/src/ntp-dev/ntpd/refclock_nmea.c

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/*
 * refclock_nmea.c - clock driver for an NMEA GPS CLOCK
 *    Michael Petry Jun 20, 1994
 *     based on refclock_heathn.c
 *
 * Updated to add support for Accord GPS Clock
 *    Venu Gopal Dec 05, 2007
 *    neo.venu@gmail.com, venugopal_d@pgad.gov.in
 *
 * Updated to process 'time1' fudge factor
 *    Venu Gopal May 05, 2008
 *
 * Converted to common PPSAPI code, separate PPS fudge time1
 * from serial timecode fudge time2.
 *    Dave Hart July 1, 2009
 *    hart@ntp.org, davehart@davehart.com
 */

#ifdef HAVE_CONFIG_H
#include <config.h>
#endif

#include "ntp_types.h"

#if defined(REFCLOCK) && defined(CLOCK_NMEA)

#define NMEA_WRITE_SUPPORT 0 /* no write support at the moment */

#include <sys/stat.h>
#include <stdio.h>
#include <ctype.h>
#ifdef HAVE_SYS_SOCKET_H
#include <sys/socket.h>
#endif

#include "ntpd.h"
#include "ntp_io.h"
#include "ntp_unixtime.h"
#include "ntp_refclock.h"
#include "ntp_stdlib.h"
#include "ntp_calendar.h"
#include "timespecops.h"

#ifdef HAVE_PPSAPI
# include "ppsapi_timepps.h"
# include "refclock_atom.h"
#endif /* HAVE_PPSAPI */


/*
 * This driver supports NMEA-compatible GPS receivers
 *
 * Prototype was refclock_trak.c, Thanks a lot.
 *
 * The receiver used spits out the NMEA sentences for boat navigation.
 * And you thought it was an information superhighway.  Try a raging river
 * filled with rapids and whirlpools that rip away your data and warp time.
 *
 * If HAVE_PPSAPI is defined code to use the PPSAPI will be compiled in.
 * On startup if initialization of the PPSAPI fails, it will fall back
 * to the "normal" timestamps.
 *
 * The PPSAPI part of the driver understands fudge flag2 and flag3. If
 * flag2 is set, it will use the clear edge of the pulse. If flag3 is
 * set, kernel hardpps is enabled.
 *
 * GPS sentences other than RMC (the default) may be enabled by setting
 * the relevent bits of 'mode' in the server configuration line
 * server 127.127.20.x mode X
 * 
 * bit 0 - enables RMC (1)
 * bit 1 - enables GGA (2)
 * bit 2 - enables GLL (4)
 * bit 3 - enables ZDA (8) - Standard Time & Date
 * bit 3 - enables ZDG (8) - Accord GPS Clock's custom sentence with GPS time 
 *           very close to standard ZDA
 * 
 * Multiple sentences may be selected except when ZDG/ZDA is selected.
 *
 * bit 4/5/6 - selects the baudrate for serial port :
 *    0 for 4800 (default) 
 *    1 for 9600 
 *    2 for 19200 
 *    3 for 38400 
 *    4 for 57600 
 *    5 for 115200 
 */
#define NMEA_MESSAGE_MASK 0x0000FF0FU
#define NMEA_BAUDRATE_MASK  0x00000070U
#define NMEA_BAUDRATE_SHIFT 4

#define NMEA_DELAYMEAS_MASK 0x80
#define NMEA_EXTLOG_MASK  0x00010000U
#define NMEA_DATETRUST_MASK 0x02000000U

#define NMEA_PROTO_IDLEN  5  /* tag name must be at least 5 chars */
#define NMEA_PROTO_MINLEN 6 /* min chars in sentence, excluding CS */
#define NMEA_PROTO_MAXLEN 80  /* max chars in sentence, excluding CS */
#define NMEA_PROTO_FIELDS 32  /* not official; limit on fields per record */

/*
 * We check the timecode format and decode its contents.  We only care
 * about a few of them, the most important being the $GPRMC format:
 *
 * $GPRMC,hhmmss,a,fddmm.xx,n,dddmmm.xx,w,zz.z,yyy.,ddmmyy,dd,v*CC
 *
 * mode (0,1,2,3) selects sentence ANY/ALL, RMC, GGA, GLL, ZDA
 * $GPGLL,3513.8385,S,14900.7851,E,232420.594,A*21
 * $GPGGA,232420.59,3513.8385,S,14900.7851,E,1,05,3.4,00519,M,,,,*3F
 * $GPRMC,232418.19,A,3513.8386,S,14900.7853,E,00.0,000.0,121199,12.,E*77
 *
 * Defining GPZDA to support Standard Time & Date
 * sentence. The sentence has the following format 
 *  
 *  $--ZDA,HHMMSS.SS,DD,MM,YYYY,TH,TM,*CS<CR><LF>
 *
 *  Apart from the familiar fields, 
 *  'TH'    Time zone Hours
 *  'TM'    Time zone Minutes
 *
 * Defining GPZDG to support Accord GPS Clock's custom NMEA 
 * sentence. The sentence has the following format 
 *  
 *  $GPZDG,HHMMSS.S,DD,MM,YYYY,AA.BB,V*CS<CR><LF>
 *
 *  It contains the GPS timestamp valid for next PPS pulse.
 *  Apart from the familiar fields, 
 *  'AA.BB' denotes the signal strength( should be < 05.00 ) 
 *  'V'     denotes the GPS sync status : 
 *     '0' indicates INVALID time, 
 *     '1' indicates accuracy of +/-20 ms
 *     '2' indicates accuracy of +/-100 ns
 *
 * Defining PGRMF for Garmin GPS Fix Data
 * $PGRMF,WN,WS,DATE,TIME,LS,LAT,LAT_DIR,LON,LON_DIR,MODE,FIX,SPD,DIR,PDOP,TDOP
 * WN  -- GPS week number (weeks since 1980-01-06, mod 1024)
 * WS  -- GPS seconds in week
 * LS  -- GPS leap seconds, accumulated ( UTC + LS == GPS )
 * FIX -- Fix type: 0=nofix, 1=2D, 2=3D
 * DATE/TIME are standard date/time strings in UTC time scale
 *
 * The GPS time can be used to get the full century for the truncated
 * date spec.
 */

/*
 * Definitions
 */
#define DEVICE    "/dev/gps%d"  /* GPS serial device */
#define PPSDEV    "/dev/gpspps%d" /* PPSAPI device override */
#define SPEED232  B4800  /* uart speed (4800 bps) */
#define PRECISION (-9)  /* precision assumed (about 2 ms) */
#define PPS_PRECISION (-20) /* precision assumed (about 1 us) */
#define REFID   "GPS\0"  /* reference id */
#define DESCRIPTION "NMEA GPS Clock" /* who we are */
#ifndef O_NOCTTY
#define M_NOCTTY  0
#else
#define M_NOCTTY  O_NOCTTY
#endif
#ifndef O_NONBLOCK
#define M_NONBLOCK  0
#else
#define M_NONBLOCK  O_NONBLOCK
#endif
#define PPSOPENMODE (O_RDWR | M_NOCTTY | M_NONBLOCK)

/* NMEA sentence array indexes for those we use */
#define NMEA_GPRMC  0  /* recommended min. nav. */
#define NMEA_GPGGA  1  /* fix and quality */
#define NMEA_GPGLL  2  /* geo. lat/long */
#define NMEA_GPZDA  3  /* date/time */
/*
 * $GPZDG is a proprietary sentence that violates the spec, by not
 * using $P and an assigned company identifier to prefix the sentence
 * identifier.  When used with this driver, the system needs to be
 * isolated from other NTP networks, as it operates in GPS time, not
 * UTC as is much more common.  GPS time is >15 seconds different from
 * UTC due to not respecting leap seconds since 1970 or so.  Other
 * than the different timebase, $GPZDG is similar to $GPZDA.
 */
#define NMEA_GPZDG  4
#define NMEA_PGRMF  5
#define NMEA_ARRAY_SIZE (NMEA_PGRMF + 1)

/*
 * Sentence selection mode bits
 */
#define USE_GPRMC   0x00000001u
#define USE_GPGGA   0x00000002u
#define USE_GPGLL   0x00000004u
#define USE_GPZDA   0x00000008u
#define USE_PGRMF   0x00000100u

/* mapping from sentence index to controlling mode bit */
static const u_int32 sentence_mode[NMEA_ARRAY_SIZE] =
{
  USE_GPRMC,
  USE_GPGGA,
  USE_GPGLL,
  USE_GPZDA,
  USE_GPZDA,
  USE_PGRMF
};

