Synchronizing Time with Linux* PTP


Time synchronization is one of the core functionalities of TSN, and it is specified by IEEE 802.1AS, also known as Generalized Precision Time Protocol (gPTP). gPTP is a profile from IEEE 1588, also known as Precision Time Protocol (PTP). gPTP consists of simplifications and constraints to PTP to optimize it to time-sensitive applications. gPTP also introduces the Grand Master (GM) role which can be played by any node in the gPTP domain (such as end-stations or bridges) and is determined by the Best Master Clock Algorithm (BMCA). The GM provides clocking information to all other nodes in the gPTP domain.

In the Linux* ecosystem, Linux PTP is the most popular implementation of PTP. It supports several profiles including gPTP and AVNU automotive profile. Linux PTP provides some tools to carry out time synchronization:

  • ptp4l: daemon that synchronizes the PTP Hardware Clock (PHC) from the NIC;
  • phc2sys: daemon that synchronizes the PHC and the System clock;
  • pmc: utility tool to configure ptp4l in run-time.

Although this tutorial was tested with an Intel(R) Ethernet Controller I210 NIC, it is hardware-agnostic and applies to any 802.1AS-capable NIC with proper device driver support. By following this tutorial, the reader will be able to synchronize time using Linux PTP.

Installing Linux PTP

Several Linux distributions provide a package for Linux PTP. This section discusses how to get Linuxptp source, build, and install the tools.

git clone linuxptp
cd linuxptp/
sudo make install

By default, Linux PTP artifacts are installed into /usr/local/sbin/.

Synchronizing the PHC

The PHC synchronization step is mandatory for all TSN systems. It guarantees the PHC from the NIC is in sync with the GM clock from the gPTP domain. This is achieved by the ptp4l daemon.

To synchronize PHC with the GM clock, run the command below. Make sure to replace eth0 by the interface name corresponding to the TSN-capable NIC in the system.

sudo ptp4l -i eth0 -f configs/gPTP.cfg --step_threshold=1 -m

The file gPTP.cfg (available in configs folder of Linux PTP source) specified by the -f option contains the configuration options to required to run ptp4l in gPTP mode while the -i option specifies the network interface this instance of ptp4l is controlling. The --step_threshold is set so ptp4l converges faster when “time jumps” occur (more on this later). The -m option enables log messages on standard output.

By default, ptp4l triggers the BMCA to determine if the PHC can be elected GM. To force a particular role, check the masterOnly and slaveOnly configuration options from ptp4l. For further details, see ptp4l(8) manpage.

Run this step in all end-points of the network.

Synchronizing the System Clock

The System clock synchronization step is mandatory only for those systems where applications rely on system clock to schedule traffic or present data (such as AVTP plugins from ALSA and GStreamer frameworks).

PHC time is set in TAI coordinate [1] while System clock time is set in UTC coordinate [2]. To ensure System clocks (CLOCK_REALTIME and CLOCK_TAI) are properly set, configure the UTC-TAI offset in the system. This is done by a run-time option from ptp4l that is set via pmc utility tool as shown below.

sudo pmc -u -b 0 -t 1 "SET GRANDMASTER_SETTINGS_NP clockClass 248 \
        clockAccuracy 0xfe offsetScaledLogVariance 0xffff \
        currentUtcOffset 37 leap61 0 leap59 0 currentUtcOffsetValid 1 \
        ptpTimescale 1 timeTraceable 1 frequencyTraceable 0 \
        timeSource 0xa0"

Once UTC-TAI offset is properly set, synchronize the System clock with PHC.

sudo phc2sys -s eth0 -c CLOCK_REALTIME --step_threshold=1 \
        --transportSpecific=1 -w -m

The -s option specifies the PHC from eth0 interface as the master clock while the -c option specifies the System clock as slave clock. In the command above PHC disciplines the System clock, that is, the system clock is adjusted. The --transportSpecific option is required when running phc2sys in a gPTP domain. The --step_threshold is set so phc2sys converges faster when “time jumps” occurs. Finally, the -w option makes phc2sys wait until ptp4l is synchronized and the -m option enables log messages on standard output. For more information about phc2sys configuration option refer to phc2sys(8) manpage.

Checking Clocks Synchronization

On ptp4l, the slave devices report out the time offset calculated from the master. This information can be used to determine whether the systems have been synchronized. The output for ptp4l is:

ptp4l[5374018.735]: rms  787 max 1208 freq -38601 +/- 1071 delay  -14 +/-   0
ptp4l[5374019.735]: rms 1314 max 1380 freq -36204 +/- 346 delay   -14 +/-   0
ptp4l[5374020.735]: rms  836 max 1106 freq -35734 +/-  31 delay   -14 +/-   0
ptp4l[5374021.736]: rms  273 max  450 freq -35984 +/-  97 delay   -14 +/-   0
ptp4l[5374022.736]: rms   50 max   82 freq -36271 +/-  64 delay   -14 +/-   0
ptp4l[5374023.736]: rms   81 max   86 freq -36413 +/-  17 delay   -14 +/-   0

The rms value reported by ptp4l once the slave has locked with the GM shows the root mean square of the time offset between the PHC and the GM clock. If ptp4l consistently reports rms lower than 100 ns, the PHC is synchronized.

