residue of 1394 Sbp-2 - forget it?

Peter Johansson PJohansson at acm.org
Tue Mar 5 14:53:48 PST 2002


At 01:57 PM 3/5/2002 -0700, Pat LaVarre wrote:

>Sbp-2 gives the host no standard way to ask for, and Sbp-2 gives the 
>target no standard way to reply with, the "residue" i.e. count of bytes of 
>the data buffer that the target chose not to move in or out.

Take a look at ANSI NCITS 325-1998 (the published standard), Pat, in 
particular Annex B.2. You'll find a standard way for SCSI command sets to 
return the same basic sense data they are able to return via the other SCSI 
transport protocols. You'll find residue defined as one of the uses of the 
INFORMATION field.

But, as David Wooten pointed out, it's really a command set issue. Commands 
typically fall into a two classes:

a) Those commands, such as disk READ or WRITE, for which GOOD status means 
that all the bytes were transferred. If the status is CHECK CONDITION, the 
command set may or may not require the return of residual information. Tape 
commands are good examples of those that return residue information when 
they fail to transfer all the data.

b) Those commands, such as INQUIRY, that may not make use of all the buffer 
space provided by the initiator even when they complete with GOOD status. 
In my experience, the data returned by these commands is self-descriptive; 
i.e., the client application may discover the length of the returned data 
by parsing the returned data.

I'll admit that SPC is weak where it should make clear that INQUIRY shall 
always return at least five (5) bytes of data, ALLOCATION LENGTH 
permitting. But, this is a command set issue, not a SCSI transport protocol 
issue.


Regards,

Peter Johansson

Congruent Software, Inc.
98 Colorado Avenue
Berkeley, CA  94707

(510) 527-3926
(510) 527-3856 FAX

PJohansson at ACM.org


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