serial: pxa: work around for errata torvalds#20

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>
Cc: stable <stable@kernel.org>
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)