esp32/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/esp32/machine_uart.c

@@ -417,15 +417,28 @@ 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 (uart_wait_tx_done(self->uart_num, 0) == ESP_OK) {
+        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_deinit), MP_ROM_PTR(&machine_uart_deinit_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_INV_TX) },
     { MP_ROM_QSTR(MP_QSTR_INV_RX), MP_ROM_INT(UART_INV_RX) },
@@ -491,6 +504,18 @@ STATIC mp_uint_t machine_uart_ioctl(mp_obj_t self_in, mp_uint_t request, uintptr
         if ((flags & MP_STREAM_POLL_WR) && 1) { // FIXME: uart_tx_any_room(self->uart_num)
             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.
+        uint32_t baudrate;
+        uart_get_baudrate(self->uart_num, &baudrate);
+        uint32_t timeout = (3 + self->txbuf) * 13000 * 2 / baudrate;
+        if (uart_wait_tx_done(self->uart_num, timeout) == ESP_OK) {
+            ret = 0;
+        } else {
+            *errcode = MP_ETIMEDOUT;
+            ret = MP_STREAM_ERROR;
+        }
     } else {
         *errcode = MP_EINVAL;
         ret = MP_STREAM_ERROR;