timesync.c

这是一个同样来自贝尔实验室的和UNIX有着渊源的操作系统, 其简洁的设计和实现易于我们学习和理解

	uchar ub[3*8], *p;
	char b[2*24+1];

	switch(ifc){
	case Ibintime:
		n = sizeof(vlong);
		if(hz != nil)
			n = 3*sizeof(vlong);
		if(ticks != nil)
			n = 2*sizeof(vlong);
		i = read(bintimefd, ub, n);
		if(i != n)
			break;
		p = ub;
		if(nsec != nil)
			be2vlong(nsec, ub);
		p += sizeof(vlong);
		if(ticks != nil)
			be2vlong((vlong*)ticks, p);
		p += sizeof(vlong);
		if(hz != nil)
			be2vlong((vlong*)hz, p);
		return 0;
	case Itiming:
		n = sizeof(vlong);
		if(ticks != nil)
			n = 2*sizeof(vlong);
		i = read(timingfd, ub, n);
		if(i != n)
			break;
		p = ub;
		if(nsec != nil)
			be2vlong(nsec, ub);
		p += sizeof(vlong);
		if(ticks != nil)
			be2vlong((vlong*)ticks, p);
		if(hz != nil){
			seek(fastclockfd, 0, 0);
			n = read(fastclockfd, b, sizeof(b)-1);
			if(n <= 0)
				break;
			b[n] = 0;
			*hz = strtoll(b+24, 0, 0);
		}
		return 0;
	case Insec:
		if(nsec != nil){
			seek(nsecfd, 0, 0);
			n = read(nsecfd, b, sizeof(b)-1);
			if(n <= 0)
				break;
			b[n] = 0;
			*nsec = strtoll(b, 0, 0);
		}
		if(ticks != nil){
			seek(fastclockfd, 0, 0);
			n = read(fastclockfd, b, sizeof(b)-1);
			if(n <= 0)
				break;
			b[n] = 0;
			*ticks = strtoll(b, 0, 0);
		}
		if(hz != nil){
			seek(fastclockfd, 0, 0);
			n = read(fastclockfd, b, sizeof(b)-1);
			if(n <= 24)
				break;
			b[n] = 0;
			*hz = strtoll(b+24, 0, 0);
		}
		return 0;
	}
	return -1;
}

static void
settime(vlong now, uvlong hz, vlong delta, int n)
{
	uchar b[1+sizeof(vlong)+sizeof(long)], *p;

	if(debug)
		fprint(2, "settime(now=%lld, hz=%llud, delta=%lld, period=%d)\n", now, hz, delta, n);
	if(impotent)
		return;
	switch(ifc){
	case Ibintime:
		if(now >= 0){
			p = b;
			*p++ = 'n';
			p = vlong2be(p, now);
			if(write(bintimefd, b, p-b) < 0)
				sysfatal("writing /dev/bintime: %r");
		}
		if(delta != 0){
			p = b;
			*p++ = 'd';
			p = vlong2be(p, delta);
			p = long2be(p, n);
			if(write(bintimefd, b, p-b) < 0)
				sysfatal("writing /dev/bintime: %r");
		}
		if(hz != 0){
			p = b;
			*p++ = 'f';
			p = vlong2be(p, hz);
			if(write(bintimefd, b, p-b) < 0)
				sysfatal("writing /dev/bintime: %r");
		}
		break;
	case Itiming:
	case Insec:
		seek(nsecfd, 0, 0);
		if(now >= 0 || delta != 0){
			if(fprint(nsecfd, "%lld %lld %d", now, delta, n) < 0)
				sysfatal("writing /dev/nsec: %r");
		}
		if(hz > 0){
			seek(fastclockfd, 0, 0);
			if(fprint(fastclockfd, "%lld", hz) < 0)
				sysfatal("writing /dev/fastclock: %r");
		}
	}
}

//
//  set priority high and wire process to a processor
//
static void
setpriority(void)
{
	int fd;
	char buf[32];

	sprint(buf, "/proc/%d/ctl", getpid());
	fd = open(buf, ORDWR);
	if(fd < 0){
		fprint(2, "can't set priority\n");
		return;
	}
	if(fprint(fd, "pri 100") < 0)
		fprint(2, "can't set priority\n");
	if(fprint(fd, "wired 2") < 0)
		fprint(2, "can't wire process\n");
	close(fd);
}

