>You won't have a PPS event without generating a delta, so why not test for
>it?

That's what the code I committed does.

> you have to do is make sure you've enabled the interrupt for DCD
>interrupts even if in local mode when PPS is enabled.

DCD interrupts seem to always be enabled. The hard interrupt
(splserial) always takes them, but takes no action for MSR-bit deltas
that aren't set in sc_msr_mask.  At least, that's what I saw when I
pulled/re-inserted a modem cable.

Long-term we could track the bits the driver cares about for flow
control and modem status, add hooks to recompute those bits given tty
state and pps state, change drivers to call that hook when tty state
or pps state changes, and then (if possible) only enable interrupts
for only the bits the driver cares about.

If the device-specifc `update status bits' function takes PPSAPI flags
for PPS state, rather than chip-specific register masks, the PPS
ioctls can be made MI, too.  Comments?