/* date formats we support */
enum date_fmt {
  DATE_1_DDMMYY,  /* use 1 field  with 2-digit year */
  DATE_3_DDMMYYYY /* use 3 fields with 4-digit year */
};

/* results for 'field_init()'
 *
 * Note: If a checksum is present, the checksum test must pass OK or the
 * sentence is tagged invalid.
 */
#define CHECK_EMPTY  -1  /* no data      */
#define CHECK_INVALID 0  /* not a valid NMEA sentence  */
#define CHECK_VALID   1  /* valid but without checksum */
#define CHECK_CSVALID 2  /* valid with checksum OK */

/*
 * Unit control structure
 */
typedef struct {
#ifdef HAVE_PPSAPI
  struct refclock_atom atom; /* PPSAPI structure */
  int ppsapi_fd;  /* fd used with PPSAPI */
  u_char  ppsapi_tried; /* attempt PPSAPI once */
  u_char  ppsapi_lit; /* time_pps_create() worked */
  u_char  ppsapi_gate;  /* system is on PPS */
#endif /* HAVE_PPSAPI */
  u_char  gps_time; /* use GPS time, not UTC */
  u_short century_cache;  /* cached current century */
  l_fp  last_reftime; /* last processed reference stamp */
  short   epoch_warp; /* last epoch warp, for logging */
  /* tally stats, reset each poll cycle */
  struct
  {
    u_int total;
    u_int accepted;
    u_int rejected;   /* GPS said not enough signal */
    u_int malformed;  /* Bad checksum, invalid date or time */
    u_int filtered;   /* mode bits, not GPZDG, same second */
    u_int pps_used;
  } 
    tally;
  /* per sentence checksum seen flag */
  u_char  cksum_type[NMEA_ARRAY_SIZE];
} nmea_unit;

/*
 * helper for faster field access
 */
typedef struct {
  char  *base;  /* buffer base    */
  char  *cptr;  /* current field ptr  */
  int    blen;  /* buffer length  */
  int    cidx;  /* current field index  */
} nmea_data;

/*
 * NMEA gps week/time information
 * This record contains the number of weeks since 1980-01-06 modulo
 * 1024, the seconds elapsed since start of the week, and the number of
 * leap seconds that are the difference between GPS and UTC time scale.
 */
typedef struct {
  u_int32 wt_time;  /* seconds since weekstart */
  u_short wt_week;  /* week number */
  short wt_leap;  /* leap seconds */
} gps_weektm;

/*
 * The GPS week time scale starts on Sunday, 1980-01-06. We need the
 * rata die number of this day.
 */
#ifndef DAY_GPS_STARTS
#define DAY_GPS_STARTS 722820
#endif

/*
 * Function prototypes
 */
static  void  nmea_init (void);
static  int nmea_start  (int, struct peer *);
static  void  nmea_shutdown (int, struct peer *);
static  void  nmea_receive  (struct recvbuf *);
static  void  nmea_poll (int, struct peer *);
#ifdef HAVE_PPSAPI
static  void  nmea_control  (int, const struct refclockstat *,
         struct refclockstat *, struct peer *);
#define   NMEA_CONTROL  nmea_control
#else
#define   NMEA_CONTROL  noentry
#endif /* HAVE_PPSAPI */
static  void  nmea_timer  (int, struct peer *);

/* parsing helpers */
static int  field_init  (nmea_data * data, char * cp, int len);
static char * field_parse (nmea_data * data, int fn);
static void field_wipe  (nmea_data * data, ...);
static u_char parse_qual  (nmea_data * data, int idx,
         char tag, int inv);
static int  parse_time  (struct calendar * jd, long * nsec,
         nmea_data *, int idx);
static int  parse_date  (struct calendar *jd, nmea_data*,
         int idx, enum date_fmt fmt);
static int  parse_weekdata  (gps_weektm *, nmea_data *,
         int weekidx, int timeidx, int leapidx);
/* calendar / date helpers */
static int  unfold_day  (struct calendar * jd, u_int32 rec_ui);
static int  unfold_century  (struct calendar * jd, u_int32 rec_ui);
static int  gpsfix_century  (struct calendar * jd, const gps_weektm * wd,
         u_short * ccentury);
static l_fp     eval_gps_time (struct peer * peer, const struct calendar * gpst,
         const struct timespec * gpso, const l_fp * xrecv);

static int  nmead_open  (const char * device);
static void     save_ltc        (struct refclockproc * const, const char * const,
         size_t);

/*
 * If we want the driver to ouput sentences, too: re-enable the send
 * support functions by defining NMEA_WRITE_SUPPORT to non-zero...
 */
#if NMEA_WRITE_SUPPORT

static  void gps_send(int, const char *, struct peer *);
# ifdef SYS_WINNT
#  undef write  /* ports/winnt/include/config.h: #define write _write */
extern int async_write(int, const void *, unsigned int);
#  define write(fd, data, octets) async_write(fd, data, octets)
# endif /* SYS_WINNT */

#endif /* NMEA_WRITE_SUPPORT */

static int32_t g_gpsMinBase;
static int32_t g_gpsMinYear;

/*
 * -------------------------------------------------------------------
 * Transfer vector
 * -------------------------------------------------------------------
 */
struct refclock refclock_nmea = {
  nmea_start,   /* start up driver */
  nmea_shutdown,    /* shut down driver */
  nmea_poll,    /* transmit poll message */
  NMEA_CONTROL,   /* fudge control */
  nmea_init,    /* initialize driver */
  noentry,    /* buginfo */
  nmea_timer    /* called once per second */
};

/*
 * -------------------------------------------------------------------
 * nmea_init - initialise data
 *
 * calculates a few runtime constants that cannot be made compile time
 * constants.
 * -------------------------------------------------------------------
 */
static void
nmea_init(void)
{
  struct calendar date;

  /* - calculate min. base value for GPS epoch & century unfolding 
   * This assumes that the build system was roughly in sync with
   * the world, and that really synchronising to a time before the
   * program was created would be unsafe or insane. If the build
   * date cannot be stablished, at least use the start of GPS
   * (1980-01-06) as minimum, because GPS can surely NOT
   * synchronise beyond it's own big bang. We add a little safety
   * margin for the fuzziness of the build date, which is in an
   * undefined time zone. */
  if (ntpcal_get_build_date(&date))
    g_gpsMinBase = ntpcal_date_to_rd(&date) - 2;
  else
    g_gpsMinBase = 0;

  if (g_gpsMinBase < DAY_GPS_STARTS)
    g_gpsMinBase = DAY_GPS_STARTS;

  ntpcal_rd_to_date(&date, g_gpsMinBase);
  g_gpsMinYear  = date.year;
  g_gpsMinBase -= DAY_NTP_STARTS;
}

/*
 * -------------------------------------------------------------------
 * nmea_start - open the GPS devices and initialize data for processing
 *
 * return 0 on error, 1 on success. Even on error the peer structures
 * must be in a state that permits 'nmea_shutdown()' to clean up all
 * resources, because it will be called immediately to do so.
 * -------------------------------------------------------------------
 */
static int
nmea_start(
  int   unit,
  struct peer * peer
  )
{
  struct refclockproc * const pp = peer->procptr;
  nmea_unit * const   up = emalloc_zero(sizeof(*up));
  char        device[20];
  size_t        devlen;
  u_int32       rate;
  int       baudrate;
  const char *      baudtext;


  /* Get baudrate choice from mode byte bits 4/5/6 */
  rate = (peer->ttl & NMEA_BAUDRATE_MASK) >> NMEA_BAUDRATE_SHIFT;

  switch (rate) {
  case 0:
    baudrate = SPEED232;
    baudtext = "4800";
    break;
  case 1:
    baudrate = B9600;
    baudtext = "9600";
    break;
  case 2:
    baudrate = B19200;
    baudtext = "19200";
    break;
  case 3:
    baudrate = B38400;
    baudtext = "38400";
    break;
#ifdef B57600
  case 4:
    baudrate = B57600;
    baudtext = "57600";
    break;
#endif
#ifdef B115200
  case 5:
    baudrate = B115200;
    baudtext = "115200";
    break;
#endif
  default:
    baudrate = SPEED232;
    baudtext = "4800 (fallback)";
    break;
  }

  /* Allocate and initialize unit structure */
  pp->unitptr = (caddr_t)up;
  pp->io.fd = -1;
  pp->io.clock_recv = nmea_receive;
  pp->io.srcclock = peer;
  pp->io.datalen = 0;
  /* force change detection on first valid message */
  memset(&up->last_reftime, 0xFF, sizeof(up->last_reftime));
  /* force checksum on GPRMC, see below */
  up->cksum_type[NMEA_GPRMC] = CHECK_CSVALID;
#ifdef HAVE_PPSAPI
  up->ppsapi_fd = -1;
#endif
  ZERO(up->tally);

