[PATCH] serial: pl011: use port lock to guard control register access

[Jon Medhurst (Tixy)]

When the pl011 is being used for a console, pl011_console_write forces
the control register (CR) to enable the UART for transmission and then
restores this to the original value afterwards. It does this while
holding the port lock.

Unfortunately, when the uart is started or shutdown - say in response to
userland using the serial device for a terminal - then this updates the
control register without any locking.

This means we can have

  pl011_console_write   Save CR
  pl011_startup         Initialise CR, e.g. enable receive
  pl011_console_write   Restore old CR with receive not enabled

this result is a serial port which doesn't respond to any input.

A similar race in reverse could happen when the device is shutdown.

We can fix these problems by taking the port lock when updating CR.

Signed-off-by: Jon Medhurst <tixy at linaro.org>
---

I can't help but think I've missed something fundamental here as it
seems wrong to be in the situation at all where were initialising
hardware if it is already being used?

Also, even with this fix I still see spurious/corrupt output on the
serial port at the point the port is initialised, so there is obviously
some other issues in this area.

 drivers/tty/serial/amba-pl011.c |    6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c
index 7203864..a0dfb86 100644
--- a/drivers/tty/serial/amba-pl011.c
+++ b/drivers/tty/serial/amba-pl011.c
@@ -1537,6 +1537,8 @@ static int pl011_startup(struct uart_port *port)
 	/*
 	 * Provoke TX FIFO interrupt into asserting.
 	 */
+	spin_lock_irq(&uap->port.lock);
+
 	cr = UART01x_CR_UARTEN | UART011_CR_TXE | UART011_CR_LBE;
 	writew(cr, uap->port.membase + UART011_CR);
 	writew(0, uap->port.membase + UART011_FBRD);
@@ -1561,6 +1563,8 @@ static int pl011_startup(struct uart_port *port)
 	cr |= UART01x_CR_UARTEN | UART011_CR_RXE | UART011_CR_TXE;
 	writew(cr, uap->port.membase + UART011_CR);
 
+	spin_unlock_irq(&uap->port.lock);
+
 	/*
 	 * initialise the old status of the modem signals
 	 */
@@ -1629,11 +1633,13 @@ static void pl011_shutdown(struct uart_port *port)
 	 * it during startup().
 	 */
 	uap->autorts = false;
+	spin_lock_irq(&uap->port.lock);
 	cr = readw(uap->port.membase + UART011_CR);
 	uap->old_cr = cr;
 	cr &= UART011_CR_RTS | UART011_CR_DTR;
 	cr |= UART01x_CR_UARTEN | UART011_CR_TXE;
 	writew(cr, uap->port.membase + UART011_CR);
+	spin_unlock_irq(&uap->port.lock);
 
 	/*
 	 * disable break condition and fifos
-- 
1.7.10.4