def setup_psd_pregenerated(workflow, tags=None):
    '''
    Setup CBC workflow to use pregenerated psd files.
    The file given in cp.get('workflow','pregenerated-psd-file-(ifo)') will
    be used as the --psd-file argument to geom_nonspinbank, geom_aligned_bank
    and pycbc_plot_psd_file.

    Parameters
    ----------
    workflow: pycbc.workflow.core.Workflow
        An instanced class that manages the constructed workflow.
    tags : list of strings
        If given these tags are used to uniquely name and identify output files
        that would be produced in multiple calls to this function.

    Returns
    --------
    psd_files : pycbc.workflow.core.FileList
        The FileList holding the gating files
    '''
    if tags is None:
        tags = []
    psd_files = FileList([])

    cp = workflow.cp
    global_seg = workflow.analysis_time
    user_tag = "PREGEN_PSD"

    # Check for one psd for all ifos
    try:
        pre_gen_file = cp.get_opt_tags('workflow-psd',
                        'psd-pregenerated-file', tags)
        pre_gen_file = resolve_url(pre_gen_file)
        file_url = urlparse.urljoin('file:',
                                     urllib.pathname2url(pre_gen_file))
        curr_file = File(workflow.ifos, user_tag, global_seg, file_url,
                                                    tags=tags)
        curr_file.PFN(file_url, site='local')
        psd_files.append(curr_file)
    except ConfigParser.Error:
        # Check for one psd per ifo
        for ifo in workflow.ifos:
            try:
                pre_gen_file = cp.get_opt_tags('workflow-psd',
                                'psd-pregenerated-file-%s' % ifo.lower(),
                                tags)
                pre_gen_file = resolve_url(pre_gen_file)
                file_url = urlparse.urljoin('file:',
                                             urllib.pathname2url(pre_gen_file))
                curr_file = File(ifo, user_tag, global_seg, file_url,
                                                            tags=tags)
                curr_file.PFN(file_url, site='local')
                psd_files.append(curr_file)

            except ConfigParser.Error:
                # It's unlikely, but not impossible, that only some ifos
                # will have pregenerated PSDs
                logging.warn("No psd file specified for IFO %s." % (ifo,))
                pass

    return psd_files