If the DMA chaning interrupt couldn't be serviced immediately (higher spl
level when kernel doing something else) a few microseconds later the NCR
controller will fill its FIFO and also interrupt the CPU.
The SCSI interrupt sees the terminal count has been reached, calls
asc_dma_intr to finish the job off. The FIFO cannot be flushed because
the block count hasn't been setup for the last dma segment (DMA chaining
still wasn't serviced).
Since the NCR 53c94 FIFO is only 16 bytes in size, any short DMA in this
size combined with the machine 'doing something else' causes the problem
to occur.
Servicing the DMA chaining interrupt before the NCR SCSI interrupt solves
this problem.
Add tests to ensure the DMA FIFO has been flushed correctly at the end of
each DMA operation just to be on the safe side.
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