Description

   This routine performs a SCSI Request Sense command, or equivalent
   if some other I/O architecture is supported. It is used by all
   MRD API routines to determine the cause of a command failure.

   The robot_info is the address of a robot_info_t structure that
   has been opened by mrd_startup(3mrd). If the medium changer isn't
   opened, the Request Sense command will fail with the operating
   system error for trying to use an unopened device.

   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.

   In typical usage by MRD, the os_status argument will be an
   operating system specific code. However, The SCSI-2 specification
   allows Request Sense to be used at any time to obtain status
   information about a device. To support this feature, the MRD
   implementation of mrd_reqeust_sense(3mrd) can be called with the
   code MRD_CHECK_SENSE to force a Request Sense command.