I think there're two ways:

1 . In function defaultParamValues(), get all data of multi selection and assign them to the input's default value:

protected function defaultParamValues()
    {
        // get data for $allMultiSelectData 
        return array(
            "multiSelect"=>$allMultiSelectData, 

2 . In function setup(), get get all data of multi selection via datastore piping and assign it to the report's params:

protected function setup()
    {
        $this->src("mydatasource")->query($query)
        ->pipe($this->dataStore("multiSelect"))
        ->requestDataSending();
        $rows = $this->dataStore("multiSelect")->data());
        $allMultiSelectData = [];
        foreach ($rows as $row) {
            $allMultiSelectData[] = $row["multiSelectColumn"];
        }
        $this->params["multiSelect"] = $allMultiSelectData;

Hope these work for you. Rgds,

Print out your $allMultiSelectData variable to see its value as well as data of the datastore for your multi select. Rgds,