Possible to limit size of DMA transfer?

[Andy Davenport <ANDY@THUBAN.AC.HMC.EDU>]

I have a video capture card with the Conexant (nee BrookTree) Bt878
chip. I am getting horizontal line defects across the video that I
believe are due to overruns in the rather small FIFO in the Bt878.
My reading of the chip spec says that I have only 10 to 13 usec to
gain access to the PCI bus before losing video samples. I also have
an Adaptec 2940U2W SCSI on the same PCI bus to which are attached
the disks for saving the video data. My presumption is that the
2940U2W grabs the PCI for far longer than 13 usec (as it rightly
should for efficiency, I suppose) and I lose video data. Even a
lowly 2048 byte transfer would take more that 15 usec, right?

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

  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?

  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?

Thanks very much.

                           Andy Davenport
                           Harvey Mudd College