Description

   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.