Dell XPS 13 9350 touchpad is busted in 4.12-rc

I just build from Linus' tree from Saturday (ef82f1ad), and touchpad
input is busted on my laptop.  The device is still there and evtest
still shows events, but something is wrong and libinput either thinks
I'm not moving at all or I'm trying to teleport to the edge of the
screen.

Is this a known problem or should I try to track it down?

Thanks,
Andy