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    RE: Addition of CmdSN in Data-out PDU



    
    	Even more common would be the DMA latency. As an initiator I would
    send the command PDU as soon as I could if there were no immediate
    data, then queue the DMA for the unsolicited data. Since there could
    be many other DMAs queued ahead of the unsolicited data it is quite
    possible that other commands, both read and write, will be sent before
    the DMA completes.
    
    	You are spot one when you say the delay in obtaining the unsolicited
    data may be large and variable. If there are multiple DMA engines it
    does become a challenge for the initiator to ensure the correct
    ordering of the unsolicited data. Note that since there may be
    multiple unsolicited data PDUs in the same burst, and those may be
    filled with separate DMAs the ordering problem propagates down to
    within individual commands.
    
    	I don't believe it is a particularly difficult problem for the
    initiator to solve, but I do agree things would be easier for the
    initiator if the ordering requirement were removed. And more so if the
    requirement to send in dataSN order were also removed, but this is
    probably too much pain for the target.
    
    	- Rod
    
    -----Original Message-----
    From: owner-ips@ece.cmu.edu [mailto:owner-ips@ece.cmu.edu]On Behalf Of
    John Hufferd
    Sent: Saturday, October 13, 2001 1:01 AM
    To: Robert Snively
    Cc: 'somesh_gupta@silverbacksystems.com'; BURBRIDGE,MATTHEW
    (HP-UnitedKingdom,ex2); 'Binford, Charles'; ips@ece.cmu.edu
    Subject: RE: Addition of CmdSN in Data-out PDU
    
    
    
    Robert,
    I think that an Initiator being able to send a waiting Read command,
    without having to wait for many large write segments -- that are being
    sent
    (as unsolicited data) -- is very useful.  And that would mean, the
    unsolicited data is waiting to be sent until the Read Commands are
    sent.
    This might be a very frequent case.
    
    .
    .
    .
    John L. Hufferd
    Senior Technical Staff Member (STSM)
    IBM/SSG San Jose Ca
    Main Office (408) 256-0403, Tie: 276-0403,  eFax: (408) 904-4688
    Home Office (408) 997-6136
    Internet address: hufferd@us.ibm.com
    
    
    Robert Snively <rsnively@Brocade.COM>@ece.cmu.edu on 10/12/2001
    03:56:06 PM
    
    Sent by:  owner-ips@ece.cmu.edu
    
    
    To:   "'somesh_gupta@silverbacksystems.com'"
          <somesh_gupta@silverbacksystems.com>, "BURBRIDGE,MATTHEW
          (HP-UnitedKingdom,ex2)" <matthew_burbridge@hp.com>, "'Binford,
          Charles'" <CBinford@pirus.com>, ips@ece.cmu.edu
    cc:
    Subject:  RE: Addition of CmdSN in Data-out PDU
    
    
    
    I have two small questions that would help me understand this.
    
    How do you know that unsolicited data is expected?  By nature,
    unsolicited data is received as a "maximum surprise" which can
    only be determined after unpacking and parsing at least a
    part of the command structure, possibly even including the
    SCSI command (although there are some hints contained in the
    command header information).
    
    How can you constrain a generic system to posting the
    unsolicited data in the same order that the commands were
    emitted?  In general, I would have expected the system to
    be emitting command followed immediately by the corresponding
    unsolicited data.  If that is not the case, it means that there
    was a delay in obtaining the unsolicited data for transfer and
    that the delay was sufficient to allow the insertion of commands.
    If the delay is that large (and probably variable), the enforcement
    of transfer of unsolicited data in the same order as the
    commands are emitted seems to me to be a significant challenge,
    and certainly shouldn't be required as normal behavior.  While it
    would make things simpler for targets (already challenged by
    unsolicited data), it seems to me that it would make things
    much more complex for initiators.
    
    Bob
    
    > -----Original Message-----
    > From: Somesh Gupta [mailto:somesh_gupta@silverbacksystems.com]
    > Sent: Friday, October 12, 2001 2:51 PM
    > To: BURBRIDGE,MATTHEW (HP-UnitedKingdom,ex2); 'Binford, Charles';
    > ips@ece.cmu.edu
    > Subject: RE: Addition of CmdSN in Data-out PDU
    >
    >
    > Matthew,
    >
    > Since the unsolicited data does not follow the
    > command, you need the link list. But since the
    > unsolicited data must be sent in the same order
    > as the commands, a link list is enough.
    >
    > Let us say that you have 8 commands. The ones
    > for which we expect unsolicted data are marked
    > as Cnn(ud). And I have marked the unsolicted
    > data PDUs as UD(nn). The (nn) with data implies
    > that it is implicit and not actually carried with
    > the PDU itself.
    >
    > C01(ud) C02(ud) C03(ud) C04 C05 C06(ud) UD(01) --->
    > --> C07(ud) UD(02) UD(03) D04 D04 D05 D05 UD(06) --->
    > --> UD(07) C08(ud) UD(08) --->
    >
    > After the target receives the command C01, C02, and C03
    > for which it expects unsolicited data, it puts them in
    > a link list. It also receives C04 and C05 for which
    > unsolicited data is not expected and they don't go
    > on the list. It then receives C06 for which unsolicited
    > data is expected, and it is added to then end of the list.
    > Then an unsolicited data PDU is received. It must go
    > with the command at the head of the list which is C01.
    > Use the ITT to make sure and you can then take C01 off
    > the list and so on.
    >
    > Somesh
    >
    >
    > > -----Original Message-----
    > > From: owner-ips@ece.cmu.edu
    > [mailto:owner-ips@ece.cmu.edu]On Behalf Of
    > > BURBRIDGE,MATTHEW (HP-UnitedKingdom,ex2)
    > > Sent: Friday, October 12, 2001 7:53 AM
    > > To: 'Binford, Charles'; ips@ece.cmu.edu
    > > Subject: RE: Addition of CmdSN in Data-out PDU
    > >
    > >
    > > Charles,
    > >
    > > As you have described the spec states that "that
    > unsolicited data MUST be
    > > sent in the same order as the commands".  This is not the same as
    > > unsolicited data must follow the command associated with
    > it:  For example:
    > >
    > > (Cx = SCSI Command PDU, Dx = The unsolicited data PDUs. The
    > x in all the
    > > example can be the ITT. It is not the CmdSN.
    > >
    > > This is allowed:
    > >
    > >   C1 C2 C3 D1 D2 C4 D3 D4
    > >
    > > and the target will have to use the ITT to associate the
    > data with the
    > > command.
    > >
    > > Matthew
    > >
    > >
    > > -----Original Message-----
    > > From: Binford, Charles [mailto:CBinford@pirus.com]
    > > Sent: Friday, October 12, 2001 2:50 PM
    > > To: ips@ece.cmu.edu
    > > Subject: RE: Addition of CmdSN in Data-out PDU
    > >
    > >
    > > I have not verify version 8 is still the same, but version 07-97
    > > had a rule
    > > that unsolicited data MUST be sent in the same order as the
    > commands.
    > >
    > > Thus, there is no need for a search on the ITT.  The target
    > just needs to
    > > keep of linked list of I/Os waiting on unsolicited data.
    > New commands are
    > > added to the tail, any unsolicited data *should* be associated
    > > with the I/O
    > > at the head of the list.  The ITT is used as a sanity check and
    > > you're done.
    > >
    > > What am I missing?
    > >
    > > Charles Binford
    > > Pirus Networks
    > > 316.315.0382 x222
    
    
    
    


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