// convert to ntp timestamps
static void
hnputts(void *p, vlong nsec)
{
	uchar *a;
	ulong tsh;
	ulong tsl;

	a = p;

	// zero is a special case
	if(nsec == 0)
		return;

	tsh = (nsec/SEC);
	nsec -= tsh*SEC;
	tsl = (nsec<<32)/SEC;
	hnputl(a, tsh+EPOCHDIFF);
	hnputl(a+4, tsl);
}

// convert from ntp timestamps
static vlong
nhgetts(void *p)
{
	uchar *a;
	ulong tsh, tsl;
	vlong nsec;

	a = p;
	tsh = nhgetl(a);
	tsl = nhgetl(a+4);
	nsec = tsl*SEC;
	nsec >>= 32;
	nsec += (tsh - EPOCHDIFF)*SEC;
	return nsec;
}

// convert to ntp 32 bit fixed point
static void
hnputfp(void *p, vlong nsec)
{
	uchar *a;
	ulong fp;

	a = p;

	fp = nsec/(SEC/((vlong)(1<<16)));
	hnputl(a, fp);
}

// convert from ntp fixed point to nanosecs
static vlong
nhgetfp(void *p)
{
	uchar *a;
	ulong fp;
	vlong nsec;

	a = p;
	fp = nhgetl(a);
	nsec = ((vlong)fp)*(SEC/((vlong)(1<<16)));
	return nsec;
}

// get network address of the server
static void
setrootid(char *d)
{
	char buf[128];
	int fd, n;
	char *p;

	snprint(buf, sizeof(buf), "%s/remote", d);
	fd = open(buf, OREAD);
	if(fd < 0)
		return;
	n = read(fd, buf, sizeof buf);
	close(fd);
	if(n <= 0)
		return;
	p = strchr(buf, '!');
	if(p != nil)
		*p = 0;
	v4parseip(rootid, buf);
}

static void
ding(void*, char *s)
{
	if(strstr(s, "alarm") != nil)
		noted(NCONT);
	noted(NDFLT);
}

static void
addntpserver(char *name)
{
	NTPserver *ns, **l;

	ns = mallocz(sizeof(NTPserver), 1);
	if(ns == nil)
		sysfatal("addntpserver: %r");
	timeserver = strdup(name);
	ns->name = name;
	for(l = &ntpservers; *l != nil; l = &(*l)->next)
		;
	*l = ns;
}

//
//  sntp client, we keep calling if the delay seems
//  unusually high, i.e., 30% longer than avg.
//
static int
ntptimediff(NTPserver *ns)
{
	int fd, tries, n;
	NTPpkt ntpin, ntpout;
	vlong dt, recvts, origts, xmitts, destts, x;
	char dir[64];

	fd = dial(netmkaddr(ns->name, "udp", "ntp"), 0, dir, 0);
	if(fd < 0){
		syslog(0, logfile, "can't reach %s: %r", ns->name);
		return -1;
	}
	setrootid(dir);
	notify(ding);

	memset(&ntpout, 0, sizeof(ntpout));
	ntpout.mode = 3 | (3 << 3);

	for(tries = 0; tries < 3; tries++){
		alarm(2*1000);

		gettime(&x, 0, 0);
		hnputts(ntpout.xmitts, x);
		if(write(fd, &ntpout, NTPSIZE) < 0){
			alarm(0);
			continue;
		}

		n = read(fd, &ntpin, sizeof(ntpin));
		alarm(0);
		gettime(&destts, 0, 0);
		if(n >= NTPSIZE){
			close(fd);

			// we got one, use it
			recvts = nhgetts(ntpin.recvts);
			origts = nhgetts(ntpin.origts);
			xmitts = nhgetts(ntpin.xmitts);
			dt = ((recvts - origts) + (xmitts - destts))/2;

			// save results
			ns->rtt = ((destts - origts) - (xmitts - recvts))/2;
			ns->dt = dt;
			ns->stratum = ntpin.stratum;
			ns->precision = ntpin.precision;
			ns->rootdelay = nhgetfp(ntpin.rootdelay);
			ns->rootdisp = nhgetfp(ntpin.rootdisp);

			if(debug)
				fprint(2, "ntp %s stratum %d ntpdelay(%lld)\n", 
					ns->name, ntpin.stratum, ns->rtt);
			return 0;
		}

