[PATCH 1/2] serial: stm32: Wait TC bit before performing initialization

[Patrice Chotard]

In case there is still chars from previous bootstage to transmit, wait
for TC (Transmission Complete) bit to be set which ensure that the last
data written in the USART_TDR has been transmitted out of the shift
register.

Signed-off-by: Patrice Chotard <patrice.chotard at foss.st.com>
---

 drivers/serial/serial_stm32.c | 15 +++++++++++++++
 drivers/serial/serial_stm32.h |  1 +
 2 files changed, 16 insertions(+)

diff --git a/drivers/serial/serial_stm32.c b/drivers/serial/serial_stm32.c
index 2ba92bf9c48..93f70947eec 100644
--- a/drivers/serial/serial_stm32.c
+++ b/drivers/serial/serial_stm32.c
@@ -18,6 +18,7 @@
 #include <dm/device_compat.h>
 #include <linux/bitops.h>
 #include <linux/delay.h>
+#include <linux/iopoll.h>
 #include "serial_stm32.h"
 #include <dm/device_compat.h>
 
@@ -181,9 +182,12 @@ static int stm32_serial_probe(struct udevice *dev)
 	struct stm32x7_serial_plat *plat = dev_get_plat(dev);
 	struct clk clk;
 	struct reset_ctl reset;
+	u32 isr;
 	int ret;
+	bool stm32f4;
 
 	plat->uart_info = (struct stm32_uart_info *)dev_get_driver_data(dev);
+	stm32f4 = plat->uart_info->stm32f4;
 
 	ret = clk_get_by_index(dev, 0, &clk);
 	if (ret < 0)
@@ -195,6 +199,17 @@ static int stm32_serial_probe(struct udevice *dev)
 		return ret;
 	}
 
+	/*
+	 * before uart initialization, wait for TC bit (Transmission Complete)
+	 * in case there is still chars from previous bootstage to transmit
+	 */
+	ret = read_poll_timeout(readl, isr, isr & USART_ISR_TC, 10, 150,
+				plat->base + ISR_OFFSET(stm32f4));
+	if (ret) {
+		clk_disable(&clk);
+		return ret;
+	}
+
 	ret = reset_get_by_index(dev, 0, &reset);
 	if (!ret) {
 		reset_assert(&reset);
diff --git a/drivers/serial/serial_stm32.h b/drivers/serial/serial_stm32.h
index 5bee68fa9c2..b7e7a90b931 100644
--- a/drivers/serial/serial_stm32.h
+++ b/drivers/serial/serial_stm32.h
@@ -66,6 +66,7 @@ struct stm32x7_serial_plat {
 #define USART_CR3_OVRDIS		BIT(12)
 
 #define USART_ISR_TXE			BIT(7)
+#define USART_ISR_TC			BIT(6)
 #define USART_ISR_RXNE			BIT(5)
 #define USART_ISR_ORE			BIT(3)
 #define USART_ISR_FE			BIT(1)
-- 
2.25.1