serial: pxa: work around for errata torvalds#20

commit e44aabd upstream.

Errata E20: UART: Character Timeout interrupt remains set under certain
software conditions.

Implication: The software servicing the UART can be trapped in an infinite loop.

Signed-off-by: Marcus Folkesson <marcus.folkesson@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/tty/serial/pxa.c

@@ -100,6 +100,16 @@ static inline void receive_chars(struct uart_pxa_port *up, int *status)
 	int max_count = 256;
 
 	do {
+		/* work around Errata #20 according to
+		 * Intel(R) PXA27x Processor Family
+		 * Specification Update (May 2005)
+		 *
+		 * Step 2
+		 * Disable the Reciever Time Out Interrupt via IER[RTOEI]
+		 */
+		up->ier &= ~UART_IER_RTOIE;
+		serial_out(up, UART_IER, up->ier);
+
 		ch = serial_in(up, UART_RX);
 		flag = TTY_NORMAL;
 		up->port.icount.rx++;
@@ -156,6 +166,16 @@ static inline void receive_chars(struct uart_pxa_port *up, int *status)
 		*status = serial_in(up, UART_LSR);
 	} while ((*status & UART_LSR_DR) && (max_count-- > 0));
 	tty_flip_buffer_push(tty);
+
+	/* work around Errata #20 according to
+	 * Intel(R) PXA27x Processor Family
+	 * Specification Update (May 2005)
+	 *
+	 * Step 6:
+	 * No more data in FIFO: Re-enable RTO interrupt via IER[RTOIE]
+	 */
+	up->ier |= UART_IER_RTOIE;
+	serial_out(up, UART_IER, up->ier);
 }
 
 static void transmit_chars(struct uart_pxa_port *up)