another QAS approach for simple devices

[Elliott, Robert (Hou)]

* From the T10 Reflector (t10 at t10.org), posted by:
* "Elliott, Robert (Hou)" <Robert.Elliott at compaq.com>
*
Here's another attempt at solving the QAS problem for
simple devices:

What if we let devices like tapes and enclosure services 
devices participate in QAS but not generate QAS REQUEST 
messages themselves?  

They would snoop for the QAS REQUEST message after a DT DATA 
phase, participate in QAS arbitration, and understand QAS 
selection.  The only difference would be that they would 
proceed in non-packetized mode rather than enter packetized 
mode, and would go BUS FREE rather than generate a 
QAS REQUEST message when finished.
 
This should be compatible with existing QAS designs.  
It is simpler for simple devices to implement than 
full packetized mode.

SPC-n/SPI-4 changes would include:
* INQUIRY QAS bit means the device can participate in QAS 
  but does not necessarily initiate QAS arbitration
* PPR "QAS_REQ" bit means QAS participation is enabled.  
* Devices with QAS enabled may send QAS_ REQUEST 
  messages.  They are not required to do so.

An initiator which chooses this option might break existing
QAS target designs.  The target will need to understand 
initiators that ask for QAS without IU and enter message 
phase after selection rather than DT DATA phase.  We could
only allow targets to implement this subset if that is
a problem.

A target which chooses this option should not break
existing initiator designs.  Software might need to relax
some rule checks about QAS_REQ and IU_REQ.  Hardware must
not complain about not entering DT DATA phase after a QAS.

I'll put this into 00-252r1 if nobody sees any problems.

---
PC: Robert.Elliott at compaq.com
UNIX: relliott at hobbit.eng.hou.compaq.com
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