  /* Initialize miscellaneous variables */
  peer->precision = PRECISION;
  pp->clockdesc = DESCRIPTION;
  memcpy(&pp->refid, REFID, 4);

  /* Open serial port. Use CLK line discipline, if available. */
  devlen = snprintf(device, sizeof(device), DEVICE, unit);
  if (devlen >= sizeof(device)) {
    msyslog(LOG_ERR, "%s clock device name too long",
      refnumtoa(&peer->srcadr));
    return FALSE; /* buffer overflow */
  }
  pp->io.fd = refclock_open(device, baudrate, LDISC_CLK);
  if (0 >= pp->io.fd) {
    pp->io.fd = nmead_open(device);
    if (-1 == pp->io.fd)
      return FALSE;
  }
  LOGIF(CLOCKINFO, (LOG_NOTICE, "%s serial %s open at %s bps",
        refnumtoa(&peer->srcadr), device, baudtext));

  /* succeed if this clock can be added */
  return io_addclock(&pp->io) != 0;
}


/*
 * -------------------------------------------------------------------
 * nmea_shutdown - shut down a GPS clock
 * 
 * NOTE this routine is called after nmea_start() returns failure,
 * as well as during a normal shutdown due to ntpq :config unpeer.
 * -------------------------------------------------------------------
 */
static void
nmea_shutdown(
  int           unit,
  struct peer * peer
  )
{
  struct refclockproc * const pp = peer->procptr;
  nmea_unit     * const up = (nmea_unit *)pp->unitptr;

  UNUSED_ARG(unit);

  if (up != NULL) {
#ifdef HAVE_PPSAPI
    if (up->ppsapi_lit)
      time_pps_destroy(up->atom.handle);
    if (up->ppsapi_tried && up->ppsapi_fd != pp->io.fd)
      close(up->ppsapi_fd);
#endif
    free(up);
  }
  pp->unitptr = (caddr_t)NULL;
  if (-1 != pp->io.fd)
    io_closeclock(&pp->io);
  pp->io.fd = -1;
}

/*
 * -------------------------------------------------------------------
 * nmea_control - configure fudge params
 * -------------------------------------------------------------------
 */
#ifdef HAVE_PPSAPI
static void
nmea_control(
  int                         unit,
  const struct refclockstat * in_st,
  struct refclockstat       * out_st,
  struct peer               * peer
  )
{
  struct refclockproc * const pp = peer->procptr;
  nmea_unit     * const up = (nmea_unit *)pp->unitptr;

  char   device[32];
  size_t devlen;
  
  UNUSED_ARG(in_st);
  UNUSED_ARG(out_st);

  /*
   * PPS control
   *
   * If /dev/gpspps$UNIT can be opened that will be used for
   * PPSAPI.  Otherwise, the GPS serial device /dev/gps$UNIT
   * already opened is used for PPSAPI as well. (This might not
   * work, in which case the PPS API remains unavailable...)
   */

  /* Light up the PPSAPI interface if not yet attempted. */
  if ((CLK_FLAG1 & pp->sloppyclockflag) && !up->ppsapi_tried) {
    up->ppsapi_tried = TRUE;
    devlen = snprintf(device, sizeof(device), PPSDEV, unit);
    if (devlen < sizeof(device)) {
      up->ppsapi_fd = open(device, PPSOPENMODE,
               S_IRUSR | S_IWUSR);
    } else {
      up->ppsapi_fd = -1;
      msyslog(LOG_ERR, "%s PPS device name too long",
        refnumtoa(&peer->srcadr));
    }
    if (-1 == up->ppsapi_fd)
      up->ppsapi_fd = pp->io.fd;  
    if (refclock_ppsapi(up->ppsapi_fd, &up->atom)) {
      /* use the PPS API for our own purposes now. */
      up->ppsapi_lit = refclock_params(
        pp->sloppyclockflag, &up->atom);
      if (!up->ppsapi_lit) {
        /* failed to configure, drop PPS unit */
        time_pps_destroy(up->atom.handle);
        msyslog(LOG_WARNING,
          "%s set PPSAPI params fails",
          refnumtoa(&peer->srcadr));        
      }
      /* note: the PPS I/O handle remains valid until
       * flag1 is cleared or the clock is shut down. 
       */
    } else {
      msyslog(LOG_WARNING,
        "%s flag1 1 but PPSAPI fails",
        refnumtoa(&peer->srcadr));
    }
  }

  /* shut down PPS API if activated */
  if (!(CLK_FLAG1 & pp->sloppyclockflag) && up->ppsapi_tried) {
    /* shutdown PPS API */
    if (up->ppsapi_lit)
      time_pps_destroy(up->atom.handle);
    up->atom.handle = 0;
    /* close/drop PPS fd */
    if (up->ppsapi_fd != pp->io.fd)
      close(up->ppsapi_fd);
    up->ppsapi_fd = -1;

    /* clear markers and peer items */
    up->ppsapi_gate  = FALSE;
    up->ppsapi_lit   = FALSE;
    up->ppsapi_tried = FALSE;

    peer->flags &= ~FLAG_PPS;
    peer->precision = PRECISION;
  }
}
#endif  /* HAVE_PPSAPI */

/*
 * -------------------------------------------------------------------
 * nmea_timer - called once per second
 *    this only polls (older?) Oncore devices now
 *
 * Usually 'nmea_receive()' can get a timestamp every second, but at
 * least one Motorola unit needs prompting each time. Doing so in
 * 'nmea_poll()' gives only one sample per poll cycle, which actually
 * defeats the purpose of the median filter. Polling once per second
 * seems a much better idea.
 * -------------------------------------------------------------------
 */
static void
nmea_timer(
  int       unit,
  struct peer * peer
  )
{
#if NMEA_WRITE_SUPPORT
    
  struct refclockproc * const pp = peer->procptr;

  UNUSED_ARG(unit);

  if (-1 != pp->io.fd) /* any mode bits to evaluate here? */
    gps_send(pp->io.fd, "$PMOTG,RMC,0000*1D\r\n", peer);
#else
  
  UNUSED_ARG(unit);
  UNUSED_ARG(peer);
  
#endif /* NMEA_WRITE_SUPPORT */
}

#ifdef HAVE_PPSAPI
/*
 * -------------------------------------------------------------------
 * refclock_ppsrelate(...) -- correlate with PPS edge
 *
 * This function is used to correlate a receive time stamp and a
 * reference time with a PPS edge time stamp. It applies the necessary
 * fudges (fudge1 for PPS, fudge2 for receive time) and then tries to
 * move the receive time stamp to the corresponding edge. This can warp
 * into future, if a transmission delay of more than 500ms is not
 * compensated with a corresponding fudge time2 value, because then the
 * next PPS edge is nearer than the last. (Similiar to what the PPS ATOM
 * driver does, but we deal with full time stamps here, not just phase
 * shift information.) Likewise, a negative fudge time2 value must be
 * used if the reference time stamp correlates with the *following* PPS
 * pulse.
 *
 * Note that the receive time fudge value only needs to move the receive
 * stamp near a PPS edge but that close proximity is not required;
 * +/-100ms precision should be enough. But since the fudge value will
 * probably also be used to compensate the transmission delay when no
 * PPS edge can be related to the time stamp, it's best to get it as
 * close as possible.
 *
 * It should also be noted that the typical use case is matching to the
 * preceeding edge, as most units relate their sentences to the current
 * second.
 *
 * The function returns PPS_RELATE_NONE (0) if no PPS edge correlation
 * can be fixed; PPS_RELATE_EDGE (1) when a PPS edge could be fixed, but
 * the distance to the reference time stamp is too big (exceeds
 * +/-400ms) and the ATOM driver PLL cannot be used to fix the phase;
 * and PPS_RELATE_PHASE (2) when the ATOM driver PLL code can be used.
 *
 * On output, the receive time stamp is replaced with the corresponding
 * PPS edge time if a fix could be made; the PPS fudge is updated to
 * reflect the proper fudge time to apply. (This implies that
 * 'refclock_process_offset()' must be used!)
 * -------------------------------------------------------------------
 */
#define PPS_RELATE_NONE  0  /* no pps correlation possible    */
#define PPS_RELATE_EDGE  1  /* recv time fixed, no phase lock */
#define PPS_RELATE_PHASE 2  /* recv time fixed, phase lock ok */

static int
refclock_ppsrelate(
  const struct refclockproc  * pp     , /* for sanity   */
  const struct refclock_atom * ap     , /* for PPS io   */
  const l_fp       * reftime ,
  l_fp         * rd_stamp,  /* i/o read stamp */
  double           pp_fudge,  /* pps fudge    */
  double         * rd_fudge /* i/o read fudge */
  )
{
  pps_info_t  pps_info;
  struct timespec timeout;
  l_fp    pp_stamp, pp_delta;
  double    delta, idelta;

  if (pp->leap == LEAP_NOTINSYNC)
    return PPS_RELATE_NONE; /* clock is insane, no chance */

