MRIE behavior in SPC-2 vs SSC

Robert Elliott relliott at hobbit.eng.hou.compaq.com
Fri Jun 4 13:58:37 PDT 1999


* From the T10 Reflector (t10 at symbios.com), posted by:
* relliott at hobbit.eng.hou.compaq.com (Robert Elliott)
*
There is a slight difference in the SPC-2 and SSC definitions of the 
Method for Reporting Informational Exceptions (MRIE) bit in the 
Informational Exceptions mode page 02h.  (MMC-2 includes portions
of the SPC-2 description)

See SPC-2 revision 10 section 8.3.6 and SSC revision 17 section 5.3.4.6.

For MRIE = 4, for example, SPC-2 says:
  This method instructs the device server to report informational 
  exception conditions, regardless of the value of the per bit of 
  the error recovery parameters mode page, by returning a 
  CHECK CONDITION status on any command.

SSC says:
  This method instructs the device server to report information 
  exception conditions (regardless of the value of the per bit of 
  the error recovery parameters mode page) by returning a 
  CHECK CONDITION status on the next SCSI command (excluding Inquiry 
  and Request Sense) after an informational exception condition 
  was detected.


There are two differences:
1.  SSC exempts INQUIRY and REQUEST SENSE.  SPC-2 does not.   This 
occurs for MRIE values 2, 3, 4, and 5.

The only other way to cause INQUIRY and REQUEST SENSE to return CHECK
CONDITION is to use a reserved bit.  SSC's exemption seems appropriate. 


2.  SSC says the CHECK CONDITION will be returned on the next command.
SPC-2 says "any" command.  This occurs for MRIE = 4 only.

In case of an actual hardware exception condition, you can't tell when 
it happened so this doesn't matter.  If the TEST bit is being used to 
trigger a fake exception condition, however, the SSC requirement is 
more useful since the command that must cause the CHECK CONDITION 
can be predicted.


So, I have these questions:

1.  Was this difference intentionally introduced?

2.  Would the non-tape devices accept tightening the SPC-2 description
to match the new description in SSC?  Do disk drives and other devices
already behave this way?


-- 
Rob Elliott      UNIX mailto:relliott at hobbit.eng.hou.compaq.com    
Houston, TX        PC mailto:Robert.Elliott at compaq.com
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