fix(lp_uart): Added lp_uart flush feature

This commit adds the lp_core_uart_flush() API to flush the LP UART Tx
FIFO. This API is automatically called once the program returns from the
main function().

Closes https://github.com/espressif/esp-idf/issues/14530

components/ulp/lp_core/lp_core/include/ulp_lp_core_uart.h

@@ -1,5 +1,5 @@
 /*
- * SPDX-FileCopyrightText: 2023 Espressif Systems (Shanghai) CO LTD
+ * SPDX-FileCopyrightText: 2023-2024 Espressif Systems (Shanghai) CO LTD
  *
  * SPDX-License-Identifier: Apache-2.0
  */
@@ -59,6 +59,16 @@ esp_err_t lp_core_uart_write_bytes(uart_port_t lp_uart_num, const void *src, siz
  */
 int lp_core_uart_read_bytes(uart_port_t lp_uart_num, void *buf, size_t size, int32_t timeout);
 
+/**
+ * @brief Flush LP UART Tx FIFO
+ *
+ * This function is automatically called before the LP core powers down once the main() function returns.
+ * It can also be called manually in the application to flush the Tx FIFO.
+ *
+ * @param lp_uart_num   LP UART port number
+ */
+void lp_core_uart_tx_flush(uart_port_t lp_uart_num);
+
 #ifdef __cplusplus
 }
 #endif

components/ulp/lp_core/lp_core/lp_core_uart.c

@@ -1,5 +1,5 @@
 /*
- * SPDX-FileCopyrightText: 2023 Espressif Systems (Shanghai) CO LTD
+ * SPDX-FileCopyrightText: 2023-2024 Espressif Systems (Shanghai) CO LTD
  *
  * SPDX-License-Identifier: Apache-2.0
  */
@@ -63,6 +63,24 @@ int lp_core_uart_tx_chars(uart_port_t lp_uart_num, const void *src, size_t size)
     return tx_len;
 }
 
+void lp_core_uart_tx_flush(uart_port_t lp_uart_num)
+{
+    (void)lp_uart_num;
+    int loop_cnt = 0;
+
+    if (uart_ll_is_enabled(LP_UART_NUM_0) && !uart_hal_is_tx_idle(&hal)) {
+        /* Wait for the Tx FIFO to be empty */
+        while (!(uart_hal_get_intraw_mask(&hal) & (LP_UART_TX_INT_FLAG | LP_UART_ERR_INT_FLAG))) {
+            loop_cnt++;
+            if (loop_cnt > 10000) {
+                /* Bail out */
+                break;
+            }
+        }
+        uart_hal_clr_intsts_mask(&hal, LP_UART_TX_INT_FLAG | LP_UART_ERR_INT_FLAG);
+    }
+}
+
 esp_err_t lp_core_uart_write_bytes(uart_port_t lp_uart_num, const void *src, size_t size, int32_t timeout)
 {
     (void)lp_uart_num;

examples/system/ulp/lp_core/lp_uart/lp_uart_print/main/lp_core/main.c

@@ -7,6 +7,7 @@
 #include <stdint.h>
 #include "ulp_lp_core_print.h"
 #include "ulp_lp_core_utils.h"
+#include "ulp_lp_core_uart.h"
 
 int main (void)
 {
@@ -17,6 +18,7 @@ int main (void)
     lp_core_printf("This program has run %d times\r\n", ++iteration);
     lp_core_printf("%s", separator);
     lp_core_printf("\n");
+    lp_core_uart_tx_flush(LP_UART_NUM_0);
 
     return 0;
 }