  ZERO(timeout);
  ZERO(pps_info);
  if (time_pps_fetch(ap->handle, PPS_TSFMT_TSPEC,
         &pps_info, &timeout) < 0)
    return PPS_RELATE_NONE; /* can't get time stamps */

  /* get last active PPS edge before receive */
  if (ap->pps_params.mode & PPS_CAPTUREASSERT)
    timeout = pps_info.assert_timestamp;
  else if (ap->pps_params.mode & PPS_CAPTURECLEAR)
    timeout = pps_info.clear_timestamp;
  else
    return PPS_RELATE_NONE; /* WHICH edge, please?!? */

  /* get delta between receive time and PPS time */
  pp_stamp = tspec_stamp_to_lfp(timeout);
  pp_delta = *rd_stamp;
  L_SUB(&pp_delta, &pp_stamp);
  LFPTOD(&pp_delta, delta);
  delta += pp_fudge - *rd_fudge;
  if (fabs(delta) > 1.5)
    return PPS_RELATE_NONE; /* PPS timeout control */
  
  /* eventually warp edges, check phase */
  idelta    = floor(delta + 0.5);
  pp_fudge -= idelta;
  delta  -= idelta;
  if (fabs(delta) > 0.45)
    return PPS_RELATE_NONE; /* dead band control */

  /* we actually have a PPS edge to relate with! */
  *rd_stamp = pp_stamp;
  *rd_fudge = pp_fudge;

  /* if whole system out-of-sync, do not try to PLL */
  if (sys_leap == LEAP_NOTINSYNC)
    return PPS_RELATE_EDGE; /* cannot PLL with atom code */

  /* check against reftime if ATOM PLL can be used */
  pp_delta = *reftime;
  L_SUB(&pp_delta, &pp_stamp);
  LFPTOD(&pp_delta, delta);
  delta += pp_fudge;
  if (fabs(delta) > 0.45)
    return PPS_RELATE_EDGE; /* cannot PLL with atom code */

  /* all checks passed, gets an AAA rating here! */
  return PPS_RELATE_PHASE; /* can PLL with atom code */
}
#endif  /* HAVE_PPSAPI */

/*
 * -------------------------------------------------------------------
 * nmea_receive - receive data from the serial interface
 *
 * This is the workhorse for NMEA data evaluation:
 *
 * + it checks all NMEA data, and rejects sentences that are not valid
 *   NMEA sentences
 * + it checks whether a sentence is known and to be used
 * + it parses the time and date data from the NMEA data string and
 *   augments the missing bits. (century in dat, whole date, ...)
 * + it rejects data that is not from the first accepted sentence in a
 *   burst
 * + it eventually replaces the receive time with the PPS edge time.
 * + it feeds the data to the internal processing stages.
 * -------------------------------------------------------------------
 */
static void
nmea_receive(
  struct recvbuf * rbufp
  )
{
  /* declare & init control structure ptrs */
  struct peer     * const peer = rbufp->recv_peer;
  struct refclockproc * const pp = peer->procptr;
  nmea_unit     * const up = (nmea_unit*)pp->unitptr;

  /* Use these variables to hold data until we decide its worth keeping */
  nmea_data rdata;
  char    rd_lastcode[BMAX];
  l_fp    rd_timestamp, rd_reftime;
  int   rd_lencode;
  double    rd_fudge;

  /* working stuff */
  struct calendar date; /* to keep & convert the time stamp */
  struct timespec tofs; /* offset to full-second reftime */
  gps_weektm      gpsw; /* week time storage */
  /* results of sentence/date/time parsing */
  u_char    sentence; /* sentence tag */
  int   checkres;
  char *    cp;
  int   rc_date;
  int   rc_time;

  /* make sure data has defined pristine state */
  ZERO(tofs);
  ZERO(date);
  ZERO(gpsw);

  /* 
   * Read the timecode and timestamp, then initialise field
   * processing. The <CR><LF> at the NMEA line end is translated
   * to <LF><LF> by the terminal input routines on most systems,
   * and this gives us one spurious empty read per record which we
   * better ignore silently.
   */
  rd_lencode = refclock_gtlin(rbufp, rd_lastcode,
            sizeof(rd_lastcode), &rd_timestamp);
  checkres = field_init(&rdata, rd_lastcode, rd_lencode);
  switch (checkres) {

  case CHECK_INVALID:
    DPRINTF(1, ("%s invalid data: '%s'\n",
      refnumtoa(&peer->srcadr), rd_lastcode));
    refclock_report(peer, CEVNT_BADREPLY);
    return;

  case CHECK_EMPTY:
    return;

  default:
    DPRINTF(1, ("%s gpsread: %d '%s'\n",
      refnumtoa(&peer->srcadr), rd_lencode,
      rd_lastcode));
    break;
  }
  up->tally.total++;

  /* 
   * --> below this point we have a valid NMEA sentence <--
   *
   * Check sentence name. Skip first 2 chars (talker ID) in most
   * cases, to allow for $GLGGA and $GPGGA etc. Since the name
   * field has at least 5 chars we can simply shift the field
   * start.
   */
  cp = field_parse(&rdata, 0);
  if      (strncmp(cp + 2, "RMC,", 4) == 0)
    sentence = NMEA_GPRMC;
  else if (strncmp(cp + 2, "GGA,", 4) == 0)
    sentence = NMEA_GPGGA;
  else if (strncmp(cp + 2, "GLL,", 4) == 0)
    sentence = NMEA_GPGLL;
  else if (strncmp(cp + 2, "ZDA,", 4) == 0)
    sentence = NMEA_GPZDA;
  else if (strncmp(cp + 2, "ZDG,", 4) == 0)
    sentence = NMEA_GPZDG;
  else if (strncmp(cp,   "PGRMF,", 6) == 0) 
    sentence = NMEA_PGRMF;
  else
    return; /* not something we know about */

  /* Eventually output delay measurement now. */
  if (peer->ttl & NMEA_DELAYMEAS_MASK) {
    mprintf_clock_stats(&peer->srcadr, "delay %0.6f %.*s",
       ldexp(rd_timestamp.l_uf, -32),
       (int)(strchr(rd_lastcode, ',') - rd_lastcode),
       rd_lastcode);
  }
  
  /* See if I want to process this message type */
  if ((peer->ttl & NMEA_MESSAGE_MASK) &&
      !(peer->ttl & sentence_mode[sentence])) {
    up->tally.filtered++;
    return;
  }

  /* 
   * make sure it came in clean
   *
   * Apparently, older NMEA specifications (which are expensive)
   * did not require the checksum for all sentences.  $GPMRC is
   * the only one so far identified which has always been required
   * to include a checksum.
   *
   * Today, most NMEA GPS receivers checksum every sentence.  To
   * preserve its error-detection capabilities with modern GPSes
   * while allowing operation without checksums on all but $GPMRC,
   * we keep track of whether we've ever seen a valid checksum on
   * a given sentence, and if so, reject future instances without
   * checksum.  ('up->cksum_type[NMEA_GPRMC]' is set in
   * 'nmea_start()' to enforce checksums for $GPRMC right from the
   * start.)
   */
  if (up->cksum_type[sentence] <= (u_char)checkres) {
    up->cksum_type[sentence] = (u_char)checkres;
  } else {
    DPRINTF(1, ("%s checksum missing: '%s'\n",
      refnumtoa(&peer->srcadr), rd_lastcode));
    refclock_report(peer, CEVNT_BADREPLY);
    up->tally.malformed++;
    return;
  }

  /*
   * $GPZDG provides GPS time not UTC, and the two mix poorly.
   * Once have processed a $GPZDG, do not process any further UTC
   * sentences (all but $GPZDG currently).
   */ 
  if (up->gps_time && NMEA_GPZDG != sentence) {
    up->tally.filtered++;
    return;
  }

  DPRINTF(1, ("%s processing %d bytes, timecode '%s'\n",
    refnumtoa(&peer->srcadr), rd_lencode, rd_lastcode));

