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Linux Pulse Per Second Stuff
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                    Linux Pulse Per Second Stuff

                     (c) 2017 Andreas Steinmetz

This is all about getting precision timing from a serial line PPS signal.

For this stuff to be of any use you first need a GPS receiver with a PPS output and a host system with a real (i.e. no USB) RS232 interface (at least 4wire which means RxD, TxD, RTS and CTS) as well as a not too historic kernel. Preferably the system should have NICs with hardware timestamping support. Furthermore you need to have chrony and gpsd installed.

Personally I'm using a Navilock NL-8005P with a Sub-D9 adapter. In my experience this u-blox 8 based GPS receiver is sensitive enough for indoor (near window) use.

Though the communication of this receiver is RS232 level, the PPS output is open drain. To convert this to RS232 you have to heat up your soldering iron and create a little gadget based on a MAX233A (if you can't get one a MAX233 will do) like shown in the PNG image. With this gadget you convert 3.3V or 5V logic based open drain or open collector output to RS232 level.

Now if you're unlucky, chances are your serial interface is not fully wired and missing the DCD input which is the kernel's PPS source. With the supplied kernel patch you can use CTS instead of DCD. CTS should be available even on crippled interfaces. Note that the provided patch is against Linux 4.13. It is preliminary. After applying the patch and rebooting, there will be a file named /sys/class/tty/<device>/pps_4wire. Assuming that you're using /dev/ttyS2 you will have to do the following to use CTS instead of DCD for PPS input:

echo 1 > /sys/class/tty/ttyS2/pps_4wire

Assuming that your kernel is properly configured with PPS support and that your GPS device is attached to /dev/ttyS2 you now can create a basic configuration for chronyd (/etc/chrony/chrony.conf):

refclock SOCK /run/chrony.ttyS2.sock refid GPS poll 5 filter 80 precision 1e-9
corrtimeratio 3.2

Then first start chronyd followed by gpsd:

chronyd -f /etc/chrony/chrony.conf
gpsd -n -D 0 -F /run/gpsd.sock -P /run/ /dev/ttyS2

If you look at the output of "chronyc sources" after a few minutes of stabilization you will be disappointed. Time precision will be well and far worse than 1us, even more like 10us. This is due to the aggressive power management of modern processors. The typical answer to this problem is to completely disable power management which, however, causes excessive heat and an ugly power bill. If you can accept a precision which is not optimal but quite close to it there is a power saving alternative.

"unidled", which is included here, switches either all cores or, if the intel_idle driver is in use, a single core to poll mode for 0.2% of the time and let the power management handle the remaining 99.8%. This keeps the CPU cool and your power bill healthy. For an initial test after compiling unidled (see the comment in the source) just start unidled after chronyd and gpsd:

unidled -f /run/ -d /dev/ttyS2 -a

Check, that unidled is running and wait again a few minutes for chrony to settle. You should now find that the precision displayed by "chronyc sources" is quite better than 1us, typically you should see a precision better than 500ns.

If things work out well and you now are a happy camper, you can start to refine your setup e.g. by using realtime priorities for gpsd and chronyd (the realtime priority for unidled must be higher than those). In case of a CPU that uses the intel_idle driver you can configure the serial device interrupt as well as unidled to be served by the same core (beware of irqbalance) and remove the "-a" option for unidled (you may have to assert that chronyd as well as gpsd run on the same core, too). This way, only one core will have the forced 0.2% of poll time.

In case you're using irqbalance have a look at the provided script (you at least need to modify the core selection). Start irqbalance with "-l path-to-script" and irqbalance will not mess with the serial line interrupt.

Note that you will have to configure chrony's temperature compensation to prevent a wide frequency adjustment range which in turn causes loss of precision. You need to get a list of temperature and frequency offset pairs. You can use the provided "heatppm" utility for this.

First have a look at what clock sources are in use by looking at the contents of /sys/devices/system/clocksource/clocksource*/current_clocksource. Then look at the contents of /sys/class/hwmon/hwmon*/name to check which temperature measurement sources are available and check the jitter of the temperatures provided by /sys/class/hwmon/hwmon*/temp*_input. In my case with TSC being the clock source I decided to use the temperature provided by "acpitz" which shows quite less jitter than "coretemp".

