This was the main impetus behind adding the PCI IRQ shim.

In order to properly order DMA writes wrt. interrupts, you have to
write to a PCI controller register, then poll for that bit clearing.
There is one bit for each interrupt source, and setting this register
bit tells Tomatillo to drain all pending DMA from that device.

Furthermore, Tomatillo's with revision less than 4 require us to do a
block store due to some memory transaction ordering issues it has on
JBUS.
Signed-off-by: David S. Miller <davem@davemloft.net>