  /*
   * Grab fields depending on clock string type and possibly wipe
   * sensitive data from the last timecode.
   */
  switch (sentence) {

  case NMEA_GPRMC:
    /* Check quality byte, fetch data & time */
    rc_time  = parse_time(&date, &tofs.tv_nsec, &rdata, 1);
    pp->leap = parse_qual(&rdata, 2, 'A', 0);
    rc_date  = parse_date(&date, &rdata, 9, DATE_1_DDMMYY)
      && unfold_century(&date, rd_timestamp.l_ui);
    if (CLK_FLAG4 & pp->sloppyclockflag)
      field_wipe(&rdata, 3, 4, 5, 6, -1);
    break;

  case NMEA_GPGGA:
    /* Check quality byte, fetch time only */
    rc_time  = parse_time(&date, &tofs.tv_nsec, &rdata, 1);
    pp->leap = parse_qual(&rdata, 6, '0', 1);
    rc_date  = unfold_day(&date, rd_timestamp.l_ui);
    if (CLK_FLAG4 & pp->sloppyclockflag)
      field_wipe(&rdata, 2, 4, -1);
    break;

  case NMEA_GPGLL:
    /* Check quality byte, fetch time only */
    rc_time  = parse_time(&date, &tofs.tv_nsec, &rdata, 5);
    pp->leap = parse_qual(&rdata, 6, 'A', 0);
    rc_date  = unfold_day(&date, rd_timestamp.l_ui);
    if (CLK_FLAG4 & pp->sloppyclockflag)
      field_wipe(&rdata, 1, 3, -1);
    break;
  
  case NMEA_GPZDA:
    /* No quality.  Assume best, fetch time & full date */
    pp->leap = LEAP_NOWARNING;
    rc_time  = parse_time(&date, &tofs.tv_nsec, &rdata, 1);
    rc_date  = parse_date(&date, &rdata, 2, DATE_3_DDMMYYYY);
    break;

  case NMEA_GPZDG:
    /* Check quality byte, fetch time & full date */
    rc_time  = parse_time(&date, &tofs.tv_nsec, &rdata, 1);
    rc_date  = parse_date(&date, &rdata, 2, DATE_3_DDMMYYYY);
    pp->leap = parse_qual(&rdata, 4, '0', 1);
    tofs.tv_sec = -1; /* GPZDG is following second */
    break;

  case NMEA_PGRMF:
    /* get date, time, qualifier and GPS weektime. We need
     * date and time-of-day for the century fix, so we read
     * them first.
     */
    rc_date  = parse_weekdata(&gpsw, &rdata, 1, 2, 5)
            && parse_date(&date, &rdata, 3, DATE_1_DDMMYY);
    rc_time  = parse_time(&date, &tofs.tv_nsec, &rdata, 4);
    pp->leap = parse_qual(&rdata, 11, '0', 1);    
    rc_date  = rc_date
            && gpsfix_century(&date, &gpsw, &up->century_cache);
    if (CLK_FLAG4 & pp->sloppyclockflag)
      field_wipe(&rdata, 6, 8, -1);
    break;
    
  default:
    INVARIANT(0);  /* Coverity 97123 */
    return;
  }

  /* Check sanity of time-of-day. */
  if (rc_time == 0) { /* no time or conversion error? */
    checkres = CEVNT_BADTIME;
    up->tally.malformed++;
  }
  /* Check sanity of date. */
  else if (rc_date == 0) {/* no date or conversion error? */
    checkres = CEVNT_BADDATE;
    up->tally.malformed++;
  }
  /* check clock sanity; [bug 2143] */
  else if (pp->leap == LEAP_NOTINSYNC) { /* no good status? */
    checkres = CEVNT_BADREPLY;
    up->tally.rejected++;
  }
  else
    checkres = -1;

  if (checkres != -1) {
    save_ltc(pp, rd_lastcode, rd_lencode);
    refclock_report(peer, checkres);
    return;
  }

  DPRINTF(1, ("%s effective timecode: %04u-%02u-%02u %02d:%02d:%02d\n",
    refnumtoa(&peer->srcadr),
    date.year, date.month, date.monthday,
    date.hour, date.minute, date.second));

  /* Check if we must enter GPS time mode; log so if we do */
  if (!up->gps_time && (sentence == NMEA_GPZDG)) {
    msyslog(LOG_INFO, "%s using GPS time as if it were UTC",
      refnumtoa(&peer->srcadr));
    up->gps_time = 1;
  }
  
  /*
   * Get the reference time stamp from the calendar buffer.
   * Process the new sample in the median filter and determine the
   * timecode timestamp, but only if the PPS is not in control.
   * Discard sentence if reference time did not change.
   */
  rd_reftime = eval_gps_time(peer, &date, &tofs, &rd_timestamp);
  if (L_ISEQU(&up->last_reftime, &rd_reftime)) {
    /* Do not touch pp->a_lastcode on purpose! */
    up->tally.filtered++;
    return;
  }
  up->last_reftime = rd_reftime;
  rd_fudge = pp->fudgetime2;

  DPRINTF(1, ("%s using '%s'\n",
        refnumtoa(&peer->srcadr), rd_lastcode));

  /* Data will be accepted. Update stats & log data. */
  up->tally.accepted++;
  save_ltc(pp, rd_lastcode, rd_lencode);
  pp->lastrec = rd_timestamp;

#ifdef HAVE_PPSAPI
  /*
   * If we have PPS running, we try to associate the sentence
   * with the last active edge of the PPS signal.
   */
  if (up->ppsapi_lit)
    switch (refclock_ppsrelate(
        pp, &up->atom, &rd_reftime, &rd_timestamp,
        pp->fudgetime1, &rd_fudge))
    {
    case PPS_RELATE_PHASE:
      up->ppsapi_gate = TRUE;
      peer->precision = PPS_PRECISION;
      peer->flags |= FLAG_PPS;
      DPRINTF(2, ("%s PPS_RELATE_PHASE\n",
            refnumtoa(&peer->srcadr)));
      up->tally.pps_used++;
      break;
      
    case PPS_RELATE_EDGE:
      up->ppsapi_gate = TRUE;
      peer->precision = PPS_PRECISION;
      DPRINTF(2, ("%s PPS_RELATE_EDGE\n",
            refnumtoa(&peer->srcadr)));
      break;
      
    case PPS_RELATE_NONE:
    default:
      /*
       * Resetting precision and PPS flag is done in
       * 'nmea_poll', since it might be a glitch. But
       * at the end of the poll cycle we know...
       */
      DPRINTF(2, ("%s PPS_RELATE_NONE\n",
            refnumtoa(&peer->srcadr)));
      break;
    }
#endif /* HAVE_PPSAPI */

  refclock_process_offset(pp, rd_reftime, rd_timestamp, rd_fudge);
}


/*
 * -------------------------------------------------------------------
 * nmea_poll - called by the transmit procedure
 *
 * Does the necessary bookkeeping stuff to keep the reported state of
 * the clock in sync with reality.
 *
 * We go to great pains to avoid changing state here, since there may
 * be more than one eavesdropper receiving the same timecode.
 * -------------------------------------------------------------------
 */
static void
nmea_poll(
  int           unit,
  struct peer * peer
  )
{
  struct refclockproc * const pp = peer->procptr;
  nmea_unit     * const up = (nmea_unit *)pp->unitptr;
  
  /*
   * Process median filter samples. If none received, declare a
   * timeout and keep going.
   */
#ifdef HAVE_PPSAPI
  /*
   * If we don't have PPS pulses and time stamps, turn PPS down
   * for now.
   */
  if (!up->ppsapi_gate) {
    peer->flags &= ~FLAG_PPS;
    peer->precision = PRECISION;
  } else {
    up->ppsapi_gate = FALSE;
  }
#endif /* HAVE_PPSAPI */

  /*
   * If the median filter is empty, claim a timeout. Else process
   * the input data and keep the stats going.
   */
  if (pp->coderecv == pp->codeproc) {
    refclock_report(peer, CEVNT_TIMEOUT);
  } else {
    pp->polls++;
    pp->lastref = pp->lastrec;
    refclock_receive(peer);
  }
  
  /*
   * If extended logging is required, write the tally stats to the
   * clockstats file; otherwise just do a normal clock stats
   * record. Clear the tally stats anyway.
  */
  if (peer->ttl & NMEA_EXTLOG_MASK) {
    /* Log & reset counters with extended logging */
    const char *nmea = pp->a_lastcode;
    if (*nmea == '\0') nmea = "(none)";
    mprintf_clock_stats(
      &peer->srcadr, "%s  %u %u %u %u %u %u",
      nmea,
      up->tally.total, up->tally.accepted,
      up->tally.rejected, up->tally.malformed,
      up->tally.filtered, up->tally.pps_used);
  } else {
    record_clock_stats(&peer->srcadr, pp->a_lastcode);
  }
  ZERO(up->tally);
}

