def valid_configs(self):
        """ Return a list of slots having a valid configurtion. Requires firmware 2.1. """
        if self.ykver() < (2,1,0):
            raise YubiKeyUSBHIDError('Valid configs unsupported in firmware %s' % (self.version()))
        res = []
        if self.touch_level & self.CONFIG1_VALID == self.CONFIG1_VALID:
            res.append(1)
        if self.touch_level & self.CONFIG2_VALID == self.CONFIG2_VALID:
            res.append(2)
        return res