My
take is this:
The overflow/underflow is determined by
the SCSI layer. This is because the "expected data transfer length"
gives the size of the initiators buffer and the CDB gives the size of the "attempted
transfer". If the target is attempting to transfer too much, that would
be an overflow.
In some transports (like parallel SCSI)
this is not determined by the target but is determined by the initiator driver
or host adapter. In this case the target will try to stay in data-in phase to
transfer all of the data. The initiator sees this as an overflow.
Eddy
-----Original Message-----
From: Dean Scoville
[mailto:dean.scoville@qlogic.com]
Sent: Tuesday, July 22, 2003 12:43
PM
To: Julian Satran
Cc: ips@ece.cmu.edu
Subject: CDB/PDU inconsistencies
and residual counts
Julian,
The
overflow/underflow and residual count values in Data-in and
SCSI
Response PDUs are set by the target based on the expected
transfer
length for the command, but should these values be set based
on the
CDB (SCSI layer) or based on the PDU "expected data transfer length"
field
(iSCSI layer)?
For example, suppose an initiator sends a 4 block
SCSI Read command
but
for some reason thinks the block size is 1024 bytes when in actuality
the
disk has 512 byte blocks. The SCSI Command PDU would have an
"expected
data transfer length" of value of 4096 bytes, but the target would
actually
return 2048 bytes (based on the true block size). Should the target
set
the underflow bit and return a residual count (based on the iSCSI PDU),
or
should it return no underflow and no residual count (based on the SCSI CDB)?
The question comes up because I was running some
iSCSI protocol test
scripts
where the PDU "expected transfer length" and the transfer length
in the
CDB were inconsistent in one of the scripts.
Another example would be if the "allocation
length" field in the CDB for
an
Inquiry command doesn't match the "expected data transfer length"
field
in the
SCSI Command PDU.
The script was in error and should be fixed, is there
a desired target
behavior
in such situations, given that the CDB fields are not typically parsed
by the
iSCSI layer, the SCSI device may know nothing about iSCSI PDUs,
and some
middle-layer may or may not know about both.
If the initiator detects that an underflow has
occurred, based solely on the
amount
of data received and the "expected data transfer field" in the iSCSI
PDU,
but
the target did not report an underflow or residual count in the SCSI response,
is the
initiator allowed/expected to report an underflow to the upper layers?
Thanks,
Dean Scoville
