def read_block_data(self, addr, cmd):
        """read_block_data(addr, cmd) -> results

        Perform SMBus Read Block Data transaction.
        """
        # XXX untested, the raspberry pi i2c driver does not support this
        # command
        self._set_addr(addr)
        data = ffi.new("union i2c_smbus_data *")
        if SMBUS.i2c_smbus_access(self._fd,
                                  int2byte(SMBUS.I2C_SMBUS_READ),
                                  ffi.cast("__u8", cmd),
                                  SMBUS.I2C_SMBUS_BLOCK_DATA,
                                  data):
            raise IOError(ffi.errno)
        return smbus_data_to_list(data)