The `timer` trigger will periodically change the LED brightness between
        0 and the current brightness setting. The `on` time can
        be specified via `delay_on` attribute in milliseconds.

def delay_on(self):
        """
        The `timer` trigger will periodically change the LED brightness between
        0 and the current brightness setting. The `on` time can
        be specified via `delay_on` attribute in milliseconds.
        """

        # Workaround for ev3dev/ev3dev#225.
        # 'delay_on' and 'delay_off' attributes are created when trigger is set
        # to 'timer', and destroyed when it is set to anything else.
        # This means the file cache may become outdated, and we may have to
        # reopen the file.
        for retry in (True, False):
            try:
                self._delay_on, value = self.get_attr_int(self._delay_on, 'delay_on')
                return value
            except OSError:
                if retry:
                    self._delay_on = None
                else:
                    raise