rp2/machine_uart.c: Test for an invalid baud rate.

The baud rate requested in the init call may not be achievable. This
happens at least if the baud rate is too low or too high, but may
as well happen at other baud rates. If the deviation is larger than
2 %, an error is raised.

Signed-off-by: robert-hh <robert@hammelrath.com>

ports/rp2/machine_uart.c

@@ -367,7 +367,11 @@ static void mp_machine_uart_init_helper(machine_uart_obj_t *self, size_t n_args,
             self->timeout_char = min_timeout_char;
         }
 
-        uart_init(self->uart, self->baudrate);
+        uint32_t new_baudrate = uart_init(self->uart, self->baudrate);
+        uint32_t baudrate_diff = new_baudrate > self->baudrate ? new_baudrate - self->baudrate : self->baudrate - new_baudrate;
+        if (baudrate_diff > (self->baudrate / 50)) {
+            mp_raise_ValueError(MP_ERROR_TEXT("Invalid baud rate"));
+        }
         uart_set_format(self->uart, self->bits, self->stop, self->parity);
         __DSB(); // make sure UARTLCR_H register is written to
         uart_set_fifo_enabled(self->uart, true);