>From what I've read, ISA SCSI cards can do DMA only into memory
addresses < 16 MB, while EISA SCSI cards do not have this restriction.
In Linux 2.0.24, file drivers/scsi/sd.h, ISA_DMA_THRESHOLD is
#defined to 16 MB, and this is used in sd.c, which makes it apply
to _all_ SCSI cards, including non-ISA cards.
Why is this? Or am I missing something here?
You're missing something. Drivers have control over this by setting the
unchecked_isa_dma flag in the SCSI Host structure. If this flag is not set,
then the card is assumed to be able to access the entire memory space, and so
bounce buffers are not used. On Alpha, Linux actually doesn't handle the case
of PCI access to > 4GB memory.
Leonard