SAS-1.1 Disable First Burst problem

Elliott, Robert (Server Storage) elliott at hp.com
Mon Sep 15 10:53:08 PDT 2003


* From the T10 Reflector (t10 at t10.org), posted by:
* "Elliott, Robert (Server Storage)" <elliott at hp.com>
*
While I was in the SAS phy WG meeting, the SAS protocol WG passed a
proposal adding a DISABLE FIRST BURST bit to the COMMAND frame
(03-249r2).  Unfortunately, this proposal makes SAS-1.0 and SAS-1.1
incompatible, mainly since targets are allowed (although not required)
to check reserved fields in the SSP COMMAND frame headers.

The possible responses to the DISABLE FIRST BURST bit being set to 0
(per SAS-1.0) are:
* SAS-1.0 or SAS-1.1 target
  * with first burst: FAILS (mismatch)
  * without first burst: SUCCEEDS

The possible responses to DISABLE FIRST BURST being set to 1 (per
SAS-1.1) are:
* SAS-1.0 target
  * without first burst
    * does not check reserved fields: SUCCEEDS (no first burst)
    * checks reserved fields: FAILS (get a RESPONSE frame indicating
INVALID FRAME)
  * with first burst
    * does not check reserved fields: FAILS (mismatch)
    * checks reserved fields: FAILS (get a RESPONSE frame indicating
INVALID FRAME)
* SAS-1.1 target
  * all cases: SUCCEEDS (no first burst)

I think it works better if the sense is ENABLE FIRST BURST.  This only
breaks when first burst is used, not when it is not used:
ENABLE FIRST BURST set to 0:
* SAS-1.0 target
  * without first burst: SUCCEEDS (no first burst)
  * with first burst: FAILS (mismatch)
* SAS-1.1 target
  * all cases: SUCCEEDS (no first burst)

ENABLE FIRST BURST set to 1:
* any kind of target: doesn't happen since no initiator supports this
misplaced feature
(but assuming an initiator does support it:)
* SAS-1.0 target
  * without first burst
    * does not check reserved fields: FAILS (mismatch)
    * checks reserved fields: FAILS (get a RESPONSE frame indicating
INVALID FRAME)
  * with first burst 
    * does not check reserved fields: SUCCEEDS
    * checks reserved fields: FAILS (get a RESPONSE frame indicating
INVALID FRAME)
* SAS-1.1 target
  * all cases: SUCCEEDS (honors the mode page)

I would like to incorporate the proposal into SAS-1.1 revision 1 with an
editor's note advising that the sense of the bit may change, and propose
changing the sense in the November CAP meeting.

--
Rob Elliott, elliott at hp.com
Hewlett-Packard Industry Standard Server Storage Advanced Technology
https://ecardfile.com/id/RobElliott



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