I've discussed the recent patches for PPS support with people from
FreeBSD, and the xntpd maintainers.  Their preferred interface is
rather like ours, only different:

      typedef u_int pps_seq_t;			/* at least 32 bits */
      typedef struct ppsinfo {
              pps_seq_t   sequence;            /* event sequence # */
              struct timespec timestamp;       /* time of event */
              int     flags;                   /* flag bits */
      } ppsinfo_t;

      #define PPSGETTIME    _IOR('t', 75, struct ppsinfo)
      #define PPSSETMODE    _IOW('t', 76, int)
      #define PPSGETMODE    _IOR('t', 77, int)

with PPS mode bits as follows:

      #define PPSMODE_NONE            0x00
      #define PPSMODE_ONASSERT        0x01
      #define PPSMODE_ONCLEAR         0x02
      #define PPSMODE_ONBOTH          (PPSMODE_ONASSERT|PPSMODE_ONCLEAR)
      #define PPSMODE_HARDPPS         0x04

This API is an Internet Draft (draft-mogul-pps-api-01.txt), and may be
widely deployed; I propose we adopt it, since its semantically very
extremely close to the one that was settled on here.

Poul Henning-Kamp pointed out that this API has problems with DCF77
receivers, where one needs both the timestamp of the PPS pulse and
also its width. (since higher-epoch times than the minute or second
epochs are encoded in the width of each of the PPS pulses in any given
minute, more or less.)

Also, PPSMODE_HARDPPS is used to explicitly enable hardpps() for
multi-clock configurations, something phk and I had noted as missing
in the earlier `NetBSD API'.  It's not clear how PPSMODE_ONBOTH is
supposed to interact with PPSMODE_HARDPPS: it may change

Assuuming we go with this API, where should the struct and constants
be defined They're in the 't' group, which suggests <ttycom.h>, but
theyre' outside the TIOC namespace.  I think struct ppsinfo doesn't
belong in ttycom.h: <sys/timex.h> seems more sensible.
(The draft API is silent on this point).

Or should I post the reworked code?

NB: version 4 of ntpd should support this API eventually, but for the
stable codebase (xntpd), TIOCDCDTIMESTAMP is still it.