AutoSense Message Proposal

Greg Alvey KD6NNM at gnn.com
Sat Oct 26 10:06:31 PDT 1996


* From the SCSI Reflector (scsi at symbios.com), posted by:
* Greg Alvey <KD6NNM at gnn.com>
*

Date:    October 25, 1996
To:      X3T10 Membership
From:  	 Greg Alvey - Solution Technology
Subject: Proposal for New SCSI-3 "AutoSense" Message

Discussion:
CAM (Section 7.7)  discusses AutoSense, but in a way that only saves the 
upper layers of the system (application client) from having to do the work 
of building and issuing a REQUEST SENSE command.  This still involves another 
"middle" layer on the host side having to intercept the status (i.e. CHECK 
CONDITION), building a REQUEST SENSE command, connecting to the faulted device 
to issue the REQUEST SENSE, returning the sense data, and disconnecting again.  
This does nothing to reduce overhead on the bus itself.  

This proposal is intended to provide a way to reduce bus overhead by delivering 
the sense data during the same connection as the original faulted command.  

AutoSense Data (Sense data delivered automatically) can be returned during the 
same connection as the faulted command by adding support for one two-byte 
message, "AUTOSENSE DATA FOLLOWS".  

Purpose:
To reduce overhead required to retrieve Sense Data from parallel SCSI devices.  
This may also apply to other types of physical interconnect systems.

Attachments:
Attachments are provided for proposed changes to SIP, SAM, & SPC.

------------------------------------------------------------------------------
Proposed changes in SIP:
Message Section - Add the following:

X.X.X AUTOSENSE DATA FOLLOWS
The AUTOSENSE DATA FOLLOWS message (see table xx) is sent from the target role 
agent to the initiator role agent, immediately following a CHECK CONDITION or 
COMMAND TERMINATED status, and requests that the initiator role agent prepare 
to accept sense data related to the currently faulted command.  The target role 
agent then sends the sense data associated with the faulted command followed 
immedately by the TASK COMPLETE message.  

Any ACA which may have been generated for the faulted command is cleared when 
the sense data has been transferred, according to the AutoSense rules 
(see SCSI-3 Architecture Model).

The target role agent shall only use this message for tasks where the 
Enable AutoSense (EAS) bit has been set to one in the Control byte of the 
current CDB (see SCSI-3 Architecture Model - Section 5.1.2).

An application client can determine whether a device server implements this 
message by examining the (ASense) bit  in the standard INQUIRY data 
(see SCSI-3 Primary Commands Standard).

The format of the message is shown below:

            AutoSense data follows message format
 +--------+-------------------------------------------------+
 | byte 0 |          Message code (25h - proposed)          |
 +--------+-------------------------------------------------+
 | byte 1 |     Number of Sense Data bytes (following)      |
 +--------+-------------------------------------------------+

The value in the "Number of Sense Data bytes" field is the number of data 
bytes that will be transferred immediately after this message.

The contents of the sense data bytes are identical to what would be 
transferred if a REQUEST SENSE command were issued for the currently faulted 
command.
------------------------------------------------------------------------------

------------------------------------------------------------------------------
Proposed changes in SAM
SAM - Control Byte (Section 5.1.2) - Add the following:

Change in Control Byte of CDB
 +--------+-------+-------+-------+-------+-------+-------+-------+-------+
 |     bit| bit 7 | bit 6 | bit 5 | bit 4 | bit 3 | bit 2 | bit 1 | bit 0 |
 +--------+-------+-------+-------+-------+-------+-------+-------+-------+
 | byte n |      VS       |    Reserved   |  EAS  |  NACA |  Flag |  Link |
 +--------+-------+-------+-------+-------+-------+-------+-------+-------+

The EAS (Enable AutoSense) bit is used to control the rules for delivering 
sense data related to an ACA condition caused by the command. If the EAS bit 
is one, the device server may use the AutoSense rules (see clause 5.6.4.2) 
 
The ability to support a Enable AutoSense value of one is indicated in standard 
INQUIRY data.
------------------------------------------------------------------------------

------------------------------------------------------------------------------
Proposed changes in SPC
INQUIRY command - Section 7.5.1 
Add the AutoSense (ASense) bit to the Standard INQUIRY data format as shown 
below:

 +--------+-------+-------+-------+-------+-------+-------+-------+-------+
 |     bit| bit 7 | bit 6 | bit 5 | bit 4 | bit 3 | bit 2 | bit 1 | bit 0 |
 +--------+-------+-------+-------+-------+-------+-------+-------+-------+
 | byte 3 |  AERC | TrmTsk|NormACA| ASense|      Response data format     |
 +--------+-------+-------+-------+-------+-------+-------+-------+-------+

The AutoSense bit (ASense) of one indicates that the device server supports 
setting the EAS bit to one in the Control Byte of the CDB (as defined in SAM). 
An ASense bit of zero indicates that the device server does not support setting 
the EAS bit to one.
------------------------------------------------------------------------------

------------------------------------------------------------------------------
The following is what AutoSense MIGHT look like on some SCSI bus analyzer.
Example:

 Bus Free Detected
   Arbitration Start    7	
   Arb_win              7
                                (Atn Asserted) ATN
     Selection Start    7    4                 ATN
     Selection Complete	                       ATN
                                 (Atn Negated) ATN
       Message Out      C0
         Command Out    08 00 01 00 01 08  (Enable AutoSense)
           Data In      00 01 02 03 04 05
                        06 07 08 09 0A 0B
                        0C OD OE 0F 10 11
         Status In      02                 (Check Condition)
       Message In       25                 (AutoSense Follows)
       Message In       12                 (18 bytes of Sense Data)
           Data In      70 00 02 00 00 00
                        00 0A 00 00 00 00
                        18 01 00 80 00 85	
       Message In       00		
 Bus Free Detected					

------------------------------------------------------------------------------
Greg Alvey
KD6NNM at gnn.com

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