path: root/ppsthread.c

/*
 * ppsthread.c - manage PPS watcher threads
 *
 * To enable KPPS, this file needs to be compiled with -DHAVE_SYS_TIMEPPS_H
 *
 * If you are not good at threads do not touch this file!
 * For example: errno is thread safe; strerror() is not.
 *
 * It helps to know that there are two PPS measurement methods in
 * play.  One is defined by RFC2783 and typically implemented in the
 * kernel.  It is available on FreeBSD, Linux, and NetBSD.  In gpsd it
 * is referred to as KPPS.  KPPS is accessed on Linux via /dev/ppsN
 * devices.  On BSD it is accessed via the same device as the serial
 * port.  This mechanism is preferred as it should provide the smallest
 * latency and jitter from control line transition to timestamp.
 *
 * The other mechanism is user-space PPS, which uses the (not
 * standardized) TIOCMIWAIT ioctl to wait for PPS transitions on
 * serial port control lines.  It is implemented on Linux and OpenBSD.
 *
 * On Linux, RFC2783 PPS requires root permissions for initialization;
 * user-space PPS does not.  User-space PPS loses some functionality
 * when not initialized as root.  In gpsd, user-space PPS is referred
 * to as "plain PPS".
 *
 * On {Free,Net}BSD, RFC2783 PPS should only require access to the
 * serial port, but details have not yet been tested and documented
 * here.
 *
 * Note that for easy debugging all logging from this file is prefixed
 * with PPS or KPPS.
 *
 * To use the thread manager, you need to first fill in the
 * devicefd, devicename, and the four hook function members in the thread
 * context structure. The void *context member is available for your hook
 * functions to use; the thread-monitor code doesn't touch it.
 *
 * After this setup, you can call pps_thread_activate() and the
 * thread will launch.  It is OK to do this before the device is open,
 * the thread will wait on that.
 *
 * WARNING!  Loss of precision
 * UNIX time to nanoSec precision is 62 significant bits
 * UNIX time to nanoSec precision after 2038 is 63 bits
 * a double is only 53 significant bits.
 *
 * You cannot do PPS math with doubles
 *
 * This file is Copyright (c) 2013 by the GPSD project. BSD terms
 * apply: see the file COPYING in the distribution root for details.
 */

#include <string.h>
#include <stdio.h>
#include <stdbool.h>
#include <limits.h>
#include <errno.h>
#include <math.h>
#include <sys/socket.h>
#include <unistd.h>
#include <pthread.h>		/* pacifies OpenBSD's compiler */

/* use RFC 2783 PPS API */
/* this needs linux >= 2.6.34 and
 * CONFIG_PPS=y
 * CONFIG_PPS_DEBUG=y  [optional to kernel log pulses]
 * CONFIG_PPS_CLIENT_LDISC=y
 */
#if defined(HAVE_SYS_TIMEPPS_H)
// include unistd.h here as it is missing on older pps-tools releases.
// 'close' is not defined otherwise.
#include <unistd.h>
#include <sys/time.h>
#include <sys/timepps.h>
#endif

#include "timespec.h"
#include "ppsthread.h"

/*
 * Tell GCC that we want thread-safe behavior with _REENTRANT;
 * in particular, errno must be thread-local.
 * Tell POSIX-conforming implementations with _POSIX_THREAD_SAFE_FUNCTIONS.
 * See http://www.unix.org/whitepapers/reentrant.html
 */
#ifndef _REENTRANT
#define _REENTRANT
#endif
#ifndef _POSIX_THREAD_SAFE_FUNCTIONS
#define _POSIX_THREAD_SAFE_FUNCTIONS
#endif

/*
 * Warning: This is a potential portability problem.
 * It's needed so that TIOCMIWAIT will be defined and the plain PPS
 * code will work, but it's not a SuS/POSIX standard header.  We're
 * going to include it unconditionally here because we expect both
 * Linux and BSD to have it and we want compilation to break with
 * an audible snapping sound if it's not present.
 */
#include <sys/ioctl.h>

#if defined(HAVE_SYS_TIMEPPS_H)
#include <glob.h>
#include <fcntl.h>	/* needed for open() and friends */
#endif

#if defined(TIOCMIWAIT)
static int get_edge_tiocmiwait( volatile struct pps_thread_t *,
                         struct timespec *, int *,
                         volatile struct timedelta_t *);
#endif /* TIOCMIWAIT */

struct inner_context_t {
    volatile struct pps_thread_t	*pps_thread;
    bool pps_canwait;                   /* can RFC2783 wait? */
#if defined(HAVE_SYS_TIMEPPS_H)
    int pps_caps;                       /* RFC2783 getcaps() */
    pps_handle_t kernelpps_handle;
#endif /* defined(HAVE_SYS_TIMEPPS_H) */
};

#if defined(HAVE_SYS_TIMEPPS_H)
static int get_edge_rfc2783(struct inner_context_t *,
                         struct timespec *,
                         int *,
                         struct timespec *,
                         int *,
                         volatile struct timedelta_t *);
#endif  /* defined(HAVE_SYS_TIMEPPS_H) */

static pthread_mutex_t ppslast_mutex = PTHREAD_MUTEX_INITIALIZER;

static void thread_lock(volatile struct pps_thread_t *pps_thread)
{
    int pthread_err = pthread_mutex_lock(&ppslast_mutex);
    if ( 0 != pthread_err ) {
	char errbuf[BUFSIZ] = "unknown error";
	(void)strerror_r(errno, errbuf, sizeof(errbuf));
	pps_thread->log_hook(pps_thread, THREAD_ERROR,
		"PPS:%s pthread_mutex_lock() : %s\n",
	        pps_thread->devicename, errbuf);
    }
}

static void thread_unlock(volatile struct pps_thread_t *pps_thread)
{
    int pthread_err = pthread_mutex_unlock(&ppslast_mutex);
    if ( 0 != pthread_err ) {
	char errbuf[BUFSIZ] = "unknown error";
	(void)strerror_r(errno, errbuf, sizeof(errbuf));
	pps_thread->log_hook(pps_thread, THREAD_ERROR,
		    "TPPS:%s pthread_mutex_unlock() : %s\n",
		    pps_thread->devicename, errno, errbuf);
    }
}

