, istat;
+	u32 istat;
 	struct mxs_auart_port *s = context;
 	u32 stat = readl(s->port.membase + AUART_STAT);
 
-	istatus = istat = readl(s->port.membase + AUART_INTR);
+	istat = readl(s->port.membase + AUART_INTR);
+
+	/* ack irq */
+	writel(istat & (AUART_INTR_RTIS
+		| AUART_INTR_TXIS
+		| AUART_INTR_RXIS
+		| AUART_INTR_CTSMIS),
+			s->port.membase + AUART_INTR_CLR);
 
 	if (istat & AUART_INTR_CTSMIS) {
 		uart_handle_cts_change(&s->port, stat & AUART_STAT_CTS);
@@ -702,12 +709,6 @@ static irqreturn_t mxs_auart_irq_handle(int irq, void *context)
 		istat &= ~AUART_INTR_TXIS;
 	}
 
-	writel(istatus & (AUART_INTR_RTIS
-		| AUART_INTR_TXIS
-		| AUART_INTR_RXIS
-		| AUART_INTR_CTSMIS),
-			s->port.membase + AUART_INTR_CLR);
-
 	return IRQ_HANDLED;
 }