serial: stm32: add support for "flush_buffer" ops

Add the support for "flush_buffer" ops in order to flush any write buffers,
reset any DMA state and stop any ongoing DMA transfers when the
port->state->xmit circular buffer is cleared.

Signed-off-by: Erwan Le Ray <erwan.leray@foss.st.com>
Link: https://lore.kernel.org/r/20210304162308.8984-14-erwan.leray@foss.st.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/stm32-usart.c

@@ -597,6 +597,19 @@ static void stm32_usart_start_tx(struct uart_port *port)
 	stm32_usart_transmit_chars(port);
 }
 
+/* Flush the transmit buffer. */
+static void stm32_usart_flush_buffer(struct uart_port *port)
+{
+	struct stm32_port *stm32_port = to_stm32_port(port);
+	const struct stm32_usart_offsets *ofs = &stm32_port->info->ofs;
+
+	if (stm32_port->tx_ch) {
+		dmaengine_terminate_async(stm32_port->tx_ch);
+		stm32_usart_clr_bits(port, ofs->cr3, USART_CR3_DMAT);
+		stm32_port->tx_dma_busy = false;
+	}
+}
+
 /* Throttle the remote when input buffer is about to overflow. */
 static void stm32_usart_throttle(struct uart_port *port)
 {
@@ -992,6 +1005,7 @@ static const struct uart_ops stm32_uart_ops = {
 	.break_ctl	= stm32_usart_break_ctl,
 	.startup	= stm32_usart_startup,
 	.shutdown	= stm32_usart_shutdown,
+	.flush_buffer	= stm32_usart_flush_buffer,
 	.set_termios	= stm32_usart_set_termios,
 	.pm		= stm32_usart_pm,
 	.type		= stm32_usart_type,