#if defined(HAVE_SYS_TIMEPPS_H)
static int init_kernel_pps(struct inner_context_t *inner_context)
/* return handle for kernel pps, or -1; requires root privileges */
{
    pps_params_t pp;
    int ret;
#ifdef __linux__
    /* These variables are only needed by Linux to find /dev/ppsN. */
    int ldisc = 18;   /* the PPS line discipline */
    glob_t globbuf;
#endif
    char path[PATH_MAX] = "";
    volatile struct pps_thread_t *pps_thread = inner_context->pps_thread;

    inner_context->kernelpps_handle = -1;
    inner_context->pps_canwait = false;

    /*
     * This next code block abuses "ret" by storing the filedescriptor
     * to use for RFC2783 calls.
     */
#ifndef __clang_analyzer__
    ret = -1;  /* this ret will not be unneeded when the 'else' part
                * of the followinng ifdef becomes an #elif */
#endif /* __clang_analyzer__ */
#ifdef __linux__
    /*
     * Some Linuxes, like the RasbPi's, have PPS devices preexisting.
     * Other OS have no way to automatically determine the proper /dev/ppsX.
     * Allow user to pass in an explicit PPS device path.
     *
     * (We use strncpy() here because this might be compiled where
     * strlcpy() is not available.)
     */
    if (strncmp(pps_thread->devicename, "/dev/pps", 8) == 0)
	(void)strncpy(path, pps_thread->devicename, sizeof(path));
    else {
	char pps_num = '\0';  /* /dev/pps[pps_num] is our device */
	size_t i;             /* to match type of globbuf.gl_pathc */
	/*
	 * Otherwise one must make calls to associate a serial port with a
	 * /dev/ppsN device and then grovel in system data to determine
	 * the association.
	 */

	/* Attach the line PPS discipline, so no need to ldattach */
	/* This activates the magic /dev/pps0 device */
	/* Note: this ioctl() requires root, and device is a tty */
	if ( 0 > ioctl(pps_thread->devicefd, TIOCSETD, &ldisc)) {
	    char errbuf[BUFSIZ] = "unknown error";
	    strerror_r(errno, errbuf, sizeof(errbuf));
	    pps_thread->log_hook(pps_thread, THREAD_INF,
				 "KPPS:%s cannot set PPS line discipline %s\n",
				 pps_thread->devicename, errbuf);
	    return -1;
	}

	/* uh, oh, magic file names!, RFC2783 neglects to specify how
	 * to associate the serial device and pps device names */
	/* need to look in /sys/devices/virtual/pps/pps?/path
	 * (/sys/class/pps/pps?/path is just a link to that)
	 * to find the /dev/pps? that matches our serial port.
	 * this code fails if there are more then 10 pps devices.
	 *
	 * yes, this could be done with libsysfs, but trying to keep
	 * the number of required libs small, and libsysfs would still
	 * be linux only */
	memset( (void *)&globbuf, 0, sizeof(globbuf));
	(void)glob("/sys/devices/virtual/pps/pps?/path", 0, NULL, &globbuf);

	memset( (void *)&path, 0, sizeof(path));
	for ( i = 0; i < globbuf.gl_pathc; i++ ) {
	    int fd = open(globbuf.gl_pathv[i], O_RDONLY);
	    if ( 0 <= fd ) {
		ssize_t r = read( fd, path, sizeof(path) -1);
		if ( 0 < r ) {
		    path[r - 1] = '\0'; /* remove trailing \x0a */
		}
		(void)close(fd);
	    }
	    pps_thread->log_hook(pps_thread, THREAD_PROG,
				 "KPPS:%s checking %s, %s\n",
				 pps_thread->devicename,
				 globbuf.gl_pathv[i], path);
	    if ( 0 == strncmp( path, pps_thread->devicename, sizeof(path))) {
		/* this is the pps we are looking for */
		/* FIXME, now build the proper pps device path */
		pps_num = globbuf.gl_pathv[i][28];
		break;
	    }
	    memset( (void *)&path, 0, sizeof(path));
	}
	/* done with blob, clear it */
	globfree(&globbuf);

	if ( 0 == (int)pps_num ) {
	    pps_thread->log_hook(pps_thread, THREAD_INF,
				 "KPPS:%s device not found.\n",
				 pps_thread->devicename);
	    return -1;
	}
	/* construct the magic device path */
	(void)snprintf(path, sizeof(path), "/dev/pps%c", pps_num);
    }

    /* root privs are probably required for this device open
     * do not bother to check uid, just go for the open() */

    ret = open(path, O_RDWR);
    if ( 0 > ret ) {
	char errbuf[BUFSIZ] = "unknown error";
	(void)strerror_r(errno, errbuf, sizeof(errbuf));
	pps_thread->log_hook(pps_thread, THREAD_INF,
		    "KPPS:%s cannot open %s: %s\n",
		    pps_thread->devicename,
                    path, errbuf);
    	return -1;
    }
#else /* not __linux__ */
    /*
     * On BSDs that support RFC2783, one uses the API calls on serial
     * port file descriptor.
     *
     * FIXME! need more specific than 'not linux'
     */
    (void)strlcpy(path, pps_thread->devicename, sizeof(path));
    // cppcheck-suppress redundantAssignment
    ret  = pps_thread->devicefd;
#endif
    /* assert(ret >= 0); */
    pps_thread->log_hook(pps_thread, THREAD_INF,
		"KPPS:%s RFC2783 path:%s, fd is %d\n",
	        pps_thread->devicename, path,
		ret);

    /* RFC 2783 implies the time_pps_setcap() needs priviledges *
     * keep root a tad longer just in case */
    if ( 0 > time_pps_create(ret, (pps_handle_t *)&inner_context->kernelpps_handle )) {
	char errbuf[BUFSIZ] = "unknown error";
	(void)strerror_r(errno, errbuf, sizeof(errbuf));
	pps_thread->log_hook(pps_thread, THREAD_INF,
		    "KPPS:%s time_pps_create(%d) failed: %s\n",
	            pps_thread->devicename,
		    ret, errbuf);
    	return -1;
    }

