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
