serial: stm32: fix incorrect characters on console

Incorrect characters are observed on console during boot. This issue occurs
when init/main.c is modifying termios settings to open /dev/console on the
rootfs.

This patch adds a waiting loop in set_termios to wait for TX shift register
empty (and TX FIFO if any) before stopping serial port.

Fixes: 48a6092fb41f ("serial: stm32-usart: Add STM32 USART Driver")
Signed-off-by: Erwan Le Ray <erwan.leray@foss.st.com>
Link: https://lore.kernel.org/r/20210304162308.8984-4-erwan.leray@foss.st.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/serial/stm32-usart.c b/drivers/tty/serial/stm32-usart.c
index c6ca8f964c696269283a6b9ab325bc9d9b5c7c76..eae54b8cf5e250ef6f1f02c9e76ab5e3a57d6f89 100644 (file)
--- a/drivers/tty/serial/stm32-usart.c
+++ b/drivers/tty/serial/stm32-usart.c
@@ -738,8 +738,9 @@ static void stm32_usart_set_termios(struct uart_port *port,
        unsigned int baud, bits;
        u32 usartdiv, mantissa, fraction, oversampling;
        tcflag_t cflag = termios->c_cflag;
-       u32 cr1, cr2, cr3;
+       u32 cr1, cr2, cr3, isr;
        unsigned long flags;
+       int ret;
 
        if (!stm32_port->hw_flow_control)
                cflag &= ~CRTSCTS;
@@ -748,6 +749,15 @@ static void stm32_usart_set_termios(struct uart_port *port,
 
        spin_lock_irqsave(&port->lock, flags);
 
+       ret = readl_relaxed_poll_timeout_atomic(port->membase + ofs->isr,
+                                               isr,
+                                               (isr & USART_SR_TC),
+                                               10, 100000);
+
+       /* Send the TC error message only when ISR_TC is not set. */
+       if (ret)
+               dev_err(port->dev, "Transmission is not complete\n");
+
        /* Stop serial port and reset value */
        writel_relaxed(0, port->membase + ofs->cr1);