Description of rounding rules for PPR transfer agreements

Gerry.Houlder at Gerry.Houlder at
Thu Dec 20 13:05:21 PST 2001

* From the T10 Reflector (t10 at, posted by:
* Gerry.Houlder at

I don't see wording in the standard (even rev 6) that expresses what I
remember about this rule. I suspect the wording was removed at a much
earlier revision or I might just be remembering a discussion at a meeting
and the result of the discussion (at that time) was never written into the

One discussion revolved around having a period of 8 or 9 (which requires DT
data transfer) and having the DT_REQ bit cleared. This case looks
"roundable" (i.e., just round the period value) so this was not a problem.

Other cases were not so clear. What if QAS was set and IU_REQ is not? What
if period is 8 (320 MB paced transfer) and IU_REQ is not set? What if
IU_REQ is set and width is set to 0? There may be more than one way to
"round these down". There was talk of just reverting to the default
agreement if these cases were encountered instead of rounding. This seems
to be an allowed rounding value for any case, but will initiators depend on
exact rounding behavior in the drives and could strange things happen if
different drive vendors do the rounding differently?

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