def _udev(udev_info, key):
    """
    Return the value for a udev key.

    The `key` parameter is a lower case text joined by dots. For
    example, 'e.id_bus' will represent the key for
    `udev_info['E']['ID_BUS']`.

    """
    k, _, r = key.partition(".")
    if not k:
        return udev_info
    if not isinstance(udev_info, dict):
        return "n/a"
    if not r:
        return udev_info.get(k.upper(), "n/a")
    return _udev(udev_info.get(k.upper(), {}), r)