and signal the root hub interrupt only once we are succesfully able
to open the device node. This makes it possible to insert a device
after the rump kernel was booted and have it succesfully attach
(does not make detach possible yet, though, as there are some
ugen and host kernel uhci/ohci/ehci evil crashies with that).
XXX: optimally, match would fail if there is a permanent error in
opening. However, it is difficult to figure out the difference
between the device backing ugen not being present, a transient
error in opening and a permanent error in opening. For example,
which of the latter two would EPERM be? And, ugen returns ENXIO
if the device is not present, but how would be know that's really
the case and not some other ENXIO from elsewhere in the stack?
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