    /* have kernel PPS handle */
    /* get RFC2783 features supported */
    inner_context->pps_caps = 0;
    if ( 0 > time_pps_getcap(inner_context->kernelpps_handle,
				&inner_context->pps_caps)) {
	char errbuf[BUFSIZ] = "unknown error";
	inner_context->pps_caps = 0;
	(void)strerror_r(errno, errbuf, sizeof(errbuf));
	pps_thread->log_hook(pps_thread, THREAD_INF,
		    "KPPS:%s time_pps_getcap() failed: %.100s\n",
		    pps_thread->devicename, errbuf);
    	return -1;
    } else {
	pps_thread->log_hook(pps_thread, THREAD_INF,
		    "KPPS:%s pps_caps 0x%02X\n",
		    pps_thread->devicename,
		    inner_context->pps_caps);
    }

    /* construct the setparms structure */
    memset( (void *)&pp, 0, sizeof(pps_params_t));
    pp.api_version = PPS_API_VERS_1;  /* version 1 protocol */
    if ( 0 == (PPS_TSFMT_TSPEC & inner_context->pps_caps ) ) {
       /* PPS_TSFMT_TSPEC means return a timespec
        * mandatory for driver to implement, require it */
	pps_thread->log_hook(pps_thread, THREAD_WARN,
		    "KPPS:%s fail, missing mandatory PPS_TSFMT_TSPEC\n",
		    pps_thread->devicename);
        return -1;
    }
    if ( 0 != (PPS_CANWAIT & inner_context->pps_caps ) ) {
       /* we can wait! so no need for TIOCMIWAIT */
       pps_thread->log_hook(pps_thread, THREAD_INF,
                   "KPPS:%s have PPS_CANWAIT\n",
                   pps_thread->devicename);
        inner_context->pps_canwait = true;
    }

    pp.mode = PPS_TSFMT_TSPEC;
    switch ( (PPS_CAPTUREASSERT | PPS_CAPTURECLEAR) & inner_context->pps_caps ) {
    case PPS_CAPTUREASSERT:
	pps_thread->log_hook(pps_thread, THREAD_WARN,
		    "KPPS:%s missing PPS_CAPTURECLEAR, pulse may be offset\n",
		    pps_thread->devicename);
	pp.mode |= PPS_CAPTUREASSERT;
        break;
    case PPS_CAPTURECLEAR:
	pps_thread->log_hook(pps_thread, THREAD_WARN,
		    "KPPS:%s missing PPS_CAPTUREASSERT, pulse may be offset\n",
		    pps_thread->devicename);
	pp.mode |= PPS_CAPTURECLEAR;
        break;
    case PPS_CAPTUREASSERT | PPS_CAPTURECLEAR:
	pp.mode |= PPS_CAPTUREASSERT | PPS_CAPTURECLEAR;
        break;
    default:
        /* THREAD_ERR in the calling routine */
	pps_thread->log_hook(pps_thread, THREAD_INF,
		    "KPPS:%s missing PPS_CAPTUREASSERT and CLEAR\n",
		    pps_thread->devicename);
        return -1;
    }

    if ( 0 > time_pps_setparams(inner_context->kernelpps_handle, &pp)) {
	char errbuf[BUFSIZ] = "unknown error";
	(void)strerror_r(errno, errbuf, sizeof(errbuf));
	pps_thread->log_hook(pps_thread, THREAD_ERROR,
	    "KPPS:%s time_pps_setparams(mode=0x%02X) failed: %s\n",
	    pps_thread->devicename, pp.mode,
	    errbuf);
	(void)time_pps_destroy(inner_context->kernelpps_handle);
	return -1;
    }
    return 0;
}
#endif /* defined(HAVE_SYS_TIMEPPS_H) */

#if defined(TIOCMIWAIT)
/* wait for, and get, an edge using TIOCMIWAIT
 * return -1 for error
 *         0 for OK
 */
static int get_edge_tiocmiwait( volatile struct pps_thread_t *thread_context,
                         struct timespec *clock_ts,
                         int *state,
                         volatile struct timedelta_t *last_fixtime)
{
    char ts_str[TIMESPEC_LEN];

    /* we are lucky to have TIOCMIWAIT, so wait for next edge */
    #define PPS_LINE_TIOC (TIOCM_CD|TIOCM_RI|TIOCM_CTS|TIOCM_DSR)
    /*
     * DB9  DB25  Name      Full name
     * ---  ----  ----      --------------------
     *  3     2    TXD  --> Transmit Data
     *  2     3    RXD  <-- Receive Data
     *  7     4    RTS  --> Request To Send
     *  8     5    CTS  <-- Clear To Send
     *  6     6    DSR  <-- Data Set Ready
     *  4    20    DTR  --> Data Terminal Ready
     *  1     8    DCD  <-- Data Carrier Detect
     *  9    22    RI   <-- Ring Indicator
     *  5     7    GND      Signal ground
     *
     * Wait for status change on any handshake line.  Just one edge,
     * we do not want to be spinning waiting for the trailing edge of
     * a pulse. The only assumption here is that no GPS lights up more
     * than one of these pins.  By waiting on all of them we remove a
     * configuration switch.
     *
     * Note that it only makes sense to wait on handshake lines
     * activated from the receive side (DCE->DTE) here; in this
     * context "DCE" is the GPS. {CD,RI,CTS,DSR} is the
     * entire set of these.
     *
     */

    if (ioctl(thread_context->devicefd, TIOCMIWAIT, PPS_LINE_TIOC) != 0) {
	char errbuf[BUFSIZ] = "unknown error";
	(void)strerror_r(errno, errbuf, sizeof(errbuf));
	thread_context->log_hook(thread_context, THREAD_WARN,
		"TPPS:%s ioctl(TIOCMIWAIT) failed: %d %.40s\n",
		thread_context->devicename, errno, errbuf);
	return -1;;
    }

    /*
     * Start of time critical section
     * Only error reporting, not success reporting in critical section
     */

