mimxrt/machine_uart: Implement uart.flush() and uart.txdone().

uart.flush()

flush() will wait until all characters have been sent.To avoid a
permanent lock, a timeout applies depending on the size of txbuf
and the baud rate.

ret = uart.txdone()

ret is True if no transfer is in progress.
ret is False otherwise.

ports/mimxrt/machine_uart.c

@@ -308,17 +308,30 @@ STATIC mp_obj_t machine_uart_sendbreak(mp_obj_t self_in) {
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_1(machine_uart_sendbreak_obj, machine_uart_sendbreak);
 
+STATIC mp_obj_t machine_uart_txdone(mp_obj_t self_in) {
+    machine_uart_obj_t *self = MP_OBJ_TO_PTR(self_in);
+
+    if (self->tx_status == kStatus_LPUART_TxIdle) {
+        return mp_const_true;
+    } else {
+        return mp_const_false;
+    }
+}
+STATIC MP_DEFINE_CONST_FUN_OBJ_1(machine_uart_txdone_obj, machine_uart_txdone);
+
 STATIC const mp_rom_map_elem_t machine_uart_locals_dict_table[] = {
     { MP_ROM_QSTR(MP_QSTR_init), MP_ROM_PTR(&machine_uart_init_obj) },
 
     { MP_ROM_QSTR(MP_QSTR_any), MP_ROM_PTR(&machine_uart_any_obj) },
 
+    { MP_ROM_QSTR(MP_QSTR_flush), MP_ROM_PTR(&mp_stream_flush_obj) },
     { MP_ROM_QSTR(MP_QSTR_read), MP_ROM_PTR(&mp_stream_read_obj) },
     { MP_ROM_QSTR(MP_QSTR_readline), MP_ROM_PTR(&mp_stream_unbuffered_readline_obj) },
     { MP_ROM_QSTR(MP_QSTR_readinto), MP_ROM_PTR(&mp_stream_readinto_obj) },
     { MP_ROM_QSTR(MP_QSTR_write), MP_ROM_PTR(&mp_stream_write_obj) },
 
     { MP_ROM_QSTR(MP_QSTR_sendbreak), MP_ROM_PTR(&machine_uart_sendbreak_obj) },
+    { MP_ROM_QSTR(MP_QSTR_txdone), MP_ROM_PTR(&machine_uart_txdone_obj) },
 
     { MP_ROM_QSTR(MP_QSTR_INV_TX), MP_ROM_INT(UART_INVERT_TX) },
     { MP_ROM_QSTR(MP_QSTR_INV_RX), MP_ROM_INT(UART_INVERT_RX) },
@@ -433,6 +446,21 @@ STATIC mp_uint_t machine_uart_ioctl(mp_obj_t self_in, mp_uint_t request, mp_uint
         if ((flags & MP_STREAM_POLL_WR)) {
             ret |= MP_STREAM_POLL_WR;
         }
+    } else if (request == MP_STREAM_FLUSH) {
+        // The timeout is estimated using the buffer size and the baudrate.
+        // Take the worst case assumptions at 13 bit symbol size times 2.
+        uint64_t timeout = (uint64_t)(3 + self->txbuf_len) * 13000000ll * 2 /
+            self->config.baudRate_Bps + ticks_us64();
+
+        do {
+            if (machine_uart_txdone((mp_obj_t)self) == mp_const_true) {
+                return 0;
+            }
+            MICROPY_EVENT_POLL_HOOK
+        } while (ticks_us64() < timeout);
+
+        *errcode = MP_ETIMEDOUT;
+        ret = MP_STREAM_ERROR;
     } else {
         *errcode = MP_EINVAL;
         ret = MP_STREAM_ERROR;