You haven't got it right. Physical address > 4GB does not mean
your 32-bit device cannot DMA to it. Stop thinking about
implementation, that's the whole point of the abstraction :-)
On 64-bit platforms that don't set CONFIG_HIGHMEM, they have MMU's on
the PCI bus that can map arbitrary 64-bit physical addresses to 32-bit
PCI bus addresses. So on these platforms you may pass any pointer
from kmalloc()/alloc_page() whatsoever into the pci_map_foo()
routines.
In order to handle highmem pages, you have to set your DMA mask
appropriately (to indicate 64-bit addressing capability) and
use pci_map_page() instead of pci_map_single().
Look at other drivers using the DMA interfaces like the two aic7xxx
and all of the sym53c8xx drivers, they get it right.
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