    /* duplicate copy in get_edge_rfc2783 */
    /* quick, grab a copy of last_fixtime before it changes */
    thread_lock(thread_context);
    *last_fixtime = thread_context->fix_in;
    thread_unlock(thread_context);
    /* end duplicate copy in get_edge_rfc2783 */

    /* get the time after we just woke up */
    if ( 0 > clock_gettime(CLOCK_REALTIME, clock_ts) ) {
	/* uh, oh, can not get time! */
	char errbuf[BUFSIZ] = "unknown error";
	(void)strerror_r(errno, errbuf, sizeof(errbuf));
	thread_context->log_hook(thread_context, THREAD_ERROR,
		    "TPPS:%s clock_gettime() failed: %.100s\n",
		    thread_context->devicename, errbuf);
	return -1;;
    }

    /* got the edge, got the time just after the edge, now quickly
     * get the edge state */
    if (ioctl(thread_context->devicefd, (unsigned long)TIOCMGET, state) != 0) {
	char errbuf[BUFSIZ] = "unknown error";
	(void)strerror_r(errno, errbuf, sizeof(errbuf));
	thread_context->log_hook(thread_context, THREAD_ERROR,
		    "TPPS:%s ioctl(TIOCMGET) failed: %.100s\n",
		    thread_context->devicename, errbuf);
	return -1;
    }
    /* end of time critical section */
    /* mask for monitored lines */

    *state &= PPS_LINE_TIOC;

    timespec_str( clock_ts, ts_str, sizeof(ts_str) );
    thread_context->log_hook(thread_context, THREAD_PROG,
		"TPPS:%s ioctl(TIOCMIWAIT) succeeded, time:%s,  state: %d\n",
		thread_context->devicename, ts_str, *state);

    return 0;
}

#endif /* TIOCMIWAIT */

#if defined(HAVE_SYS_TIMEPPS_H)
/* wait for, and get, last two edges using RFC2783
 * return -1 for error
 *         0 for OK
 *         1 no edge found, continue
 *
 * on a quad core 2.4GHz Xeon using KPPS timestamp instead of plain
 * PPS timestamp removes about 20uS of latency, and about +/-5uS
 * of jitter
 */
static int get_edge_rfc2783(struct inner_context_t *inner_context,
                         struct timespec *prev_clock_ts,
                         int *prev_edge,
                         struct timespec *clock_ts,
                         int *edge,
                         volatile struct timedelta_t *last_fixtime)
{
    pps_info_t pi;
    char ts_str1[TIMESPEC_LEN], ts_str2[TIMESPEC_LEN];
    struct timespec kernelpps_tv;
    volatile struct pps_thread_t *thread_context = inner_context->pps_thread;

    if ( inner_context->pps_canwait ) {
	/*
	 * RFC2783 specifies that a NULL timeval means to wait, if
	 * PPS_CANWAIT is available.
	 *
	 * since we pps_canwait, we skipped the TIOMCIWAIT
	 *
	 * 3 second time out, some GPS output 0.5Hz and some RFC2783
	 * can only trigger on one edge
	 * a better and more complex solution would be to wait
	 * for 1/20 second and suffer the cycles
	 */
	kernelpps_tv.tv_sec = 3;
	kernelpps_tv.tv_nsec = 0;
    } else {
	/*
	 * We use of a non-NULL zero timespec here,
	 * which means to return immediately with -1 (section
	 * 3.4.3).  This is because we know we just got a pulse because
	 * TIOCMIWAIT just woke up.
	 * The timestamp has already been captured in the kernel, and we
	 * are merely fetching it here.
	 */
	memset( (void *)&kernelpps_tv, 0, sizeof(kernelpps_tv));
    }
    memset( (void *)&pi, 0, sizeof(pi)); /* paranoia */
    if ( 0 > time_pps_fetch(inner_context->kernelpps_handle, PPS_TSFMT_TSPEC
	, &pi, &kernelpps_tv)) {

	char errbuf[BUFSIZ] = "unknown error";
	(void)strerror_r(errno, errbuf, sizeof(errbuf));
	thread_context->log_hook(thread_context, THREAD_ERROR,
		    "KPPS:%s kernel PPS failed %s\n",
		    thread_context->devicename, errbuf);
	if ( ETIMEDOUT == errno || EINTR == errno ) {
		/* just a timeout */
		return 1;
	}
	return 0;
    }
    if ( inner_context->pps_canwait ) {
        /* get_edge_tiocmiwait() got this if !pps_canwait */

	/* quick, grab a copy of last fixtime before it changes */
	thread_lock(thread_context);
	*last_fixtime = thread_context->fix_in;
	thread_unlock(thread_context);
    }


    // find the last edge
    if ( pi.assert_timestamp.tv_sec > pi.clear_timestamp.tv_sec ) {
        /* assert 1 sec or more after than clear */
	*edge = 1;
    } else if ( pi.assert_timestamp.tv_sec < pi.clear_timestamp.tv_sec ) {
        /* assert 1 sec or more before than clear */
	*edge = 0;
    } else if ( pi.assert_timestamp.tv_nsec > pi.clear_timestamp.tv_nsec ) {
        /* assert less than 1 sec after clear */
	*edge = 1;
    } else {
        /* assert less than 1 sec before clear */
	*edge = 0;
    }
    if ( 1 == *edge ) {
        /* assert after clear */
	*prev_edge = 0;
	if ( 0 == pi.clear_timestamp.tv_sec ) {
                /* brain damaged pps-gpio sometimes never fills in clear
                 * so make it look like an invisible pulse
                 * if clear is the leading edge, then we are off by the
                 * pulse width */
		*prev_clock_ts = pi.assert_timestamp;
	} else {
		*prev_clock_ts = pi.clear_timestamp;
        }
	*clock_ts = pi.assert_timestamp;
    } else {
        /* assert before clear */
	*prev_edge = 1;
	*prev_clock_ts = pi.assert_timestamp;
	*clock_ts = pi.clear_timestamp;
    }
    /*
     * pps_seq_t is uint32_t on NetBSD, so cast to
     * unsigned long as a wider-or-equal type to
     * accomodate Linux's type.
     */
    timespec_str( &pi.assert_timestamp, ts_str1, sizeof(ts_str1) );
    timespec_str( &pi.clear_timestamp, ts_str2, sizeof(ts_str2) );
    thread_context->log_hook(thread_context, THREAD_PROG,
		"KPPS:%s assert %s, sequence: %lu, "
		"clear  %s, sequence: %lu - using: %.10s\n",
		thread_context->devicename,
		ts_str1,
		(unsigned long) pi.assert_sequence,
		ts_str2,
		(unsigned long) pi.clear_sequence,
		*edge ? "assert" : "clear");