After having selected the clock source make sure that the system is as idle as possible while keeping chronyd, gpsd and unidled running. Let the system cool down as far as possible, then start heatppm, e.g. as (adapt the temperature source!):

heatppm -t /sys/class/hwmon/hwmon0/temp1_input

Wait for several hours. heatppm will slowly but steadily output a list of temperature and frequency offset pairs. When either heatppm terminates or you interrupt heatppm, copy and paste the generated list to a file. In the following I'll assume it is named "tempcomp.txt".

You then can either create a frequency offset correction file from the list or you can generate the parameters for the frequency offset correction function. I'll describe the latter.

Make sure that you have gnuplot as well as tempcomp.txt available on an arbitrary system. Start gnuplot. Issue the following commands (replace "value-of-row1-column1-of-tempcomp.txt" with the actual value):

f1(x) = k0+(x-t0)*k1+(x-t0)*(x-t0)*k2
fit f1(x) 'tempcomp.txt' using 1:2 via k0, k1, k2

From the generated output you need the "Final set of parameters". Ignore k0 but remember the k1 and k2 values. To get visible data you can now issue:

plot "tempcomp.txt" using 1:2,k0+(x-t0)*k1+(x-t0)*(x-t0)*k2

When you're satisfied, creating a temperature compensation configuration for chronyd is easy:

tempcomp /sys/class/hwmon/hwmon0/temp1_input 1 t0 0.0 -k1 -k2

t0, k1 and k2 are the values entered into or generated by gnuplot. Note the sign inversion for k1 and k2!

Another point: if you have hardware timestamping available, use it! Probably most if not all onboard NICs supported by the "igb" or "e1000e" drivers should support hardware timestamping. For other NICs look for "IEEE 1588" advertisements as well as linux hardware timestamping support.

If your time server doesn't support NIC hardware timestamping, stop here and use NTP to distribute time in your LAN.

If, however, your time server does support NIC hardware timestamping and you want higher precision time in your LAN, read on.

To distribute the precision time in your LAN use PTP and thus have a look at linuxptp and ptpd2. You will probably need both. Now, PTP as well as GPS actually use TAI time. TAI is effectively UTC without any leap seconds applied. As for GPS the signal contains a TAI to UTC offset which is why you get UTC time from a GPS receiver by default. PTP, however, needs either to be corrected for every leap second or has to be configured to use UTC instead of TAI. Well, I'm lazy and don't want to track leap second changes. So I'm going to use UTC for PTP and describe that here.

The utilities you need on your time server are "phc2sys" and "ptp4l" which are both part of linuxptp. First you need to run php2sys as a kind of sys2phc, i.e. you make your NIC clocks follow your system clock. For every NIC you need to start a phc2sys instance (replace "eth0" with your NIC device):

phc2sys -s CLOCK_REALTIME -c eth0 -O 0 -R 10 -N 2 -E linreg -L 50000000 -n 0 -l 0 -q -m

Run phc2sys with realtime privilege, otherwise it will spill errors sooner or later as it seemingly can't handle longer scheduling delays.

Then you have to distribute the time using the PTP protocol aka IEEE 1588. To do so, create a configuration file for ptp4l, let's assume you use /etc/ptp4l.conf:

transportSpecific 0x0
delay_mechanism E2E
network_transport L2
gmCapable 1
slaveOnly 0
priority1 64
priority2 64
# clock class 13 means application specific, leave as is
clockClass 13
# use one of the following values for clock accuracy:
# 0x20 25ns    0x24 2.5us    0x28 250us    0x2c 25ms    0x30 10s
# 0x21 100ns   0x25 10us     0x29 1ms      0x2d 100ms   0x31 more than 10s
# 0x22 250ns   0x26 25us     0x2a 2.5ms    0x2e 250ms   0xfe unknown
# 0x23 1us     0x27 100us    0x2b 10ms     0x2f 1s
clockAccuracy 0x22
domainNumber 0
free_running 0
#udp scope 5 means site local (see rfc7346)
udp6_scope 0x05
dscp_event 46
dscp_general 34
logging_level 0
verbose 0
use_syslog 0
# use one of the following for time source:
# 0x10 atomic clock  0x30 terrestrial radio  0x50 NTP       0x90 other
# 0x20 GPS           0x40 PTP                0x60 hand set  0xa0 int. oscillator
timeSource 0x20
time_stamping hardware
# clear faults at once, helps if there's NIC timestamp driver problems
fault_reset_interval ASAP
boundary_clock_jbod 0
hybrid_e2e 1

Add a section for every interface you may want to distribute time using PTP. If you use VLANs use the VLAN interfaces , e.g. [eth1.5]. Add the "hybrid_e2e 1" to every interface section. Then start ptp4l as:

ptp4l -f /etc/ptp4l.conf

Running ptp4l with realtime privilege doesn't hurt, either.

