establish the interrupt handler before enabling interrupts, so that
we will not get an interrupt storm that keeps the system from making
any progress. (We may have only been susceptible to the storm if
cbb(4) shared an interrupt with, say, uhci(4)). Also, finish
setting up the pcic_handle before enabling interrupts, so that the
Pcic_read() call in pccbbintr() will not jump to address 0!
Now, pccbb_intrinit() encapsulates all of the interrupt initialization.
Explicitly enable card-status changed (CSTSCHG) events.
Paranoia: ensure that writes to the socket event register (interrupt
acknowledgements) are flushed to the h/w by reading the register
(gross). We really need something like
bus_space_sync(BUS_SPACE_BARRIER_WRITE_BEFORE_INTERRUPT).
More paranoia: in pccbbintr(), if the device is inactive, get out.
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