[sg toup] drivers: serial: si32: Support UART line control API

Implementing this API allows PPP to (temporally) prevent its peer (Linux
in our case) from sending more bytes. Without this, Linux may send data
too fast, causing messages to be lost on the radio module side.

This commit should be part of our (future) SiM3U upstreaming efforts.

(cherry picked from commit 46ecdd1)

Author: rettichschnidi

drivers/serial/uart_si32_usart.c

@@ -209,6 +209,28 @@ static void usart_si32_irq_handler(const struct device *dev)
 }
 #endif /* CONFIG_UART_INTERRUPT_DRIVEN */
 
+#ifdef CONFIG_UART_LINE_CTRL
+
+static int usart_si32_line_ctrl_set(const struct device *dev, uint32_t ctrl, uint32_t val)
+{
+	const struct usart_si32_config *config = dev->config;
+
+	switch (ctrl) {
+	case UART_LINE_CTRL_RTS:
+		/* When inversion is not enabled, clear the RTS bit to signal readyness */
+		if (val) {
+			SI32_USART_A_clear_rts(config->usart);
+		} else {
+			SI32_USART_A_set_rts(config->usart);
+		}
+		return 0;
+	}
+
+	return -ENOTSUP;
+}
+
+#endif /* CONFIG_UART_LINE_CTRL */
+
 static const struct uart_driver_api usart_si32_driver_api = {
 	.poll_in = usart_si32_poll_in,
 	.poll_out = usart_si32_poll_out,
@@ -229,6 +251,10 @@ static const struct uart_driver_api usart_si32_driver_api = {
 	.irq_update = usart_si32_irq_update,
 	.irq_callback_set = usart_si32_irq_callback_set,
 #endif /* CONFIG_UART_INTERRUPT_DRIVEN */
+
+#ifdef CONFIG_UART_LINE_CTRL
+	.line_ctrl_set = usart_si32_line_ctrl_set,
+#endif
 };
 
 static int usart_si32_init(const struct device *dev)
@@ -286,10 +312,17 @@ static int usart_si32_init(const struct device *dev)
 
 	SI32_USART_A_exit_loopback_mode(config->usart);
 
+	/* Configure RTS to be pulled low when we are ready to receive data */
+	SI32_USART_A_disable_rts_inversion(config->usart);
+
+	/* Hardware flow control can only be enabled (or disabled) for both, RTS and CTS in the
+	 * device tree. As a workaround, when CONFIG_UART_LINE_CTRL is enabled, configure CTS only.
+	 */
 	if (config->hw_flow_control) {
+#if !defined(CONFIG_UART_LINE_CTRL)
 		SI32_USART_A_enable_rts(config->usart);
 		SI32_USART_A_select_rts_deassert_on_byte_free(config->usart);
-		SI32_USART_A_disable_rts_inversion(config->usart);
+#endif
 
 		SI32_USART_A_enable_cts(config->usart);
 		SI32_USART_A_disable_cts_inversion(config->usart);
@@ -328,6 +361,9 @@ static int usart_si32_init(const struct device *dev)
 	SI32_USART_A_disable_rx_signal_inversion(config->usart);
 	SI32_USART_A_select_rx_fifo_threshold_1(config->usart);
 	SI32_USART_A_enable_rx(config->usart);
+#if defined(CONFIG_UART_LINE_CTRL)
+	SI32_USART_A_clear_rts(config->usart);
+#endif
 
 	SI32_USART_A_flush_tx_fifo(config->usart);
 	SI32_USART_A_flush_rx_fifo(config->usart);