/*
 * -------------------------------------------------------------------
 * Save the last timecode string, making sure it's properly truncated
 * if necessary and NUL terminated in any case.
 */
static void
save_ltc(
  struct refclockproc * const pp,
  const char * const          tc,
  size_t                      len
  )
{
  if (len >= sizeof(pp->a_lastcode))
    len = sizeof(pp->a_lastcode) - 1;
  pp->lencode = (u_short)len;
  memcpy(pp->a_lastcode, tc, len);
  pp->a_lastcode[len] = '\0';
}


#if NMEA_WRITE_SUPPORT
/*
 * -------------------------------------------------------------------
 *  gps_send(fd, cmd, peer) Sends a command to the GPS receiver.
 *   as in gps_send(fd, "rqts,u", peer);
 *
 * If 'cmd' starts with a '$' it is assumed that this command is in raw
 * format, that is, starts with '$', ends with '<cr><lf>' and that any
 * checksum is correctly provided; the command will be send 'as is' in
 * that case. Otherwise the function will create the necessary frame
 * (start char, chksum, final CRLF) on the fly.
 *
 * We don't currently send any data, but would like to send RTCM SC104
 * messages for differential positioning. It should also give us better
 * time. Without a PPS output, we're Just fooling ourselves because of
 * the serial code paths
 * -------------------------------------------------------------------
 */
static void
gps_send(
  int           fd,
  const char  * cmd,
  struct peer * peer
  )
{
  /* $...*xy<CR><LF><NUL> add 7 */
  char        buf[NMEA_PROTO_MAXLEN + 7];
  int       len;
  u_char        dcs;
  const u_char *beg, *end;

  if (*cmd != '$') {
    /* get checksum and length */
    beg = end = (const u_char*)cmd;
    dcs = 0;
    while (*end >= ' ' && *end != '*')
      dcs ^= *end++;
    len = end - beg;
    /* format into output buffer with overflow check */
    len = snprintf(buf, sizeof(buf), "$%.*s*%02X\r\n",
             len, beg, dcs);
    if ((size_t)len >= sizeof(buf)) {
      DPRINTF(1, ("%s gps_send: buffer overflow for command '%s'\n",
            refnumtoa(&peer->srcadr), cmd));
      return; /* game over player 1 */
    }
    cmd = buf;
  } else {
    len = strlen(cmd);
  }

  DPRINTF(1, ("%s gps_send: '%.*s'\n", refnumtoa(&peer->srcadr),
    len - 2, cmd));

  /* send out the whole stuff */
  if (write(fd, cmd, len) == -1)
    refclock_report(peer, CEVNT_FAULT);
}
#endif /* NMEA_WRITE_SUPPORT */

/*
 * -------------------------------------------------------------------
 * helpers for faster field splitting
 * -------------------------------------------------------------------
 *
 * set up a field record, check syntax and verify checksum
 *
 * format is $XXXXX,1,2,3,4*ML
 *
 * 8-bit XOR of characters between $ and * noninclusive is transmitted
 * in last two chars M and L holding most and least significant nibbles
 * in hex representation such as:
 *
 *   $GPGLL,5057.970,N,00146.110,E,142451,A*27
 *   $GPVTG,089.0,T,,,15.2,N,,*7F
 *
 * Some other constraints:
 * + The field name must at least 5 upcase characters or digits and must
 *   start with a character.
 * + The checksum (if present) must be uppercase hex digits.
 * + The length of a sentence is limited to 80 characters (not including
 *   the final CR/LF nor the checksum, but including the leading '$')
 *
 * Return values:
 *  + CHECK_INVALID
 *  The data does not form a valid NMEA sentence or a checksum error
 *  occurred.
 *  + CHECK_VALID
 *  The data is a valid NMEA sentence but contains no checksum.
 *  + CHECK_CSVALID
 *  The data is a valid NMEA sentence and passed the checksum test.
 * -------------------------------------------------------------------
 */
static int
field_init(
  nmea_data * data, /* context structure           */
  char    * cptr, /* start of raw data           */
  int     dlen  /* data len, not counting trailing NUL */
  )
{
  u_char cs_l;  /* checksum local computed  */
  u_char cs_r;  /* checksum remote given  */
  char * eptr;  /* buffer end end pointer */
  char   tmp; /* char buffer      */
  
  cs_l = 0;
  cs_r = 0;
  /* some basic input constraints */
  if (dlen < 0)
    dlen = 0;
  eptr = cptr + dlen;
  *eptr = '\0';
  
  /* load data context */  
  data->base = cptr;
  data->cptr = cptr;
  data->cidx = 0;
  data->blen = dlen;

  /* syntax check follows here. check allowed character
   * sequences, updating the local computed checksum as we go.
   *
   * regex equiv: '^\$[A-Z][A-Z0-9]{4,}[^*]*(\*[0-9A-F]{2})?$'
   */

  /* -*- start character: '^\$' */
  if (*cptr == '\0')
    return CHECK_EMPTY;
  if (*cptr++ != '$')
    return CHECK_INVALID;

  /* -*- advance context beyond start character */
  data->base++;
  data->cptr++;
  data->blen--;
  
  /* -*- field name: '[A-Z][A-Z0-9]{4,},' */
  if (*cptr < 'A' || *cptr > 'Z')
    return CHECK_INVALID;
  cs_l ^= *cptr++;
  while ((*cptr >= 'A' && *cptr <= 'Z') ||
         (*cptr >= '0' && *cptr <= '9')  )
    cs_l ^= *cptr++;
  if (*cptr != ',' || (cptr - data->base) < NMEA_PROTO_IDLEN)
    return CHECK_INVALID;
  cs_l ^= *cptr++;

  /* -*- data: '[^*]*' */
  while (*cptr && *cptr != '*')
    cs_l ^= *cptr++;
  
  /* -*- checksum field: (\*[0-9A-F]{2})?$ */
  if (*cptr == '\0')
    return CHECK_VALID;
  if (*cptr != '*' || cptr != eptr - 3 ||
      (cptr - data->base) >= NMEA_PROTO_MAXLEN)
    return CHECK_INVALID;

  for (cptr++; (tmp = *cptr) != '\0'; cptr++) {
    if (tmp >= '0' && tmp <= '9')
      cs_r = (cs_r << 4) + (tmp - '0');
    else if (tmp >= 'A' && tmp <= 'F')
      cs_r = (cs_r << 4) + (tmp - 'A' + 10);
    else
      break;
  }

  /* -*- make sure we are at end of string and csum matches */
  if (cptr != eptr || cs_l != cs_r)
    return CHECK_INVALID;

  return CHECK_CSVALID;
}

/*
 * -------------------------------------------------------------------
 * fetch a data field by index, zero being the name field. If this
 * function is called repeatedly with increasing indices, the total load
 * is O(n), n being the length of the string; if it is called with
 * decreasing indices, the total load is O(n^2). Try not to go backwards
 * too often.
 * -------------------------------------------------------------------
 */
static char *
field_parse(
  nmea_data * data,
  int       fn
  )
{
  char tmp;

  if (fn < data->cidx) {
    data->cidx = 0;
    data->cptr = data->base;
  }
  while ((fn > data->cidx) && (tmp = *data->cptr) != '\0') {
    data->cidx += (tmp == ',');
    data->cptr++;
  }
  return data->cptr;
}

/*
 * -------------------------------------------------------------------
 * Wipe (that is, overwrite with '_') data fields and the checksum in
 * the last timecode.  The list of field indices is given as integers
 * in a varargs list, preferrably in ascending order, in any case
 * terminated by a negative field index.
 *
 * A maximum number of 8 fields can be overwritten at once to guard
 * against runaway (that is, unterminated) argument lists.
 *
 * This function affects what a remote user can see with
 *
 * ntpq -c clockvar <server>
 *
 * Note that this also removes the wiped fields from any clockstats
 * log.  Some NTP operators monitor their NMEA GPS using the change in
 * location in clockstats over time as as a proxy for the quality of
 * GPS reception and thereby time reported.
 * -------------------------------------------------------------------
 */
static void
field_wipe(
  nmea_data * data,
  ...
  )
{
  va_list va;   /* vararg index list */
  int fcnt;   /* safeguard against runaway arglist */
  int fidx;   /* field to nuke, or -1 for checksum */
  char  * cp;   /* overwrite destination */
  
  fcnt = 8;
  cp = NULL;
  va_start(va, data);
  do {
    fidx = va_arg(va, int);
    if (fidx >= 0 && fidx <= NMEA_PROTO_FIELDS) {
      cp = field_parse(data, fidx);
    } else {
      cp = data->base + data->blen;
      if (data->blen >= 3 && cp[-3] == '*')
        cp -= 2;
    }
    for ( ; '\0' != *cp && '*' != *cp && ',' != *cp; cp++)
      if ('.' != *cp)
        *cp = '_';
  } while (fcnt-- && fidx >= 0);
  va_end(va); 
}

