Highmem SCSI driver

We are trying to test 64-bit PCI DMA for a SCSI driver on a Xeon box, RH9 2.4.20-8 bigmem kernel, 6GB RAM. The machine shows 6GB in top, we set highmem_io in the driver, PCI DMA mask covers 64-bit range, etc.

Of course we're trying to make sure that the system does not create bounce buffers unnecessarily. On a 64-bit box (AMD64) everything works as expected. On the Xeon, no matter what we try, we never see I/Os mapped above 4GB.

Any ideas on how we can drive I/Os mapped above 4GB down to our driver?

Thanks,
-Mark

can you give an URL to the driver so that we can see why it breaks ?