    return 0;
}
#endif  /* defined(HAVE_SYS_TIMEPPS_H) */

/* gpsd_ppsmonitor()
 *
 * the core loop of the PPS thread.
 * All else is initialization, cleanup or subroutine
 */
static void *gpsd_ppsmonitor(void *arg)
{
    char ts_str1[TIMESPEC_LEN], ts_str2[TIMESPEC_LEN];
    struct inner_context_t inner_context = *((struct inner_context_t *)arg);
    volatile struct pps_thread_t *thread_context = inner_context.pps_thread;
    /* the GPS time and system clock timme, to the nSec,
     * when the last fix received
     * using a double would cause loss of precision */
    volatile struct timedelta_t last_fixtime = {{0, 0}, {0, 0}};
    struct timespec clock_ts = {0, 0};
    time_t last_second_used = 0;
    long long cycle = 0, duration = 0;
    /* state is the last state of the tty control signals */
    int state = 0;
    /* count of how many cycles unchanged data */
    int  unchanged = 0;
    /* state_last is previous state */
    int state_last = 0;
    /* edge, used as index into pulse to find previous edges */
    int edge = 0;       /* 0 = clear edge, 1 = assert edge */

#if defined(TIOCMIWAIT)
    int edge_tio = 0;
    long long cycle_tio = 0;
    long long duration_tio = 0;
    int state_tio = 0;
    int state_last_tio = 0;
    struct timespec clock_ts_tio = {0, 0};
    /* pulse stores the time of the last two edges */
    struct timespec pulse_tio[2] = { {0, 0}, {0, 0} };
#endif /* TIOCMIWAIT */

#if defined(HAVE_SYS_TIMEPPS_H)
    long long cycle_kpps = 0, duration_kpps = 0;
    /* kpps_pulse stores the time of the last two edges */
    struct timespec pulse_kpps[2] = { {0, 0}, {0, 0} };
#endif /* defined(HAVE_SYS_TIMEPPS_H) */
    bool not_a_tty = false;

    /* before the loop, figure out how we can detect edges:
     * TIOMCIWAIT, which is linux specifix
     * RFC2783, a.k.a kernel PPS (KPPS)
     * or if KPPS is deficient a combination of the two */
    if ( isatty(thread_context->devicefd) == 0 ) {
	thread_context->log_hook(thread_context, THREAD_INF,
            "KPPS:%s gps_fd:%d not a tty\n",
            thread_context->devicename,
            thread_context->devicefd);
        /* why do we care the device is a tty? so as not to ioctl(TIO..)
        * /dev/pps0 is not a tty and we need to use it */
        not_a_tty = true;
    }
    /* if no TIOCMIWAIT, we hope to have PPS_CANWAIT */

    if ( not_a_tty && !inner_context.pps_canwait ) {
	/* for now, no way to wait for an edge, in the future maybe figure out
         * a sleep */
    }

    /*
     * this is the main loop, exit and never any further PPS processing.
     *
     * Four stages to the loop,
     * an unwanted condition at any point and the loop restarts
     * an error condition and we exit for all time.
     *
     * Stage One: wait for the next edge.
     *      If we have KPPS
     *         If we have PPS_CANWAIT
     *             use KPPS and PPS_CANWAIT - this is the most accurate
     *         else
     *             use KPPS and TIOMCIWAIT together - this is pretty accurate
     *      else If we have TIOMCIWAIT
     *         use TIOMCIWAIT - this is the least accurate
     *      else
     *         give up
     *
     * Success is we have a good edge, otherwise loop some more
     *
     * On a successul stage one, we know this about the exact moment
     * of current pulse:
     *      GPS (real) time
     *      system (clock) time
     *      edge type: Assert (rising) or Clear (falling)
     *
     * From the above 3 items, we can compute:
     *      cycle length - elapsed time from the previous edge of the same type
     *      pulse length (duration) - elapsed time from the previous edge
     *                            (the previous edge would be the opposite type)
     *
     * Stage Two:  Categorize the current edge
     *      Decide if we have 0.5Hz, 1Hz, 5 Hz cycle time
     *      knowing cycle time determine if we have the leading or trailing edge
     *      restart the loop if the edge looks dodgy
     *
     * Stage Three: Calculate
     *	    Calculate the offset (difference) between the system time
     *      and the GPS time at the pulse moment
     *      restart the loop if the offset looks dodgy
     *
     * Stage Four: Tell ntpd, chronyd, or gpsmon what we learned
     *       a few more sanity checks
     *       call the report hook with our PPS report
     */
    while (thread_context->report_hook != NULL) {
	bool ok = false;
	char *log = NULL;
        char *edge_str = "";

	if (++unchanged == 10) {
            /* last ten edges no good, stop spinning, just wait 10 seconds */
	    unchanged = 0;
	    thread_context->log_hook(thread_context, THREAD_WARN,
		    "PPS:%s unchanged state, ppsmonitor sleeps 10\n",
		    thread_context->devicename);
	    (void)sleep(10);
        }

        /* Stage One; wait for the next edge */
#if defined(TIOCMIWAIT)
        if ( !not_a_tty && !inner_context.pps_canwait ) {
            int ret;