/*
 * -------------------------------------------------------------------
 * PARSING HELPERS
 * -------------------------------------------------------------------
 *
 * Check sync status
 *
 * If the character at the data field start matches the tag value,
 * return LEAP_NOWARNING and LEAP_NOTINSYNC otherwise. If the 'inverted'
 * flag is given, just the opposite value is returned. If there is no
 * data field (*cp points to the NUL byte) the result is LEAP_NOTINSYNC.
 * -------------------------------------------------------------------
 */
static u_char
parse_qual(
  nmea_data * rd,
  int         idx,
  char        tag,
  int         inv
  )
{
  static const u_char table[2] =
        { LEAP_NOTINSYNC, LEAP_NOWARNING };
  char * dp;

  dp = field_parse(rd, idx);
  
  return table[ *dp && ((*dp == tag) == !inv) ];
}

/*
 * -------------------------------------------------------------------
 * Parse a time stamp in HHMMSS[.sss] format with error checking.
 *
 * returns 1 on success, 0 on failure
 * -------------------------------------------------------------------
 */
static int
parse_time(
  struct calendar * jd, /* result calendar pointer */
  long    * ns, /* storage for nsec fraction */
  nmea_data       * rd,
  int     idx
  )
{
  static const unsigned long weight[4] = {
    0, 100000000, 10000000, 1000000
  };

  int rc;
  u_int h;
  u_int m;
  u_int s;
  int p1;
  int p2;
  u_long  f;
  char  * dp;

  dp = field_parse(rd, idx);
  rc = sscanf(dp, "%2u%2u%2u%n.%3lu%n", &h, &m, &s, &p1, &f, &p2);
  if (rc < 3 || p1 != 6) {
    DPRINTF(1, ("nmea: invalid time code: '%.6s'\n", dp));
    return FALSE;
  }
  
  /* value sanity check */
  if (h > 23 || m > 59 || s > 60) {
    DPRINTF(1, ("nmea: invalid time spec %02u:%02u:%02u\n",
          h, m, s));
    return FALSE;
  }

  jd->hour   = (u_char)h;
  jd->minute = (u_char)m;
  jd->second = (u_char)s;
  /* if we have a fraction, scale it up to nanoseconds. */
  if (rc == 4)
    *ns = f * weight[p2 - p1 - 1];
  else
    *ns = 0;

  return TRUE;
}

/*
 * -------------------------------------------------------------------
 * Parse a date string from an NMEA sentence. This could either be a
 * partial date in DDMMYY format in one field, or DD,MM,YYYY full date
 * spec spanning three fields. This function does some extensive error
 * checking to make sure the date string was consistent.
 *
 * returns 1 on success, 0 on failure
 * -------------------------------------------------------------------
 */
static int
parse_date(
  struct calendar * jd, /* result pointer */
  nmea_data       * rd,
  int     idx,
  enum date_fmt   fmt
  )
{
  int rc;
  u_int y;
  u_int m;
  u_int d;
  int p;
  char  * dp;
  
  dp = field_parse(rd, idx);
  switch (fmt) {

  case DATE_1_DDMMYY:
    rc = sscanf(dp, "%2u%2u%2u%n", &d, &m, &y, &p);
    if (rc != 3 || p != 6) {
      DPRINTF(1, ("nmea: invalid date code: '%.6s'\n",
            dp));
      return FALSE;
    }
    break;

  case DATE_3_DDMMYYYY:
    rc = sscanf(dp, "%2u,%2u,%4u%n", &d, &m, &y, &p);
    if (rc != 3 || p != 10) {
      DPRINTF(1, ("nmea: invalid date code: '%.10s'\n",
            dp));
      return FALSE;
    }
    break;

  default:
    DPRINTF(1, ("nmea: invalid parse format: %d\n", fmt));
    return FALSE;
  }

  /* value sanity check */
  if (d < 1 || d > 31 || m < 1 || m > 12) {
    DPRINTF(1, ("nmea: invalid date spec (YMD) %04u:%02u:%02u\n",
          y, m, d));
    return FALSE;
  }
  
  /* store results */
  jd->monthday = (u_char)d;
  jd->month    = (u_char)m;
  jd->year     = (u_short)y;

  return TRUE;
}

/*
 * -------------------------------------------------------------------
 * Parse GPS week time info from an NMEA sentence. This info contains
 * the GPS week number, the GPS time-of-week and the leap seconds GPS
 * to UTC.
 *
 * returns 1 on success, 0 on failure
 * -------------------------------------------------------------------
 */
static int
parse_weekdata(
  gps_weektm * wd,
  nmea_data  * rd,
  int          weekidx,
  int          timeidx,
  int          leapidx
  )
{
  u_long secs;
  int    fcnt;

  /* parse fields and count success */
  fcnt  = sscanf(field_parse(rd, weekidx), "%hu", &wd->wt_week);
  fcnt += sscanf(field_parse(rd, timeidx), "%lu", &secs);
  fcnt += sscanf(field_parse(rd, leapidx), "%hd", &wd->wt_leap);
  if (fcnt != 3 || wd->wt_week >= 1024 || secs >= 7*SECSPERDAY) {
    DPRINTF(1, ("nmea: parse_weekdata: invalid weektime spec\n"));
    return FALSE;
  }
  wd->wt_time = (u_int32)secs;

  return TRUE;
}

/*
 * -------------------------------------------------------------------
 * funny calendar-oriented stuff -- perhaps a bit hard to grok.
 * -------------------------------------------------------------------
 *
 * Unfold a time-of-day (seconds since midnight) around the current
 * system time in a manner that guarantees an absolute difference of
 * less than 12hrs.
 *
 * This function is used for NMEA sentences that contain no date
 * information. This requires the system clock to be in +/-12hrs
 * around the true time, or the clock will synchronize the system 1day
 * off if not augmented with a time sources that also provide the
 * necessary date information.
 *
 * The function updates the calendar structure it also uses as
 * input to fetch the time from.
 *
 * returns 1 on success, 0 on failure
 * -------------------------------------------------------------------
 */
static int
unfold_day(
  struct calendar * jd,
  u_int32     rec_ui
  )
{
  vint64       rec_qw;
  ntpcal_split rec_ds;

  /*
   * basically this is the peridiodic extension of the receive
   * time - 12hrs to the time-of-day with a period of 1 day.
   * But we would have to execute this in 64bit arithmetic, and we
   * cannot assume we can do this; therefore this is done
   * in split representation.
   */
  rec_qw = ntpcal_ntp_to_ntp(rec_ui - SECSPERDAY/2, NULL);
  rec_ds = ntpcal_daysplit(&rec_qw);
  rec_ds.lo = ntpcal_periodic_extend(rec_ds.lo,
             ntpcal_date_to_daysec(jd),
             SECSPERDAY);
  rec_ds.hi += ntpcal_daysec_to_date(jd, rec_ds.lo);
  return (ntpcal_rd_to_date(jd, rec_ds.hi + DAY_NTP_STARTS) >= 0);
}

/*
 * -------------------------------------------------------------------
 * A 2-digit year is expanded into full year spec around the year found
 * in 'jd->year'. This should be in +79/-19 years around the system time,
 * or the result will be off by 100 years.  The assymetric behaviour was
 * chosen to enable inital sync for systems that do not have a
 * battery-backup clock and start with a date that is typically years in
 * the past.
 *
 * Since the GPS epoch starts at 1980-01-06, the resulting year will be
 * not be before 1980 in any case.
 *
 * returns 1 on success, 0 on failure
 * -------------------------------------------------------------------
 */
static int
unfold_century(
  struct calendar * jd,
  u_int32     rec_ui
  )
{
  struct calendar rec;
  int32   baseyear;

  ntpcal_ntp_to_date(&rec, rec_ui, NULL);
  baseyear = rec.year - 20;
  if (baseyear < g_gpsMinYear)
    baseyear = g_gpsMinYear;
  jd->year = (u_short)ntpcal_periodic_extend(baseyear, jd->year,
               100);

  return ((baseyear <= jd->year) && (baseyear + 100 > jd->year));
}

/*
 * -------------------------------------------------------------------
 * A 2-digit year is expanded into a full year spec by correlation with
 * a GPS week number and the current leap second count.
 *
 * The GPS week time scale counts weeks since Sunday, 1980-01-06, modulo
 * 1024 and seconds since start of the week. The GPS time scale is based
 * on international atomic time (TAI), so the leap second difference to
 * UTC is also needed for a proper conversion.
 *
 * A brute-force analysis (that is, test for every date) shows that a
 * wrong assignment of the century can not happen between the years 1900
 * to 2399 when comparing the week signatures for different
 * centuries. (I *think* that will not happen for 400*1024 years, but I
 * have no valid proof. -*-perlinger@ntp.org-*-)
 *
 * This function is bound to to work between years 1980 and 2399
 * (inclusive), which should suffice for now ;-)
 *
 * Note: This function needs a full date&time spec on input due to the
 * necessary leap second corrections!
 *
 * returns 1 on success, 0 on failure
 * -------------------------------------------------------------------
 */
static int
gpsfix_century(
  struct calendar  * jd,
  const gps_weektm * wd,
  u_short          * century
  ) 
{
  int32 days;
  int32 doff;
  u_short week;
  u_short year;
  int     loop;

