> I think this addresses all the points so far. Three new ioctls:
>   * one to set DCD timestamping on or off
>   * one to get DCD timestamping on/off state
>   * one to just fetch the most-recent timestamp.
>     silently returns zero if timestamping is not enabled.
>     should this return an error instead?

Instead of the #ifdef PPS_TRAILING_EDGE stuff being a compile-time
decision, how about making everything else use three three states:  off /
on (rising edge) / on (falling edge).

--apb (Alan Barrett)