            /* we are a tty, so can TIOCMIWAIT */
            /* we have no PPS_CANWAIT, so must TIOCMIWAIT */

	    ret = get_edge_tiocmiwait( thread_context, &clock_ts_tio,
                        &state_tio, &last_fixtime );
            if ( 0 != ret ) {
		thread_context->log_hook(thread_context, THREAD_PROG,
			    "PPS:%s die: TIOCMIWAIT Error\n",
			    thread_context->devicename);
		break;
	    }

	    edge_tio = (state_tio > state_last_tio) ? 1 : 0;

            /* three things now known about the current edge:
             * clock_ts - time of the edge
             * state - the serial line input states
             * edge - rising edge (1), falling edge (0) or invisble edge (0)
             */

	    /* calculate cycle and duration from previous edges */
	    cycle_tio = timespec_diff_ns(clock_ts_tio, pulse_tio[edge_tio]);
	    cycle_tio /= 1000;  /* nsec to usec */
	    duration_tio = timespec_diff_ns(clock_ts_tio,
			pulse_tio[edge_tio ? 0 : 1])/1000;

	    /* save this edge so we know next cycle time */
	    pulse_tio[edge_tio] = clock_ts_tio;

            /* use this data */
            ok = true;
	    clock_ts = clock_ts_tio;
	    state = edge_tio;
	    edge = edge_tio;
            edge_str = edge ? "Assert" : "Clear";
	    cycle = cycle_tio;
	    duration = duration_tio;

	    timespec_str( &clock_ts, ts_str1, sizeof(ts_str1) );
	    thread_context->log_hook(thread_context, THREAD_PROG,
		    "TPPS:%s %.10s cycle: %d, duration: %d @ %s\n",
		    thread_context->devicename, edge_str, cycle, duration,
                    ts_str1);

        }
#endif /* TIOCMIWAIT */

	/* ok and log used by KPPS and TIOCMIWAIT */
	log = NULL;
#if defined(HAVE_SYS_TIMEPPS_H)
        if ( 0 <= inner_context.kernelpps_handle ) {
	    int ret;
	    int edge_kpps = 0;       /* 0 = clear edge, 1 = assert edge */
            /* time of the last edge */
	    struct timespec clock_ts_kpps = {0, 0};
            /* time of the edge before the last edge */
	    struct timespec prev_clock_ts = {0, 0};
            /* direction of next to last edge 1 = assert, 0 = clear */
            int prev_edge = 0;

	    /* get last and previsou edges, in order
             * optionally wait for goood data
             */
	    ret = get_edge_rfc2783(&inner_context,
                         &prev_clock_ts,
                         &prev_edge,
                         &clock_ts_kpps,
                         &edge_kpps,
			 &last_fixtime);

            if ( -1 == ret ) {
		/* error, so break */
		thread_context->log_hook(thread_context, THREAD_ERROR,
			    "PPS:%s die: RFC2783 Error\n",
			    thread_context->devicename);
		break;
            }

            if ( 1 == ret ) {
		/* no edge found, so continue */
                /* maybe use TIOCMIWAIT edge instead?? */
		continue;
            }
            /* for now, as we have been doing all of gpsd 3.x, just
             *use the last edge, not the previous edge */

            /* compute time from previous saved similar edge */
	    cycle_kpps = timespec_diff_ns(clock_ts_kpps, pulse_kpps[edge_kpps]);
	    cycle_kpps /= 1000;
            /* compute time from previous saved dis-similar edge */
	    duration_kpps = timespec_diff_ns(clock_ts_kpps, prev_clock_ts)/1000;

	    /* save for later */
	    pulse_kpps[edge_kpps] = clock_ts_kpps;
	    pulse_kpps[edge_kpps ? 0 : 1] = prev_clock_ts;
            /* sanity checks are later */

            /* use this data */
	    state = edge_kpps;
	    edge = edge_kpps;
            edge_str = edge ? "Assert" : "Clear";
	    clock_ts = clock_ts_kpps;
	    cycle = cycle_kpps;
	    duration = duration_kpps;

	    timespec_str( &clock_ts_kpps, ts_str1, sizeof(ts_str1) );
	    thread_context->log_hook(thread_context, THREAD_PROG,
		"KPPS:%s %.10s cycle: %7d, duration: %7d @ %s\n",
		thread_context->devicename,
		edge_str,
		cycle_kpps, duration_kpps, ts_str1);

	}
#endif /* defined(HAVE_SYS_TIMEPPS_H) */

        if ( not_a_tty && !inner_context.pps_canwait ) {
	    /* uh, oh, no TIOMCIWAIT, nor RFC2783, die */
	    thread_context->log_hook(thread_context, THREAD_WARN,
			"PPS:%s die: no TIOMCIWAIT, nor RFC2783 CANWAIT\n",
			thread_context->devicename);
	    break;
	}
	/*
         * End of Stge One
	 * we now know this about the exact moment of current pulse:
	 *      GPS (real) time
	 *      system (clock) time
	 *      edge type: Assert (rising) or Clear (falling)
	 *
	 * we have computed:
	 *      cycle length
	 *      pulse length (duration)
	 */

	/*
	 * Stage Two:  Categorize the current edge
	 *      Decide if we have 0.5Hz, 1Hz, 5 Hz cycle time
	 *      determine if we have the leading or trailing edge
	 */

	/* FIXME! this block duplicates a lot of the next block
         * of cycle detetion code */
	if (state != state_last) {
	    thread_context->log_hook(thread_context, THREAD_RAW,
			"PPS:%s %.10s pps-detect changed to %d\n",
			thread_context->devicename, edge_str, state);
	    unchanged = 0;
        } else if ( (180000 < cycle &&  220000 > cycle)      /* 5Hz */
	        ||  (900000 < cycle && 1100000 > cycle)      /* 1Hz */
	        || (1800000 < cycle && 2200000 > cycle) ) {  /* 2Hz */

	    /* some pulses may be so short that state never changes
	     * and some RFC2783 only can detect one edge */

	    duration = 0;
	    unchanged = 0;
	    thread_context->log_hook(thread_context, THREAD_RAW,
			"PPS:%s %.10s pps-detect invisible pulse\n",
			thread_context->devicename, edge_str);
	}
        /* else, unchannged state, and weird cycle time */

	state_last = state;
	timespec_str( &clock_ts, ts_str1, sizeof(ts_str1) );
	thread_context->log_hook(thread_context, THREAD_PROG,
	    "PPS:%s %.10s cycle: %7d, duration: %7d @ %s\n",
	    thread_context->devicename,
	    edge_str,
	    cycle, duration, ts_str1);
	if (unchanged) {
	    // strange, try again
	    continue;
	}

