From b4dbc5d65a67456db9cfe874d123e1c87860301d Mon Sep 17 00:00:00 2001
From: Patrice Chotard <patrice.chotard@foss.st.com>
Date: Wed, 31 May 2023 08:01:30 +0200
Subject: [PATCH] serial: stm32: Wait TC bit before performing initialization

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@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@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 2ba92bf9c4..93f70947ee 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 5bee68fa9c..b7e7a90b93 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)