Just as an experiment, I reverted 1.5R's arch/arc/jazz/dma.[ch] to the
dma.[ch] from 1.5 release (and corrected the includes at the top).
Now, newfs no longer hangs the Magnum after some sustained writing (in
asc_dma_out), but now the machine randomly panics during sustained
scsi writes with...

asc_intr: data overrun: buflen 8192 dmalen 8192 tc 0 fifo 1                     
panic: asc_intr                                                                 

...and drops to DDB.

Obviously, we have a serious problem somewhere with DMA.

-Mark