	/*
	 * The PPS pulse is normally a short pulse with a frequency of
	 * 1 Hz, and the UTC second is defined by the front edge. But we
	 * don't know the polarity of the pulse (different receivers
	 * emit different polarities). The duration variable is used to
	 * determine which way the pulse is going.  When the duration
         * is less than 1/2 the cycle we are on the trailing edge.
	 *
	 * Some GPSes instead output a square wave that is 0.5 Hz and each
	 * edge denotes the start of a second.
	 *
	 * Some GPSes, like the Globalsat MR-350P, output a 1uS pulse.
	 * The pulse is so short that TIOCMIWAIT sees a state change
	 * but by the time TIOCMGET is called the pulse is gone.  gpsd
         * calls that an invisible pulse.
	 *
	 * A few stupid GPSes, like the Furuno GPSClock, output a 1.0 Hz
	 * square wave where the leading edge is the start of a second
         * gpsd can only guess the correct edge.
	 *
	 * 5Hz GPS (Garmin 18-5Hz) pulses at 5Hz. Set the pulse length to
	 * 40ms which gives a 160ms pulse before going high.
	 *
	 * You may think that PPS is very accurate, so the cycle time
         * valid window should be very small.  This is not the case,
         * The Rasberry Pi clock is very coarse when it starts and chronyd
         * may be doing a fast slew.  chronyd by default will slew up
         * to 8.334%!  So the cycle time as measured by the system clock
         * may be almost +/- 9%. Therefore, gpsd uses a 10% window.
	 * Don't worry, ntpd and chronyd will do further validation.
	 */

	log = "Unknown error";
        if ( 0 > cycle ) {
	    log = "Rejecting negative cycle\n";
	} else if (199000 > cycle) {
	    // too short to even be a 5Hz pulse
	    log = "Too short for 5Hz\n";
	} else if (201000 > cycle) {
	    /* 5Hz cycle */
	    /* looks like 5hz PPS pulse */
	    if (100000 > duration) {
		/* BUG: how does the code know to tell ntpd
		 * which 1/5 of a second to use?? */
		ok = true;
		log = "5Hz PPS pulse\n";
	    }
	} else if (900000 > cycle) {
            /* Yes, 10% window.  The Rasberry Pi clock is very coarse
             * when it starts and chronyd may be doing a fast slew.
             * chronyd by default will slew up to 8.334% !
             * Don't worry, ntpd and chronyd will do further sanitizing.*/
	    log = "Too long for 5Hz, too short for 1Hz\n";
	} else if (1100000 > cycle) {
            /* Yes, 10% window.  */
	    /* looks like PPS pulse or square wave */
	    if (0 == duration) {
		ok = true;
		log = "invisible pulse\n";
	    } else if (499000 > duration) {
		/* end of the short "half" of the cycle */
		/* aka the trailing edge */
		log = "1Hz trailing edge\n";
	    } else if (501000 > duration) {
		/* looks like 1.0 Hz square wave, ignore trailing edge */
		if (edge == 1) {
		    ok = true;
		    log = "square\n";
		}
	    } else {
		/* end of the long "half" of the cycle */
		/* aka the leading edge */
		ok = true;
		log = "1Hz leading edge\n";
	    }
	} else if (1999000 > cycle) {
	    log = "Too long for 1Hz, too short for 2Hz\n";
	} else if (2001000 > cycle) {
	    /* looks like 0.5 Hz square wave */
	    if (999000 > duration) {
		log = "0.5 Hz square too short duration\n";
	    } else if (1001000 > duration) {
		ok = true;
		log = "0.5 Hz square wave\n";
	    } else {
		log = "0.5 Hz square too long duration\n";
	    }
	} else {
	    log = "Too long for 0.5Hz\n";
	}

	/* end of Stage two
         * we now know what type of PPS pulse, and if we have the
         * leading edge
         */

	/* Stage Three: Calculate
	 *	Calculate the offset (difference) between the system time
	 *      and the GPS time at the pulse moment
         */

	/*
	 * If there has not yet been any valid in-band time stashed
	 * from the GPS when the PPS event was asserted, we can do
	 * nothing further.  gpsd can not tell what second this pulse is
         * in reference to.
         *
         * Some GPSes like Garmin always send a PPS, valid or not.
         * Other GPSes like some uBlox may only send PPS when time is valid.
         * It is common to get PPS, and no fixtime, while autobauding.
	 */
        if (last_fixtime.real.tv_sec == 0) {
	    /* probably should log computed offset just for grins here */
	    ok = false;
	    log = "missing last_fixtime\n";
        } else if ( ok && last_second_used >= last_fixtime.real.tv_sec ) {
	    /* uh, oh, this second already handled */
	    ok = false;
	    log = "this second already handled\n";
	}

	if ( !ok ) {
            /* can not use this pulse, reject and retry */
	    thread_context->log_hook(thread_context, THREAD_PROG,
			"PPS:%s %.10s rejected %.100s",
			thread_context->devicename, edge_str,  log);
	    continue;
        }

        /* we have validated a goood cycle, mark it */
	unchanged = 0;
	/* offset is the skew from expected to observed pulse time */
	struct timespec offset;
	/* offset as a printable string */
	char offset_str[TIMESPEC_LEN];
	/* delay after last fix */
	struct timespec  delay;
	/* delay as a printable string */
	char delay_str[TIMESPEC_LEN];
	char *log1 = "";
	/* ppstimes.real is the time we think the pulse represents  */
	struct timedelta_t ppstimes;
	thread_context->log_hook(thread_context, THREAD_RAW,
		    "PPS:%s %.10s categorized %.100s",
		    thread_context->devicename, edge_str, log);

