def nvmf_subsystem_get_controllers(client, nqn, tgt_name=None):
    """Get list of controllers of an NVMe-oF subsystem.

    Args:
        nqn: Subsystem NQN.
        tgt_name: name of the parent NVMe-oF target (optional).

    Returns:
        List of controller objects of an NVMe-oF subsystem.
    """
    params = {'nqn': nqn}

    if tgt_name:
        params['tgt_name'] = tgt_name

    return client.call('nvmf_subsystem_get_controllers', params)