tagged commands and multiple LUNs

Tom Wicklund wicklund at Intellistor.COM
Tue Dec 12 13:27:29 PST 1995

I've run into a problem accepting tag queue messages on selection
which appears to make it difficult for SCSI protocol chips to handle a
sequence of IDENTIFY, QUEUE TAG, and command before interrupting the
processor.  Since I don't think this is intentended, I'd like to know
if there's an alternate implementation which meets the standard.

Accoring to the description of queue tag messages (SCSI-2 section
6.6.17, SIP section 6.19), if a target does not implement tagged
queuing it will respond to a queue tag message with MESSAGE REJECT.

In addition, in the description of the CONTROL MODE page (SCSI-2
section it states that if queueing is disabled then the
target should respond to a tagged queue message with MESSAGE REJECT.
SPC section 8.3.1 says the protocol standard will state how to abort
tagged commands, and SIP section 6.19 is the only mention I can find.

It therefore appears that a tagged queue message must be checked
against the CONTROL MODE page setting when it is received.  If a
target chooses to implement the DQue bit then there appear to be two

1.  Implement one copy of the CONTROL MODE page so that queuing is
    globally turned on or off.

2.  Implement the CONTROL MODE page independently per target and LUN.
    It appears the SCSI protocol chip must then check the tagged queue
    message before getting the command or interrupt the processor to
    check the message.

Is there another alternative?  What do existing products do?

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