Don't use begin() method to just change baudrate

[GitHubLionel]

Ok, in that case, we need a warning for the begin() method to say that if you just want to change the Baudrate you must use updateBaudRate() method even if uartBegin() seems to allow this change.

Originally posted by @GitHubLionel in #10314

[GitHubLionel]

But I think that it will be better to correct the code in esp32-hal-uart.c since uartBegin() method should be do that.

[Jason2866]

There is nothing to correct when using wrong command for changing the UART speed.
As shown in example https://github.com/espressif/arduino-esp32/blob/master/libraries/ESP32/examples/Serial/Serial_All_CPU_Freqs/Serial_All_CPU_Freqs.ino

[SuGlider]

Up on the Arduino specification, Serial.begin(baud_rate) must correctly change the baud rate.
Therefore, I consider it as a bug.

I'll get a PR to fix it.

[SuGlider]

@GitHubLionel - please test PR #11122 and let me know.

[GitHubLionel]

Hi,
No, not work because MUTEX is misplaced.
You should replace this code:

      UART_MUTEX_LOCK();
      //User may just want to change some parameters, such as baudrate, data length, parity, stop bits or pins
      if (uart->_baudrate != baudrate) {
        if (!uartSetBaudRate(uart, baudrate)) {
          log_e("UART%d changing baudrate failed.", uart_nr);
          retCode = false;
        } else {
          log_v("UART%d changed baudrate to %d", uart_nr, baudrate);
          uart->_baudrate = baudrate;
        }
      }

By this one:

      //User may just want to change some parameters, such as baudrate, data length, parity, stop bits or pins
      if (uart->_baudrate != baudrate) {
      	retCode = uartSetBaudRate(uart, baudrate);
      }
      UART_MUTEX_LOCK();

Another thing, if you could use %ld in log message in uartSetBaudRate() for baud_rate to avoid compiler warning.
log_v("Setting UART%d baud rate to %ld.", uart->num, baud_rate);
log_e("Setting UART%d baud rate to %ld has failed.", uart->num, baud_rate);

[SuGlider]

@GitHubLionel - Thanks for testing and reviewing. All good now. This issue shall be closed when the PR is merged.