[PATCH 1/1] serial: samsung: Clear operation mode on UART shutdown

[Javier Martinez Canillas <javier.martinez@collabora.co.uk>]

Exynos serial ports operate either in a DMA-based or interrupt-based
modes. In DMA-based mode, the UART generates a transfer data request
and a Transmission (Tx) interrupt in interrupt-based mode.

The Tx IRQ is only unmasked in interrupt-based mode and it was done
in s3c24xx_serial_start_tx(). Commit ba019a3e2ad5 ("serial: samsung:
remove redundant interrupt enabling") removed the IRQ enable on that
function since it is enabled when the mode is set in enable_tx_pio().

The problem is that enable_tx_pio() is only called if the port mode
has not been set before but the mode was not cleared on .shutdown().

So if the UART was shutdown and then started up again, the mode set
will remain and the Tx IRQ won't be unmasked.

This caused a hang on at least Exynos5250, Exynos5420 and Exynos5800
when the system is rebooted or powered off.

Fixes: ba019a3e2ad5 ("serial: samsung: remove redundant interrupt enabling")
Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
---

I noticed this issue on an Exynos5250 Snow, Exynos5420 Peach Pit and Exynos
5800 Peach Pi Chromebooks. This patch fixes the issue on all of them.

The offending commit landed in v4.0-rc1 so this fix is -rc material.

 drivers/tty/serial/samsung.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/tty/serial/samsung.c b/drivers/tty/serial/samsung.c
index af821a908720..cf08876922f1 100644
--- a/drivers/tty/serial/samsung.c
+++ b/drivers/tty/serial/samsung.c
@@ -963,6 +963,7 @@ static void s3c24xx_serial_shutdown(struct uart_port *port)
 			free_irq(ourport->tx_irq, ourport);
 		tx_enabled(port) = 0;
 		ourport->tx_claimed = 0;
+		ourport->tx_mode = 0;
 	}
 
 	if (ourport->rx_claimed) {
-- 
2.1.4