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RPi5_configs/README.md
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RPi5_configs/README.md
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# Raspberry Pi 5 — GPS/PPS PTP Grandmaster
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This directory contains the files used to turn the Raspberry Pi 5 into the PTP grandmaster for the lab.
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The intended time chain is:
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```text
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GT-U7 GNSS
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├─ NMEA over UART ──> gpsd/chrony
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└─ PPS on GPIO18 ──> chrony
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│
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▼
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CLOCK_REALTIME (UTC)
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│
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phc2sys
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│
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▼
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eth0 NIC PHC
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│
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ptp4l
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│
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▼
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PTP domain 0
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```
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The Pi therefore has two jobs:
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1. discipline the Linux system clock from GNSS/PPS using chrony; and
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2. copy that time into the Ethernet PHC and advertise it as the PTP grandmaster.
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## Files in this directory
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| File | Install/use as | Purpose |
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|---|---|---|
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| `config.txt` | execute the commands in the file once | Adds the Pi 5 UART and PPS overlays to `/boot/firmware/config.txt` |
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| `ptp-gm.conf` | `/etc/linuxptp/ptp4l-gm.conf` | `ptp4l` grandmaster configuration |
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| `ptp4l-gm.service` | `/etc/systemd/system/ptp4l-gm.service` | Starts the PTP grandmaster |
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| `phc2sys-gm.service` | `/etc/systemd/system/phc2sys-gm.service` | Synchronizes the `eth0` PHC from `CLOCK_REALTIME` |
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| `ptp_nic_coalesce.service` | `/etc/systemd/system/ptp_nic_coalesce.service` | Optional NIC interrupt-coalescing tuning |
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## 1. Install the required packages
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```bash
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sudo apt update
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sudo apt install gpsd gpsd-clients pps-tools chrony linuxptp ethtool
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```
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## 2. Configure the Pi UART and PPS input
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The `config.txt` file in this directory contains shell commands that append the required settings to the real Raspberry Pi boot configuration.
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Back up the boot configuration first:
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```bash
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sudo cp /boot/firmware/config.txt /boot/firmware/config.txt.pre-ptp
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```
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Then review and execute the commands from `config.txt`.
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The resulting lines in `/boot/firmware/config.txt` should include:
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```text
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# GPS UART on GPIO14/GPIO15 for Pi 5
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dtoverlay=uart0-pi5
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init_uart_baud=9600
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# GPS PPS on physical pin 12 / GPIO18
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dtoverlay=pps-rp1,pin=18,pull-down,schmitt-trigger
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```
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`pps-rp1` is a separate Pi 5 overlay. Make sure `pps-rp1.dtbo` has already been installed in the appropriate overlays directory before rebooting.
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Reboot the Pi:
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```bash
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sudo reboot
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```
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After reboot, verify that the UART and PPS device exist:
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```bash
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ls -l /dev/ttyAMA0 /dev/pps0
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sudo ppstest /dev/pps0
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```
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`ppstest` should report a new PPS assertion once per second.
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## 3. Configure gpsd
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Configure `/etc/default/gpsd` so gpsd opens both the GT-U7 UART and PPS device:
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```bash
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START_DAEMON="true"
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USBAUTO="true"
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DEVICES="/dev/ttyAMA0 /dev/pps0"
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GPSD_OPTIONS="-n"
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```
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Restart and verify gpsd:
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```bash
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sudo systemctl restart gpsd
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systemctl status gpsd
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cgps -s
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```
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## 4. Configure chrony
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Chrony disciplines `CLOCK_REALTIME` from the GNSS receiver. NMEA identifies the UTC second and PPS supplies the precise second boundary.
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Add the following refclocks to the chrony configuration used by the Pi:
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```text
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refclock SHM 0 refid NMEA offset 0.0 precision 1e-1 poll 3 noselect
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refclock PPS /dev/pps0 refid PPS lock NMEA poll 3
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```
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If this Pi should continue serving NTP to the lab, keep the required `allow` networks, for example:
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```text
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allow 192.168.0.0/16
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allow 10.0.0.0/8
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allow 172.16.0.0/12
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```
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Also keep:
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```text
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rtcsync
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makestep 1 3
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leapseclist /usr/share/zoneinfo/leap-seconds.list
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```
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Do **not** add `local stratum 1` for the GNSS/PPS setup. A selected local refclock already makes chronyd serve the correct NTP stratum; `local` is a separate fallback/local-reference mode.
