[PATCH 3/5] serial: imx: avoid spinlock recursion deadlock

From: Andy Lowe<redacted>

The following deadlock has been observed:

imx_int() {
   imx_txint() {
     spin_lock_irqsave(&sport->port.lock,flags);
     /* ^^^uart_port spinlock taken in imx_txint */
     imx_transmit_buffer() {
       uart_write_wakeup(&sport->port) {
         tty_wakeup() {
           hci_uart_tty_wakeup() {
             hci_uart_tx_wakeup() {
               uart_write() {
                 spin_lock_irqsave(&port->lock, flags);
                 /* ^^^deadlock here when spinlock is taken again */
                   .
                   .
                   .
                 spin_unlock_irqrestore(&port->lock, flags);
               }
             }
           }
         }
       }
     }
     spin_unlock_irqrestore(&sport->port.lock,flags);
   }
}

To correct this call uart_write_wakeup() at the end of imx_txint() after
the uart_port spinlock is unlocked.

Signed-off-by: Andy Lowe<redacted>
Signed-off-by: Dirk Behme<redacted>
---
  drivers/tty/serial/imx.c |    7 ++++++-
  1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/serial/imx.c b/drivers/tty/serial/imx.c
index abe31ad..cc79706 100644
--- a/drivers/tty/serial/imx.c
+++ b/drivers/tty/serial/imx.c

@@ -636,8 +636,13 @@ static irqreturn_t imx_txint(int irq, void *dev_id)

  	imx_transmit_buffer(sport);

-	if (uart_circ_chars_pending(xmit)<  WAKEUP_CHARS)
+	if (uart_circ_chars_pending(xmit)<  WAKEUP_CHARS) {
+		spin_unlock_irqrestore(&sport->port.lock, flags);
  		uart_write_wakeup(&sport->port);
+	} else
+		spin_unlock_irqrestore(&sport->port.lock, flags);
+
+	return IRQ_HANDLED;

  out:
  	spin_unlock_irqrestore(&sport->port.lock, flags);