rp2/machine_uart: Implement uart.flush() and uart.txdone().

uart.flush()

flush() will wait until all characters have been sent. It may return
while the last character is sent. if needed, the calling code has to
add one character wait time. 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. It may return True if the
last byte of a transfer is sent.
ret is False otherwise.

ports/rp2/machine_uart.c

@@ -435,17 +435,31 @@ 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 (ringbuf_avail(&self->write_buffer) == 0 &&
+        uart_get_hw(self->uart)->fr & UART_UARTFR_TXFE_BITS) {
+        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_INVERT_TX) },
     { MP_ROM_QSTR(MP_QSTR_INV_RX), MP_ROM_INT(UART_INVERT_RX) },
@@ -538,6 +552,19 @@ STATIC mp_uint_t machine_uart_ioctl(mp_obj_t self_in, mp_uint_t request, mp_uint
         if ((flags & MP_STREAM_POLL_WR) && ringbuf_free(&self->write_buffer) > 0) {
             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 = time_us_64() +
+            (uint64_t)(33 + self->write_buffer.size) * 13000000ll * 2 / self->baudrate;
+        do {
+            if (machine_uart_txdone((mp_obj_t)self) == mp_const_true) {
+                return 0;
+            }
+            MICROPY_EVENT_POLL_HOOK
+        } while (time_us_64() < timeout);
+        *errcode = MP_ETIMEDOUT;
+        ret = MP_STREAM_ERROR;
     } else {
         *errcode = MP_EINVAL;
         ret = MP_STREAM_ERROR;