TURBOChannel cfb has a vertical-retrace interupt that cannot be disabled
in software. 4.4BSD and NetBSD don't supply an interrupt hander and
simply neverenable interrupts from slots with a cfb. This has never
ever worked with a cfb in a 3MIN (Decstation 5000/1xx), where a TURBOChannel
card in slot 0 (or 1 or 2) interrupts at the same IPL as spl0() (or spl1
or spl2), and there's also never been support for selectively enabling or
disabling those interrupts on a 3MIN, in either NetBSD or 4.4BSD.
This revision add an interrupt handler for the cfb, used only on 3MINs,
so that a 3MIN can boot with a cfb as console, and enable/disable of TC
slot interrupts can be debugged. (Serial consoles via ioasic SCCs still
don't quite work, and simply clearing the relevant bits in the R3000 cause
register is apparently over-ridden by a subsequent spl0() somewhere else.)
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