		// try again
		sleep(250);
	}
	close(fd);
	return -1;
}

static vlong
gpssample(void)
{
	vlong	l, g, d;
	int	i, n;
	char	*v[4], buf[128];

	d = -1000000000000000000LL;
	for(i = 0; i < 5; i++){
		sleep(1100);
		seek(gpsfil, 0, 0);
		n = read(gpsfil, buf, sizeof buf - 1);
		if (n <= 0)
			return(0LL);
		buf[n] = 0;
		n = tokenize(buf, v, nelem(v));
		if(n != 4)
			return(0LL);
		if(strcmp(v[3], "A") != 0)
			return(0LL);
		g = atoll(v[1]);
		l = atoll(v[2]);
		if(g-l > d)
			d = g-l;
	}
	return(d);
}

static vlong
ntpsample(void)
{
	NTPserver *tns, *ns;
	vlong metric, x;

	metric = 1000LL*SEC;
	ns = nil;
	for(tns = ntpservers; tns != nil; tns = tns->next){
		if(ntptimediff(tns) < 0)
			continue;
		x = vabs(tns->rootdisp) + (vabs(tns->rtt+tns->rootdelay)>>1);
		if(debug)
			fprint(2, "ntp %s rootdelay %lld rootdisp %lld metric %lld\n",
				tns->name, tns->rootdelay, tns->rootdisp, x);
		if(x < metric){
			metric = x;
			ns = tns;
		}
	}

	if(ns == nil)
		return 0LL;

	// save data for our server
	rootdisp = ns->rootdisp;
	rootdelay = ns->rootdelay;
	mydelay = ns->rtt;
	mydisp = avgerr;
	if(ns->stratum == 0)
		stratum = 0;
	else
		stratum = ns->stratum + 1;

	ε = abs(ns->rtt/2);
	return ns->dt;
}

//
// sample the utc file
//
static vlong
utcsample(void)
{
	vlong	s;
	int	n;
	char	*v[2], buf[128];

	s = 0;
	seek(utcfil, 0, 0);
	n = read(utcfil, buf, sizeof buf - 1);
	if (n <= 0)
		return(0LL);
	buf[n] = 0;
	n = tokenize(buf, v, nelem(v));
	if (strcmp(v[0], "0") == 0)
		return(0LL);
	if (n == 2) {
		gettime(&s, nil, nil);
		s -= atoll(v[1]);
	}
	lastutc = atoll(v[0]) + s;
	return(lastutc);
}

//
//  sntp server
//
static int
openlisten(char *net)
{
	int fd, cfd;
	char data[128];
	char devdir[40];

	sprint(data, "%s/udp!*!ntp", net);
	cfd = announce(data, devdir);
	if(cfd < 0)
		sysfatal("can't announce");
	if(fprint(cfd, "headers") < 0)
		sysfatal("can't set header mode");
	fprint(cfd, "oldheaders");

	sprint(data, "%s/data", devdir);

	fd = open(data, ORDWR);
	if(fd < 0)
		sysfatal("open udp data");
	return fd;
}
static void
ntpserver(char *servenet)
{
	int fd, n;
	NTPpkt *ntp;
	char buf[512];
	int vers, mode;
	vlong recvts, x;

	fd = openlisten(servenet);

	if (Rootid == nil)
		switch(type){
		case Fs:
			Rootid = "WWV";
			break;
		case Rtc:
			Rootid = "LOCL";
			break;
		case Utc:
			Rootid = "UTC";
			break;
		case Ntp:
			/* set by the ntp client */
			break;
		}
	if (Rootid != nil)
		memmove(rootid, Rootid, strlen(Rootid) > 4 ? 4 : strlen(Rootid));

	for(;;){
		n = read(fd, buf, sizeof(buf));
		gettime(&recvts, 0, 0);
		if(n < 0)
			return;
		if(n < OUdphdrsize + NTPSIZE)
			continue;

		ntp = (NTPpkt*)(buf+OUdphdrsize);
		mode = ntp->mode & 7;
		vers = (ntp->mode>>3) & 7;
		if(mode != 3)
			continue;