  /* Get day offset. Assumes that the input time is in range and
   * that the leap seconds do not shift more than +/-1 day.
   */
  doff = ntpcal_date_to_daysec(jd) + wd->wt_leap;
  doff = (doff >= SECSPERDAY) - (doff < 0);

  /*
   * Loop over centuries to get a match, starting with the last
   * successful one. (Or with the 19th century if the cached value
   * is out of range...)
   */
  year = jd->year % 100;
  for (loop = 5; loop > 0; loop--,(*century)++) {
    if (*century < 19 || *century >= 24)
      *century = 19;
    /* Get days and week in GPS epoch */
    jd->year = year + *century * 100;
    days = ntpcal_date_to_rd(jd) - DAY_GPS_STARTS + doff;
    week = (days / 7) % 1024;
    if (days >= 0 && wd->wt_week == week)
      return TRUE; /* matched... */
  }

  jd->year = year;
  return FALSE; /* match failed... */
}

/*
 * -------------------------------------------------------------------
 * And now the final execise: Considering the fact that many (most?)
 * GPS receivers cannot handle a GPS epoch wrap well, we try to
 * compensate for that problem by unwrapping a GPS epoch around the
 * receive stamp. Another execise in periodic unfolding, of course,
 * but with enough points to take care of.
 *
 * Note: The integral part of 'tofs' is intended to handle small(!)
 * systematic offsets, as -1 for handling $GPZDG, which gives the
 * following second. (sigh...) The absolute value shall be less than a
 * day (86400 seconds).
 * -------------------------------------------------------------------
 */
static l_fp
eval_gps_time(
  struct peer           * peer, /* for logging etc */
  const struct calendar * gpst, /* GPS time stamp  */
  const struct timespec * tofs, /* GPS frac second & offset */
  const l_fp            * xrecv /* receive time stamp */
  )
{
  struct refclockproc * const pp = peer->procptr;
  nmea_unit     * const up = (nmea_unit *)pp->unitptr;

  l_fp    retv;

  /* components of calculation */
  int32_t rcv_sec, rcv_day; /* receive ToD and day */
  int32_t gps_sec, gps_day; /* GPS ToD and day in NTP epoch */
  int32_t adj_day, weeks;   /* adjusted GPS day and week shift */

  /* some temporaries to shuffle data */
  vint64       vi64;
  ntpcal_split rs64;

  /* evaluate time stamp from receiver. */
  gps_sec = ntpcal_date_to_daysec(gpst);
  gps_day = ntpcal_date_to_rd(gpst) - DAY_NTP_STARTS;

  /* merge in fractional offset */
  retv = tspec_intv_to_lfp(*tofs);
  gps_sec += retv.l_i;

  /* If we fully trust the GPS receiver, just combine days and
   * seconds and be done. */
  if (peer->ttl & NMEA_DATETRUST_MASK) {
    retv.l_ui = ntpcal_dayjoin(gps_day, gps_sec).D_s.lo;
    return retv;
  }

  /* So we do not trust the GPS receiver to deliver a correct date
   * due to the GPS epoch changes. We map the date from the
   * receiver into the +/-512 week interval around the receive
   * time in that case. This would be a tad easier with 64bit
   * calculations, but again, we restrict the code to 32bit ops
   * when possible. */

  /* - make sure the GPS fractional day is normalised
   * Applying the offset value might have put us slightly over the
   * edge of the allowed range for seconds-of-day. Doing a full
   * division with floor correction is overkill here; a simple
   * addition or subtraction step is sufficient. Using WHILE loops
   * gives the right result even if the offset exceeds one day,
   * which is NOT what it's intented for! */
  while (gps_sec >= SECSPERDAY) {
    gps_sec -= SECSPERDAY;
    gps_day += 1;
  }
  while (gps_sec < 0) {
    gps_sec += SECSPERDAY;
    gps_day -= 1;
  }

  /* - get unfold base: day of full recv time - 512 weeks */
  vi64 = ntpcal_ntp_to_ntp(xrecv->l_ui, NULL);
  rs64 = ntpcal_daysplit(&vi64);
  rcv_sec = rs64.lo;
  rcv_day = rs64.hi - 512 * 7;

  /* - take the fractional days into account
   * If the fractional day of the GPS time is smaller than the
   * fractional day of the receive time, we shift the base day for
   * the unfold by 1. */
  if (   gps_sec  < rcv_sec
     || (gps_sec == rcv_sec && retv.l_uf < xrecv->l_uf))
    rcv_day += 1;

  /* - don't warp ahead of GPS invention! */
  if (rcv_day < g_gpsMinBase)
    rcv_day = g_gpsMinBase;

  /* - let the magic happen: */
  adj_day = ntpcal_periodic_extend(rcv_day, gps_day, 1024*7);

  /* - check if we should log a GPS epoch warp */
  weeks = (adj_day - gps_day) / 7;
  if (weeks != up->epoch_warp) {
    up->epoch_warp = weeks;
    LOGIF(CLOCKINFO, (LOG_INFO,
          "%s Changed GPS epoch warp to %d weeks",
          refnumtoa(&peer->srcadr), weeks));
  }

  /* - build result and be done */
  retv.l_ui = ntpcal_dayjoin(adj_day, gps_sec).D_s.lo;
  return retv;
}

/*
 * ===================================================================
 *
 * NMEAD support
 *
 * original nmead support added by Jon Miner (cp_n18@yahoo.com)
 *
 * See http://home.hiwaay.net/~taylorc/gps/nmea-server/
 * for information about nmead
 *
 * To use this, you need to create a link from /dev/gpsX to
 * the server:port where nmead is running.  Something like this:
 *
 * ln -s server:port /dev/gps1
 *
 * Split into separate function by Juergen Perlinger
 * (perlinger-at-ntp-dot-org)
 *
 * ===================================================================
 */
static int
nmead_open(
  const char * device
  )
{
  int fd = -1;    /* result file descriptor */
  
#ifdef HAVE_READLINK
  char  host[80];   /* link target buffer */
  char  * port;     /* port name or number  */
  int rc;     /* result code (several)*/
  int     sh;     /* socket handle  */
  struct addrinfo  ai_hint; /* resolution hint  */
  struct addrinfo *ai_list; /* resolution result  */
  struct addrinfo *ai;    /* result scan ptr  */

  fd = -1;
  
  /* try to read as link, make sure no overflow occurs */
  rc = readlink(device, host, sizeof(host));
  if ((size_t)rc >= sizeof(host))
    return fd; /* error / overflow / truncation */
  host[rc] = '\0';  /* readlink does not place NUL  */

  /* get port */
  port = strchr(host, ':');
  if (!port)
    return fd; /* not 'host:port' syntax ? */
  *port++ = '\0'; /* put in separator */
  
  /* get address infos and try to open socket
   *
   * This getaddrinfo() is naughty in ntpd's nonblocking main
   * thread, but you have to go out of your wary to use this code
   * and typically the blocking is at startup where its impact is
   * reduced. The same holds for the 'connect()', as it is
   * blocking, too...
   */
  ZERO(ai_hint);
  ai_hint.ai_protocol = IPPROTO_TCP;
  ai_hint.ai_socktype = SOCK_STREAM;
  if (getaddrinfo(host, port, &ai_hint, &ai_list))
    return fd;
  
  for (ai = ai_list; ai && (fd == -1); ai = ai->ai_next) {
    sh = socket(ai->ai_family, ai->ai_socktype,
          ai->ai_protocol);
    if (INVALID_SOCKET == sh)
      continue;
    rc = connect(sh, ai->ai_addr, ai->ai_addrlen);
    if (-1 != rc)
      fd = sh;
    else
      close(sh);
  }
  freeaddrinfo(ai_list);
#else
  fd = -1;
#endif

  return fd;
}
#else
NONEMPTY_TRANSLATION_UNIT
#endif /* REFCLOCK && CLOCK_NMEA */

Coverage Report

Created: 2023-05-19 06:16