drivers: serial: nrf_sw_lpuart: Fix case of flushing RX FIFO

In Bluetooth HCI there is a case where uart_fifo_read is called
not from the UART interrupt context. If that happens and there is
pending UART data then LPUART was not able to reset the receiver
as it remained in the blocked state.

Case like that happens when HCI device sends data to the host core
after host core got reset but before HCI device is reset by the
host. Calling uart_fifo_read not from UART interrupt handler is a
violation of the API but it has been used like that to drain
UART FIFO for years and it has been useful so LPUART adapts to
it.

Case is fixed by triggering the UART interrupt if receiver is in
blocked state.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>

Author: nordic-krch

doc/nrf/releases_and_maturity/releases/release-notes-changelog.rst

@@ -659,6 +659,8 @@ This section provides detailed lists of changes by :ref:`driver <drivers>`.
 * Updated the ``nrfx_gppi`` API to now have full support on the application core of the nRF54H20 device.
   Using DPPI does not require the devicetree configuration.
 
+* Fixed case where Low Power UART may fail after the reset if reset happened during the transmission.
+
 Wi-Fi drivers
 -------------
 

drivers/serial/uart_nrf_sw_lpuart.c

@@ -992,7 +992,7 @@ static void api_irq_rx_enable(const struct device *dev)
 	struct lpuart_data *data = get_dev_data(dev);
 
 	data->int_driven.rx_enabled = true;
-	if (int_driven_rd_available(data)) {
+	if (int_driven_rd_available(data) || data->rx_state == RX_BLOCKED) {
 		/* We need to move to the interrupt context of the same priority as UARTE. */
 		k_timer_start(&data->int_driven.trampoline_timer, K_NO_WAIT, K_NO_WAIT);
 	}