Re: Possible to limit size of DMA transfer?

["Monish Shah" <monishs_in@yahoo.com>]

> So my questions are:
>
>   Is there anything in the PCI spec that provides for imposing
>   a bus-wide limit on the size of DMA transfers?

Not exactly, but the "latency timer" comes close.

> If so, is there
>   any means in Windows NT to cause such a limit to be imposed?

I don't know, but I doubt that NT would let you control the latency timer.
It *might* be possible to set it in the BIOS.

>   Failing that, is there some kind of registry key for maximum
>   DMA transfer size in WinNT that is observed by PCI drivers?
>
>   Failing that, does anyone know if Adaptec has a way of imposing
>   such a limit on its own adapters?

I would guess that they do implement the latency timer.  I doubt that they
have any other mechanism for limiting the DMA.

>   As a parting shot, just out of curiosity, is PCI bus tuning
>   (trading off access latency, bandwidth etc) something that is
>   commonly done, or for which there is any existing literature?

I think it is nearly impossible for a vendor of a general-purpose system to
do such tuning, because they don't know what might be plugged into the
system.  If you have a system where you know exactly what is plugged in,
tuning can definitely be done.  (I've done it.)  I don't know of any
literature on the topic, though.

> Thanks very much.
>
>                            Andy Davenport
>                            Harvey Mudd College

Monish Shah
Ascent Software