private function _is_cache_worthy()
    {
        $pci_good = true;
        foreach (array_keys($this->_pci_entries) as $vendor) {
            foreach (array_keys($this->_pci_entries[$vendor]) as $dever) {
                if (!isset($this->_existing_cache_vals['hw']['pci'][$vendor][$dever])) {
                    $pci_good = false;
                    break 2;
                }
            }
        }
        $usb_good = true;
        foreach (array_keys($this->_usb_entries) as $vendor) {
            foreach (array_keys($this->_usb_entries[$vendor]) as $dever) {
                if (!isset($this->_existing_cache_vals['hw']['usb'][$vendor][$dever])) {
                    $usb_good = false;
                    break 2;
                }
            }
        }

        return array('pci' => $pci_good, 'usb' => $usb_good);
    }