[PATCH v4 1/2] 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 or baudrate change
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 proceeding 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>
Tested-by: Lothar Waßmann <LW at KARO-electronics.de>
---
 v2: Add this commit to the series
 v3: Fix typo & reordering commits for good bisectabilit
 v4: Wait for transfer completion before baudrate change as well

 drivers/serial/serial_mxc.c | 26 +++++++++++++++++++++++++-
 1 file changed, 25 insertions(+), 1 deletion(-)

diff --git a/drivers/serial/serial_mxc.c b/drivers/serial/serial_mxc.c
index 82c0d84628..9e5b987994 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);
 
@@ -169,6 +184,8 @@ static void _mxc_serial_setbrg(struct mxc_uart *base, unsigned long clk,
 {
 	u32 tmp;
 
+	_mxc_serial_flush(base);
+
 	tmp = RFDIV << UFCR_RFDIV_SHF;
 	if (use_dte)
 		tmp |= UFCR_DCEDTE;
@@ -252,10 +269,17 @@ 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