Now you have a time server distributing NTP time via chronyd (presuming proper NTP configuration) as well as PTP time via the linuxptp tools. Where NTP time is quite coarse and a fallback, PTP time is the precision time you want to use in your LAN.

Next is PTP client configuration. I'm going to configure with PTP as the main time source and NTP as the fallback. There are two methods.

The generic method which works for NICs without hardware timestamping is to use ptpd2 and ntpd for fallback timing. First you need to create a ntpd configuration file and a key file, let's call them /etc/ntp.conf and /etc/ntp.key. Here's a sample ntp.conf file (adapt example server IPs as required):

driftfile /var/lib/ntp/ntp.drift
keys /etc/ntp.key
trustedkey 1
requestkey 1
controlkey 1

#ntpdc and ptp control enablement
enable mode7

server minpoll 4 maxpoll 4
server fdf2:e35b:1a0e:2c28::1 minpoll 4 maxpoll 4

restrict ::1

Then a sample ntp.key file (replace the passphrase with your own):

1 M verysecret

Note the "enable mode7" line. This is required or ptpd2 will not be able to control ntpd. You can now start ntpd and get basic timing quality. Time to create the configuration file for ptpd2, let's call it /etc/ptpd2.conf:


Adapt the ethernet device to the one you use (never mind bridges or bonding, use the physical interface in these cases). Set the offset shift to a value that fits your system. Set the key to match the ntp key configured for ntpd. Then you can start ptpd2:

ptpd2 --global:lock_file=/run/ --global:status_file=/run/ptpd2.status -c /etc/ptpd2.conf

You now have a working PTP client that falls back to NTP time in case of PTP failure.

If the ptpd2 log file shows unexpectedly recurring (not sporadic) "Could not verify NTP status - will keep checking" messages you should apply the provided ptpd2 ntp communication patch to the ptpd sources, recompile and try again. Fixed the problem for me. YMMV.

The more precise method which can be used for clients that have a NIC with hardware timestamping. The mechanism is to run ptp4l as a client, then phc2sys in ptp4l port state following mode providing shared memory time and finally chrony using the SHM driver to update the system clock. This way PTP failures can be detected and at the same time fallback NTP servers can be used. You need to apply the provided patch to phc2sys to allow for port following mode with no time offset.

You need to create a client configuration file for ptp4l, let's assume you use /etc/ptp4l.conf:

transportSpecific 0x0
delay_mechanism E2E
delay_filter moving_average
delay_filter_length 32
tsproc_mode raw
freq_est_interval 0
clock_servo linreg
max_frequency 950000000
network_transport L2
gmCapable 1
slaveOnly 1
priority1 128
priority2 128
# clock class 255 means slave only, do not modify
clockClass 255
# clock accuracy 0xfe means unknown, do not modify
clockAccuracy 0xfe
domainNumber 0
free_running 0
#udp scope 5 means site local  (see rfc7346)
udp6_scope 0x05
dscp_event 46
dscp_general 34
logging_level 0
verbose 0
use_syslog 0
# time source 0xa0 means internal oscillator, do dot modify
timeSource 0xa0
time_stamping hardware
boundary_clock_jbod 0
hybrid_e2e 1

Replace eth0 with the physical interface you actually use. The start ptp4l (it doesn't hurt to use realtime privilege):

ptp4l -f /etc/ptp4l.conf

The start the patched phc2sys (needs realtime privilege, otherwise it tends to fail after a while):

phc2sys -A -r -E ntpshm -M 0 -R 4 -N 4 -l 0 -q

Now you need to create a chrony configuration file (/etc/chrony/chrony.conf):

refclock SHM 0 refid GPS poll 0 dpoll -2 offset -0.000045
server minpoll 4 maxpoll 4 iburst
server fdf2:e35b:1a0e:2c28::1 minpoll 4 maxpoll 4 iburst
corrtimeratio 2.0
maxslewrate 0.05
hwtimestamp eth0

Adapt the example NTP server IPs, interface name and offset time according to your system. Then start chrony:

chronyd -f /etc/chrony/chrony.conf

You may want to try to use realtime privilege for chrony.

You now have a working PTP client unsing hardware timestamping that falls back to NTP time in case of PTP failure.

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