	/* FIXME! The GR-601W at 38,400 or faster can send the
	 * serial fix before the interrupt event carrying the PPS
	 * line assertion by about 10 mSec!
	 */

	/*
	 * We get the time of the last fix recorded before the PPS came in,
	 * which is for the previous cycle.  Only works for integral cycle
         * times, but more than 1Hz is pointless.
	 */

	ppstimes.real.tv_sec = (time_t)last_fixtime.real.tv_sec + 1;
	ppstimes.real.tv_nsec = 0;  /* need to be fixed for 5Hz */
	ppstimes.clock = clock_ts;

	TS_SUB( &offset, &ppstimes.real, &ppstimes.clock);
	TS_SUB( &delay, &ppstimes.clock, &last_fixtime.clock);
	timespec_str( &delay, delay_str, sizeof(delay_str) );

	/* end Stage Three: now known about the exact edge moment:
	 *	UTC time of PPS edge
	 *      offset of system time to PS time
         */

	/* Stage Four: Tell ntpd, chronyd, or gpsmon what we learned
	 *       a few more sanity checks
	 *       call the report hook with our PPS report
         */

	if ( 0> delay.tv_sec || 0 > delay.tv_nsec ) {
	    thread_context->log_hook(thread_context, THREAD_RAW,
			"PPS:%s %.10s system clock went backwards: %.20s\n",
			thread_context->devicename,
			edge_str,
			delay_str);
	    log1 = "system clock went backwards";
	} else if ( ( 2 < delay.tv_sec)
	  || ( 1 == delay.tv_sec && 100000000 > delay.tv_nsec ) ) {
	    /* system clock could be slewing so allow 1.1 sec delay */
	    thread_context->log_hook(thread_context, THREAD_RAW,
			"PPS:%s %.10s no current GPS seconds: %.20s\n",
			thread_context->devicename,
			edge_str,
			delay_str);
	    log1 = "timestamp out of range";
	} else {
	    last_second_used = last_fixtime.real.tv_sec;
	    if (thread_context->report_hook != NULL)
		log1 = thread_context->report_hook(thread_context, &ppstimes);
	    else
		log1 = "no report hook";
	    thread_lock(thread_context);
	    thread_context->pps_out = ppstimes;
	    thread_context->ppsout_count++;
	    thread_unlock(thread_context);
	    timespec_str( &ppstimes.clock, ts_str1, sizeof(ts_str1) );
	    timespec_str( &ppstimes.real, ts_str2, sizeof(ts_str2) );
	    thread_context->log_hook(thread_context, THREAD_INF,
		"PPS:%s %.10s hooks called clock: %s real: %s: %.20s\n",
		thread_context->devicename,
		edge_str,
		ts_str1, ts_str2, log1);
	}
	timespec_str( &clock_ts, ts_str1, sizeof(ts_str1) );
	timespec_str( &offset, offset_str, sizeof(offset_str) );
	thread_context->log_hook(thread_context, THREAD_PROG,
		"PPS:%s %.10s %.30s @ %s offset %.20s\n",
		thread_context->devicename,
		edge_str,
		log1, ts_str1, offset_str);
        /* end Stage four, end of the loop, do it again */
    }
#if defined(HAVE_SYS_TIMEPPS_H)
    if (inner_context.kernelpps_handle > 0) {
	thread_context->log_hook(thread_context, THREAD_PROG,
            "PPS:%s descriptor cleaned up\n",
	    thread_context->devicename);
	(void)time_pps_destroy(inner_context.kernelpps_handle);
    }
#endif
    thread_context->log_hook(thread_context, THREAD_PROG,
		"PPS:%s gpsd_ppsmonitor exited.\n",
		thread_context->devicename);
    return NULL;
}

/*
 * Entry points begin here.
 */

void pps_thread_activate(volatile struct pps_thread_t *pps_thread)
/* activate a thread to watch the device's PPS transitions */
{
    int retval;
    pthread_t pt;
    /*
     * FIXME: this launch code is not itself thread-safe!
     * It would be if inner_context could be auto, but the monitor
     * routine gets garbage when we try that.  Ideally the body
     * of this function would be guarded by a separate mutex.
     * Either that, or this should be an exception to the no-malloc rule.
     */
    static struct inner_context_t	inner_context;

    inner_context.pps_thread = pps_thread;
#if defined(HAVE_SYS_TIMEPPS_H)
    /* some operations in init_kernel_pps() require root privs */
    (void)init_kernel_pps(&inner_context);
    if ( 0 <= inner_context.kernelpps_handle ) {
	pps_thread->log_hook(pps_thread, THREAD_INF,
		    "KPPS:%s kernel PPS will be used\n",
		    pps_thread->devicename);
    } else
#endif
    {
	pps_thread->log_hook(pps_thread, THREAD_WARN,
		    "KPPS:%s kernel PPS unavailable, PPS accuracy will suffer\n",
		    pps_thread->devicename);
    }

    memset( &pt, 0, sizeof(pt));
    retval = pthread_create(&pt, NULL, gpsd_ppsmonitor, (void *)&inner_context);
    pps_thread->log_hook(pps_thread, THREAD_PROG, "PPS:%s thread %s\n",
	        pps_thread->devicename,
		(retval==0) ? "launched" : "FAILED");
}

void pps_thread_deactivate(volatile struct pps_thread_t *pps_thread)
/* cleanly terminate PPS thread */
{
    pps_thread->report_hook = NULL;
}

void pps_thread_fixin(volatile struct pps_thread_t *pps_thread,
			      volatile struct timedelta_t *fix_in)
/* thread-safe update of last fix time - only way we pass data in */
{
    thread_lock(pps_thread);
    pps_thread->fix_in = *fix_in;
    thread_unlock(pps_thread);
}

int pps_thread_ppsout(volatile struct pps_thread_t *pps_thread,
		       volatile struct timedelta_t *td)
/* return the delta at the time of the last PPS - only way we pass data out */
{
    volatile int ret;

    thread_lock(pps_thread);
    *td = pps_thread->pps_out;
    ret = pps_thread->ppsout_count;
    thread_unlock(pps_thread);

    return ret;
}

/* end */