		ntp->mode = (vers<<3)|4;
		ntp->stratum = stratum;
		ntp->precision = myprec;
		hnputfp(ntp->rootdelay, rootdelay + mydelay);
		hnputfp(ntp->rootdisp, rootdisp + mydisp);
		hnputts(ntp->refts, lastutc);
		memmove(ntp->origts, ntp->xmitts, sizeof(ntp->origts));
		hnputts(ntp->recvts, recvts);
		memmove(ntp->rootid, rootid, sizeof(ntp->rootid));
		gettime(&x, 0, 0);
		hnputts(ntp->xmitts, x);
		write(fd, buf, NTPSIZE+OUdphdrsize);
	}
}

//
//  get the current time from the file system
//
static long
fstime(void)
{
	Dir *d;
	ulong t;

	d = dirstat("/n/boot");
	if(d != nil){
		t = d->atime;
		free(d);
	} else
		t = 0;
	return t;
}

//
//  get the current time from the real time clock
//
static long
rtctime(void)
{
	char b[20];
	static int f = -1;
	int i, retries;

	memset(b, 0, sizeof(b));
	for(retries = 0; retries < 100; retries++){
		if(f < 0)
			f = open("/dev/rtc", OREAD|OCEXEC);
		if(f < 0)
			break;
		if(seek(f, 0, 0) < 0 || (i = read(f, b, sizeof(b))) < 0){
			close(f);
			f = -1;
		} else {
			if(i != 0)
				break;
		}
	}
	return strtoul(b, 0, 10)+gmtdelta;
}


//
//  Sample a clock.  We wait for the clock to always
//  be at the leading edge of a clock period.
//
static vlong
sample(long (*get)(void))
{
	long this, last;
	vlong start, end;

	/*
	 *  wait for the second to change
	 */
	last = (*get)();
	for(;;){
		gettime(&start, 0, 0);
		this = (*get)();
		gettime(&end, 0, 0);
		if(this != last)
			break;
		last = this;
	}
	return SEC*this - (end-start)/2;
}

//
// the name of the frequency file has the method and possibly the
// server name encoded in it.
//
static int
openfreqfile(void)
{
	char *p;
	int fd;

	if(sysid == nil)
		return -1;

	switch(type){
	case Ntp:
		p = smprint("%s/ts.%s.%d.%s", dir, sysid, type, timeserver);
		break;
	default:
		p = smprint("%s/ts.%s.%d", dir, sysid, type);
		break;
	}
	fd = open(p, ORDWR);
	if(fd < 0)
		fd = create(p, ORDWR, 0666);
	free(p);
	if(fd < 0)
		return -1;
	return fd;
}

//
//  the file contains the last known frequency and the
//  number of seconds it was sampled over
//
static vlong
readfreqfile(int fd, vlong ohz, vlong minhz, vlong maxhz)
{
	int n;
	char buf[128];
	vlong hz;

	n = read(fd, buf, sizeof(buf)-1);
	if(n <= 0)
		return ohz;
	buf[n] = 0;
	hz = strtoll(buf, nil, 0);

	if(hz > maxhz || hz < minhz)
		return ohz;

	settime(-1, hz, 0, 0);
	return hz;
}

//
//  remember hz and averaging period
//
static void
writefreqfile(int fd, vlong hz, int secs, vlong diff)
{
	long now;
	static long last;

	if(fd < 0)
		return;
	now = time(0);
	if(now - last < 10*60)
		return;
	last = now;
	if(seek(fd, 0, 0) < 0)
		return;
	fprint(fd, "%lld %d %d %lld\n", hz, secs, type, diff);
}

static uvlong
vabs(vlong x)
{
	if(x < 0LL)
		return (uvlong)-x;
	else
		return (uvlong)x;
}

static void
background(void)
{
	static int inbackground;

	if(inbackground)
		return;

	if(!debug) {
		switch(rfork(RFPROC|RFFDG|RFNAMEG|RFNOTEG|RFNOWAIT)){
		case -1:
			sysfatal("forking: %r");
			break;
		case 0:
			break;
		default:
			exits(0);
		}
	}
	inbackground = 1;
}

static int
getclockprecision(vlong hz)
{
	int i;

	i = 8;
	while(hz > 0){
		i--;
		hz >>= 1;
	}
	return i;
}