Like ptp4l, phc2sys reports the time offset between PHC and System Clock, which determines if the clocks are synchronized.

phc2sys[5374168.545]: CLOCK_REALTIME phc offset   -372582 s0 freq    +246 delay   6649
phc2sys[5374169.545]: CLOCK_REALTIME phc offset   -372832 s1 freq      -4 delay   6673
phc2sys[5374170.547]: CLOCK_REALTIME phc offset        68 s2 freq     +64 delay   6640
phc2sys[5374171.547]: CLOCK_REALTIME phc offset       -20 s2 freq      -3 delay   6687
phc2sys[5374172.547]: CLOCK_REALTIME phc offset        47 s2 freq     +58 delay   6619
phc2sys[5374173.548]: CLOCK_REALTIME phc offset       -40 s2 freq     -15 delay   6680

The offset information reported by phc2sys shows the time offset between the PHC and the System clock. If phc2sys consistently reports offset lower than 100 ns, the System clock is synchronized.

To verify the TAI offset set by the pmc command above has been correctly propagated to the kernel, read this offset value using the adjtimex() system call. For more information about the adjtimex() system call, see adjtimex(2) manpage.

To automate this process, this tutorial includes the check_clocks utility tool to verify whether Linux PTP daemons (ptp4l and phc2sys) have been properly configured and the clocks have been synchronized. Run the following to compile and run the utility:

gcc -o check_clocks check_clocks.c
sudo check_clocks -d eth0

The exptected output from check_clocks is:

Clocks on this system are synchronized :)

Avnu Automotive Profile

Due to the static nature of the automotive network, AVnu has specified the Automotive Profile which does some optimizations to gPTP to improve startup time and reduce network load. The main difference from gPTP is that the BMCA is disabled so each device is statically assigned as master or slave.

Linux PTP also supports the Automotive Profile. To run ptp4l in that mode, the command-line is the same as presented in Synchronizing the PHC but with a different configuration file. In the systems playing the master role, use the automotive-master.cfg file. In all other systems, use the automotive-slave.cfg file. For illustration, see the following command-line examples:

sudo ptp4l -i eth0 -f configs/automotive-master.cfg --step_threshold=1 -m
sudo ptp4l -i eth0 -f configs/automotive-slave.cfg --step_threshold=1 -m

Both these config files are available in the configs folder in Linux PTP source code. The slave devices should also configure the servo_offset_threshold and servo_num_offset_values config options. More information about the config options is available in ptp4l(8) manpage.

Time Jumps

When a jump in time occurs in the gPTP domain, the System clock can take a considerable amount of time to converge to the new time. This happens because the clock is synchronized by adjusting the frequency. For example, if the PHC time jumps by 1 second, empirical tests have shown that System clock can take up to 30 seconds to synchronize (considering offset less than 100 ns). The phc2sys daemon provides the --step_threshold=n option which sets a threshold. If the time jump is greater than n seconds, time is adjusted by stepping the clock (that means to adjust current time) instead of changing the frequency.

However, stepping the clock has its own downsides as well. All timers set to expire between the current time and the new time expire once the time is set. This can affect the real-time behavior of the systems. So, use clock stepping carefully.


In this section we discuss some issues that we have faced when trying to synchronize time using Linux PTP in different systems

System time isn’t synchronized with PHC

If PHC offset never goes below hundreds (of nanoseconds)- or if it suddenly spikes (as seen on phc2sys log) - leaving system time out of sync, this section provides some hints on what to do.

Confirm NTP is not running

An NTP service may be running and changing the system clock. On systems with systemd, run:

timedatectl | grep NTP

If the output shows NTP service: active, disable it:

timedatectl set-ntp false

Check if NTP has been disabled and run the clock synchronization steps again and verify that the clocks are in sync.

Check NetworkManager is not messing with the NIC

When NetworkManager is running, it may reset the NIC after the qdisc setup. In this situation, PHC and the system clock may be out of sync. Do not allow the NetworkManager to manage the TSN capable NIC. Add the following to the /etc/NetworkManager/NetworkManager.conf file:



Restart NetworkManager. Run the clock synchronization steps again and verify the clocks are in sync.

Ensure qdisc setup is done before clock synchronization

Qdisc setup resets the NIC, and that can make ptp4l out of sync. If any qdisc setup needs to be done after clocks are already in sync, repeat clock synchronization steps again and verify that the clocks are still in sync.

Confirm only one instance of ptp4l or phc2sys is running

Multiple instances of ptp4l or phc2sys adjusting a single clocksource or sending out Sync messages can put the clocks out of sync. So, ensure only a single instance (per network interface) of both the daemons is running at a time. pgrep can be useful to ensure only one instance of a particular process is running. Look at pgrep(1) manpage for more details.

Check power management settings

Several power management mechanisms exist that set components (such as the CPU, NIC and the PCIe bus between CPU and NIC) into modes where the power consumption is reduced if they are inactive for some time. However, in these modes it can take 100 µs or more until they are fully active again.

In such a situation, phc2sys might report timeouts or you might measure a large difference between System clock and PHC even though phc2sys reports only small offsets.

If your BIOS supports it, enable the Intel(R) Time Coordinated Computing (TCC) mode that optimizes/disables several power management mechanisms for usage in real-time applications. Otherwise, you should at least disable PCIe ASPM, e.g. with

echo performance > /sys/module/pcie_aspm/parameters/policy