This routine performs a SCSI Read Element Status command, or
    equivalent if some other I/O architecture is supported. It is
    used by mrd_show(3mrd) and the routines doing volume tag checks.
    However, since it provides uninterpreted Read Element Status
    data, mrd_show(3mrd) will nearly always be easier to use.

    It requires that the medium changer be opened by mrd_
    startup(3mrd) and uses absolute element addresses. On SCSI
    medium changers, it maps directly to the SCSI Read Element Status
    command. Since it uses a robot_info_t structure for an open
    robot, it is suitable in applications where it is desirable to
    hold the robot open and not incur the robot startup time on each
    command.

    The type argument specifies the type of element about which
    information is to be obtained. It should be one of SLOT,
    TRANSPORT, PORT or DRIVE as defined in mrd_common.h. The start
    argument is the absolute address of the first element and count
    the number of elements for which data is to be obtained.

    The data argument is an array of unsigned characters where the
    resulting data will be copied. The length is the amount of
    space available. If more data is required than there is space
    available, the device will only data for as many element as will
    fit into length bytes.

    Medium Changers which are SCSI-2 compliant support Read Element
    Status commands which request only eight bytes of data. In this
    case the returned data will indicate how many bytes of data are
    needed for the entire request. This feature allows an application
    to find how much space is needed for a specific request, allocate
    that much and then request all of it.

    This routine uses the dev_status_t structure for handing errors.
    The dev_status_t structure includes the code, os_status, and SCSI
    error fields. The following describes how to decode errors with
    the dev_status_t structure.

    SCSI Errors

    SCSI errors are indicated when the value of the valid field of
    the SCSI error is not equal to 0. The key, asc, and ascq fields
    provide additional information to help determine the cause of the
    error.

    The code usually maps the Additional Sense Code and Additional
    Sense Code Qualifier (ASC/ASCQ) values to an MRD error. The asc
    and ascq values are copied from the request sense data returned
    by the target.

    The Additional Sense Code (asc) indicates further information
    related to the error or exception condition reported in the sense
    key field. The Additional Sense Code Qualifier (ascq) indicates
    detailed information related to the additional sense code. For
    more information, consult the SCSI-2 Specification.

    Operating System Errors

    Operating system errors are indicated when the value of the valid
    field of the SCSI error is equal to 0 and the value of the os_
    status field is not equal to 0. This result is most likely caused
    by an operating system error, and probably has a mapped error in
    MRD.

    MRD Errors

    MRD errors are indicated when the value of the os_status field is
    0, and the value of the valid field of the SCSI error is 0. This
    result is most likely caused when MRD encounters its own failure.