[PATCH v2 2/2] serial: mxc: Wait for TX completion before reset

[Loic Poulain]

The u-boot console may show some corrupted characters when
printing in board_init() due to reset of the UART (probe)
before the TX FIFO has been completely drained.

To fix this issue, and in case UART is still running, we now
try to flush the FIFO before proceding to UART reinitialization.
For this we're waiting for Transmitter Complete bit, indicating
that the FIFO and the shift register are empty.

flushing has a 4ms timeout guard, which is normally more than
enough to consume the FIFO @ low baudrate (9600bps).

Signed-off-by: Loic Poulain <loic.poulain at linaro.org>
---
 v2: Add this commit to the series

 drivers/serial/serial_mxc.c | 24 +++++++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)

diff --git a/drivers/serial/serial_mxc.c b/drivers/serial/serial_mxc.c
index f8c49865be..9899155c00 100644
--- a/drivers/serial/serial_mxc.c
+++ b/drivers/serial/serial_mxc.c
@@ -13,6 +13,7 @@
 #include <dm/platform_data/serial_mxc.h>
 #include <serial.h>
 #include <linux/compiler.h>
+#include <linux/delay.h>
 
 /* UART Control Register Bit Fields.*/
 #define URXD_CHARRDY	(1<<15)
@@ -144,8 +145,22 @@ struct mxc_uart {
 	u32 ts;
 };
 
+static void _mxc_serial_flush(struct mxc_uart *base)
+{
+	unsigned int timeout = 4000;
+
+	if (!(readl(&base->cr1) & UCR1_UARTEN) ||
+	    !(readl(&base->cr2) & UCR2_TXEN))
+		return;
+
+	while (!(readl(&base->sr2) & USR2_TXDC) && --timeout)
+		udelay(1);
+}
+
 static void _mxc_serial_init(struct mxc_uart *base, int use_dte)
 {
+	_mxc_serial_flush(base);
+
 	writel(0, &base->cr1);
 	writel(0, &base->cr2);
 
@@ -252,10 +267,17 @@ static int mxc_serial_init(void)
 	return 0;
 }
 
+static int mxc_serial_stop(void)
+{
+	_mxc_serial_flush(mxc_base);
+
+	return 0;
+}
+
 static struct serial_device mxc_serial_drv = {
 	.name	= "mxc_serial",
 	.start	= mxc_serial_init,
-	.stop	= NULL,
+	.stop	= mxc_serial_stop,
 	.setbrg	= mxc_serial_setbrg,
 	.putc	= mxc_serial_putc,
 	.puts	= default_serial_puts,
-- 
2.34.1