[PATCH 5.10 021/105] serial: stm32: prevent TDR register overwrite when sending x_char

[Greg Kroah-Hartman]

From: Valentin Caron <valentin.caron@xxxxxxxxxxx>

[ Upstream commit d3d079bde07e1b7deaeb57506dc0b86010121d17 ]

When sending x_char in stm32_usart_transmit_chars(), driver can overwrite
the value of TDR register by the value of x_char. If this happens, the
previous value that was present in TDR register will not be sent through
uart.

This code checks if the previous value in TDR register is sent before
writing the x_char value into register.

Fixes: 48a6092fb41f ("serial: stm32-usart: Add STM32 USART Driver")
Cc: stable <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Valentin Caron <valentin.caron@xxxxxxxxxxx>
Link: https://lore.kernel.org/r/20220111164441.6178-2-valentin.caron@xxxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 drivers/tty/serial/stm32-usart.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/drivers/tty/serial/stm32-usart.c b/drivers/tty/serial/stm32-usart.c
index 0eadf0547175c..6afae051ba8d1 100644
--- a/drivers/tty/serial/stm32-usart.c
+++ b/drivers/tty/serial/stm32-usart.c
@@ -420,10 +420,22 @@ static void stm32_usart_transmit_chars(struct uart_port *port)
 	struct stm32_port *stm32_port = to_stm32_port(port);
 	const struct stm32_usart_offsets *ofs = &stm32_port->info->ofs;
 	struct circ_buf *xmit = &port->state->xmit;
+	u32 isr;
+	int ret;
 
 	if (port->x_char) {
 		if (stm32_port->tx_dma_busy)
 			stm32_usart_clr_bits(port, ofs->cr3, USART_CR3_DMAT);
+
+		/* Check that TDR is empty before filling FIFO */
+		ret =
+		readl_relaxed_poll_timeout_atomic(port->membase + ofs->isr,
+						  isr,
+						  (isr & USART_SR_TXE),
+						  10, 1000);
+		if (ret)
+			dev_warn(port->dev, "1 character may be erased\n");
+
 		writel_relaxed(port->x_char, port->membase + ofs->tdr);
 		port->x_char = 0;
 		port->icount.tx++;
-- 
2.34.1