about the SAS XL state machine

Elliott, Robert (Server Storage) elliott at hp.com
Tue Dec 9 08:50:54 PST 2003

* From the T10 Reflector (t10 at t10.org), posted by:
* "Elliott, Robert (Server Storage)" <elliott at hp.com>

>    I was thinking about a motion of XL state machine with another 
> situation. I attached a pdf file that explains my situation. 
> What do you
> think of a situation when the ECM receives the Request Path after, or 
> same time, sending the Transmit Open?
> 1. An end device [A] sends the OPEN address frame to an 
> expander phy [X].
> 2. The expander phy [X] receives Arb Won and sends the Transmit Open
>    to an expander phy [Y].
> 3. An end device [B] sends the OPEN address frame to the expander
>    phy [Y].
> 4. The expander phy [Y] happens following events at same time.
>     4a. It receives the Transmit Open from the expander phy [X], and
>     4b. It goes to XL1 state.
>    In above sequence, the ECM and the expander phy [X] could not stop 
> the Transmit Open for the expander phy [Y], and the expander phy [Y] 
> could not stop going to XL1 state.
>    And I thought an outgoing frame should be sent by expander phy [Y]
> after comparing the outgoing frame and an inbound frame.

As far as the standard is concerned (this is different from real 
hardware), transitions and messages do not happen at the exact 
same time.  The message is always before or after the transition.

If it happens before, then the state machine is still in XL0 
and makes the decision to handle the Transmit Open first 
(going to XL5).

If it happens after, then the state machine sent Request Path 
|from XL0 and entered XL1.  The ECM should not send a Transmit 
Open because it has already received a Request Path.  It must
send an Arb Lost or Arb Reject before being allowed to 
send a Transmit Open after that time.

Rob Elliott, elliott at hp.com
Hewlett-Packard Industry Standard Server Storage Advanced Technology

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