Revision 1 of document accepted for Automatic Clear ACA

Bob Snively Bob.Snively at eng.sun.com
Mon Sep 19 21:31:05 PDT 1994


							X3T10/94-189, revision 1



To:		X3T10

From:		Bob Snively
		Sun Microsystems Computer Company
		Mail Stop UMPK 12-302
		2550 Garcia Ave,
		CA 94043-1100
		(415) 786-6694

Date:		September 14, 1994

Subject: 		Proposed solution for Automatic Clear of ACA

SAM Revision 15 indicates in section 6.1.2, page 45 that,  
All logical units shall implement support for at 
least one of the ACA bit values and may support both values.   
This proposal describes a protocol and 
proposes wording to include that wording in SAM and SPC.

The assumptions of this proposal are:
a)	The proposal should be compatible with SCSI-2 
	initiators and targets.
b)	The proposal should allow each command to be 
	marked as to whether or not a Check 
	Condition for that command will establish an 
	Automatic Contingent Allegiance (ACA) condition.

The control objects used by this protocol are described in the 
following table.

Object name		Location		Description, relevant values

ANSI-approved Version	INQUIRY data, 		Specified in SPC
			   byte 2, bits 2-0		Indicates implemented versio
						  2h = SCSI-2 compliant
						  3h = SCSI-3 compliant


Normal ACA Supported	INQUIRY data, 		Specified in SPC
			   location tbd			Indicates Normal ACA supported by target

						  0b = ACA Autoclear only
						  1b = Normal ACA or ACA Autoclear

Normal ACA		CDB Control Field, 	Specified in SAM
			   bit 3			Indicates Normal ACA required if 
								CHECK CONDITION occurs
						  0b = ACA Autoclear if check
						  1b = Normal ACA if check



The following behaviors are expected of targets when they indicate the 
following combinations of bits.


					Normal ACA Supported = 0 		Normal ACA Supported = 1

ANSI-approved Version = SCSI-2		SCSI-2 Contingent Allegiance		Not allowed


ANSI-approved Version = SCSI-3		Only Normal ACA = 0 allowed:		For Normal ACA = 0:
						Autoclear if check			Autoclear if check
										For Normal ACA = 1:
											Normal ACA if check



If the target identifies itself as a SCSI-2 target, it 
cannot identify itself as supporting normal ACA.  
SCSI-2 devices support Contingent Allegiance or Extended 
Contingent Allegiance as defined in the 
SCSI-2 standard.

If the target identifies itself as a SCSI-3 target, it can 
identify itself as supporting autoclear only or as 
supporting both normal ACA and autoclear.  

If a SCSI-3 target supports autoclear only, the initiator 
shall not transmit commands that contain the 
Normal ACA bit set to the Normal ACA (1) state.  If a 
command with the Normal ACA bit is transmitted 
to a target, the target  will indicate with a CHECK CONDITION 
that the control field is invalid.  

If a SCSI-3 target supports autoclear only, any command 
that ends with a CHECK CONDITION or 
COMMAND TERMINATED status establishes an Automatic Contingent 
Allegiance condition.  If the 
link protocol for that target performs autosense, the 
ACA condition is cleared automatically upon 
completion of the autosense operation.  If the link 
protocol for that target does not perform autosense, the 
ACA condition is cleared according to the rules for 
SCSI-2 Contingent Allegiance.

If a SCSI-3 target supports both normal ACA and 
autoclear, the behavior of the target when a CHECK 
CONDITION or COMMAND TERMINATED status occurs 
depends upon the state of the Normal ACA 
bit in the CDB of the failing command.  If the 
Normal ACA bit is set to zero, the target establishes an 
ACA condition.  If the link protocol for that target 
performs autosense, the ACA condition is cleared 
automatically upon completion of the autosense 
operation.  If the link protocol for that target does not 
perform autosense, the ACA condition is cleared 
according to the rules for SCSI-2 Contingent Allegiance.  
If the Normal ACA bit is set to one, the target 
establishes an ACA condition.  The ACA condition 
remains until cleared according to the rules 
specified in SAM, section 6.6.1.2.

To specify this behavior in the standards, the following 
words must be modified and added to the 
referenced standards.

Changes required for SPC:

The ANSI-approved Version field is already correctly 
described in section 7.5.1, Table 22, and associated 
text.

The following text is proposed to be added to 
section 7.5.1.  The Normal ACA Supported bit should be 
added to the proper field in the INQUIRY data format 
chart, to be determined.  An appropriate 
abbreviation is NormACA.  The text would then be added 
after the text describing the RelAddr bit.

The Normal ACA Supported bit (NormACA) of one 
indicates that the device server supports 
both the Normal ACA and the ACA Autoclear 
function.  A NormACA bit of zero indicates that 
the device server does not support Normal ACA, 
but does support the ACA Autoclear function.

Changes required for SAM:

All sections:

The bit presently named  ACA  is renamed  Normal ACA  
or  NACA  at all referenced locations.

Section 6.1.2:

The following text is removed:

All logical units shall implement support for 
at least one of the ACA bit values and may support 
both values.

and replaced with the following text:

All logical units shall implement support for 
the Normal ACA value of zero and may support the 
Normal ACA value of one.  The ability to support 
a Normal ACA value of one is indicated in 
standard INQUIRY data.

Sections 6.6.1, 6.6.1.1 are unchanged.

Section 6.6.1.2:

The following text in paragraph 3 is deleted:

If the logical unit accepts a value of zero for the ACA bit 

The first sentence will then read:

If the NACA bit is set to zero in the CDB control 
byte of the faulting command, then the SCSI-2 
rules for clearing auto contingent allegiance 
condition shall apply.

No other changes should be required.




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