>     * we have no splhard<something>() so splhardusb/splusb don't
>     follow our naming convention. Most likely we would need to make
>     splsoftusb == current splusb and make splhardusb == splusb.

One of confusing part is that interrupt handlers which are protected
by splhardusb() are called with IPL_USB (== IPL_BIO) level, and
interrupt handlers which are protected by splusb() level are called
with IPL_SOFTNET (in the __HAVE_GENERIC_SOFT_INTERRUPTS case) or
IPL_SOFTCLOCK (in !defined(__HAVE_GENERIC_SOFT_INTERRUPTS) level ;)

>     * we should probably define splusb under splvm and spltty (I'm
>       ignoring the current problem of spltty being lower priority than
>       splnet -- maybe it's time to fix this ?)

Probably this is not so simple change, because we want non-isochronous
devices to keep using lower priority, but want isochronous device
interrupt handlers to be called without lowering interrupt priority,
and also have to protect some usb controller resources from both priority
levels.
--
soda