SAM Initiator-Mode Recovery When Not Command Tag Queuing
If an error such as a check condition or hardware failure occurs, the transaction active during the error is returned with the scsi_buf.bufstruct.b_error field set to EIO.
Other transactions in the queue might be returned with the scsi_buf.bufstruct.b_error field set to ENXIO. If the adapter driver decides not to return other outstanding commands it has queued to it, then the failed transaction will be returned to the device driver with an indication that the queue for this device is not cleared by setting the SC_DID_NOT_CLEAR_Q flag in the scsi_buf.adap_q_status field. The device driver should process or recover the condition, rerunning any mode selects or device reservations to recover from this condition properly. After this recovery, it should reschedule the transaction that had the error. In many cases, the device driver only needs to retry the unsuccessful operation.
The adapter device driver should never retry a SCSI command on error after the command has successfully been given to the adapter. The consequences for retrying a command at this point range from minimal to catastrophic, depending upon the type of device. Commands for certain devices cannot be retried immediately after a failure (for example, tapes and other sequential access devices). If such an error occurs, the failed command returns an appropriate error status with an iodone call to the device driver for error recovery. Only the device driver that originally issued the command knows if the command can be retried on the device. The adapter device driver must only retry commands that were never successfully transferred to the adapter. In this case, if retries are successful, the scsi_buf status should not reflect an error. However, the adapter device driver should perform error logging on the retried condition.
The first transaction passed to the adapter device driver during error recovery must include a special flag. This SC_RESUME flag in the scsi_buf.flags field must be set to inform the adapter device driver that the device driver has recognized the fatal error and is beginning recovery operations. Any transactions passed to the adapter device driver, after the fatal error occurs and before the SC_RESUME transaction is issued, should be flushed; that is, returned with an error type of ENXIO through an iodone call.