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Restart chrony and verify that PPS becomes the selected source:
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```bash
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sudo systemctl restart chrony
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chronyc sources -v
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chronyc sourcestats
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chronyc tracking
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```
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A healthy setup should show PPS selected (`#* PPS` on common chrony versions).
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The NMEA `offset 0.0` in the example is only a starting point. If the serial NMEA sentence latency is large, measure it and adjust the NMEA offset later.
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## 5. Verify hardware timestamping on `eth0`
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Before enabling PTP, verify the Pi 5 NIC exposes a PHC and hardware TX/RX timestamping:
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```bash
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ethtool -T eth0
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```
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You want hardware TX/RX/raw timestamp support and a non-negative `PTP Hardware Clock` number.
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You can also identify the PHC with:
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```bash
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readlink -f /sys/class/net/eth0/device/ptp/ptp*
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```
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## 6. Install the PTP configuration and services
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From this directory, install the files as follows:
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```bash
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sudo install -D -m 0644 ptp-gm.conf /etc/linuxptp/ptp4l-gm.conf
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sudo install -D -m 0644 ptp4l-gm.service /etc/systemd/system/ptp4l-gm.service
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sudo install -D -m 0644 phc2sys-gm.service /etc/systemd/system/phc2sys-gm.service
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```
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The bundled PTP configuration uses:
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```text
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domainNumber 0
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network_transport UDPv4
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delay_mechanism E2E
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serverOnly 1
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clockClass 6
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clockAccuracy 0xFE
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timeSource 0x20
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utc_offset 37
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twoStepFlag 1
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```
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`clockAccuracy 0xFE` is intentionally conservative until actual accuracy has been measured. The `utc_offset` value is the PTP/TAI-to-UTC offset advertised to clients; review it if a future leap second changes the TAI-UTC difference.
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Reload systemd and enable the services:
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```bash
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sudo systemctl daemon-reload
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sudo systemctl enable --now ptp4l-gm.service
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sudo systemctl enable --now phc2sys-gm.service
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```
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The direction is important:
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```text
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chrony: GNSS/PPS -> CLOCK_REALTIME
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phc2sys-gm: CLOCK_REALTIME -> eth0 PHC
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ptp4l-gm: eth0 PHC -> PTP network
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```
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## 7. Optional NIC coalescing tuning
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`ptp_nic_coalesce.service` applies:
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```bash
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ethtool -C eth0 tx-usecs 4 rx-usecs 4
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```
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Treat this as an optional latency/jitter tuning step, not a requirement for PTP to work. First check what the driver supports:
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```bash
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ethtool -c eth0
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```
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If the settings are accepted and you want to keep them:
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```bash
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sudo install -D -m 0644 ptp_nic_coalesce.service /etc/systemd/system/ptp_nic_coalesce.service
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sudo systemctl daemon-reload
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sudo systemctl enable --now ptp_nic_coalesce.service
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```
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## 8. Verification
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Service state:
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```bash
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systemctl status chrony
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systemctl status ptp4l-gm
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systemctl status phc2sys-gm
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```
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Live logs:
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```bash
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journalctl -fu ptp4l-gm
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journalctl -fu phc2sys-gm
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```
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Query the local `ptp4l` management socket:
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```bash
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sudo pmc -u -b 0 'GET PORT_DATA_SET'
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sudo pmc -u -b 0 'GET GRANDMASTER_SETTINGS_NP'
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sudo pmc -u -b 0 'GET TIME_STATUS_NP'
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```
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Expected results include:
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```text
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portState MASTER/SERVER
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timeSource 0x20
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clockClass 6
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clockAccuracy 0xFE
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stepsRemoved 0
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```
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The `grandmasterIdentity` reported here is the identity that should remain visible through WOLF-HUNTER and NYATER.
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## 9. Troubleshooting checklist
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```bash
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# PPS exists and pulses
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sudo ppstest /dev/pps0
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# GPS data is arriving
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cgps -s
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# chrony actually selected PPS
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chronyc sources -v
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chronyc tracking
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# eth0 supports hardware PTP
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ethtool -T eth0
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# PTP services are healthy
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journalctl -u ptp4l-gm -n 100 --no-pager
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journalctl -u phc2sys-gm -n 100 --no-pager
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```
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Do not run another service that also tries to discipline the `eth0` PHC. Chrony may continue to discipline `CLOCK_REALTIME`; that is intentional on the grandmaster.
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