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New post, PTP
Signed-off-by: Joachim Wiberg <troglobit@gmail.com>
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‎_posts/2026-04-07-ptp-time-synchronization.md‎

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---
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title: "PTP Time Synchronization"
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author: troglobit
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date: 2026-04-07 10:00:00 +0200
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categories: [examples]
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tags: [ptp, ieee1588, gptp, timing, cli, marvell]
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---
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The Precision Time Protocol (PTP, IEEE 1588) synchronizes clocks across a
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network to sub-microsecond accuracy — far beyond what NTP can achieve. Infix
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supports both the standard IEEE 1588 profile and the gPTP profile (IEEE
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802.1AS) used in Time-Sensitive Networking. This post walks through the
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minimal configurations needed to get two devices talking PTP.
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For the full story (protocol concepts, port states, boundary clocks, message
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format, Wireshark tips), see the [PTP section of the User's Guide][1].
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### Hardware Timestamping Matters
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PTP accuracy depends critically on **hardware timestamping**: the NIC marks
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each packet with a precise timestamp as it crosses the wire, not in software
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where OS scheduling adds jitter in the hundreds of microseconds.
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Before we start, from the UNIX shell, verify that your interface supports
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hardware timestamps:
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```console
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admin@example:~$ sudo ethtool -T eth0
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Time stamping parameters for eth0:
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Capabilities:
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hardware-transmit (SOF_TIMESTAMPING_TX_HARDWARE)
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software-transmit (SOF_TIMESTAMPING_TX_SOFTWARE)
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hardware-receive (SOF_TIMESTAMPING_RX_HARDWARE)
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software-receive (SOF_TIMESTAMPING_RX_SOFTWARE)
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hardware-raw-clock (SOF_TIMESTAMPING_RAW_HARDWARE)
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PTP Hardware Clock: 0
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Hardware Transmit Timestamp Modes:
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off (HWTSTAMP_TX_OFF)
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on (HWTSTAMP_TX_ON)
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```
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You want `hardware-transmit` and `hardware-receive` in the output. If you
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only see the `software-*` variants, PTP will still run and the state machines
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will work, but offsets will be in the millisecond range rather than nanoseconds.
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#### Recommended Hardware
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From the [boards Infix officially supports][2], the **Marvell CN9130 CRB**
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is the recommended choice. Its Marvell 88E6393X 11-port switchcore is driven
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by the `mv88e6xxx` DSA driver, which has mature PTP hardware timestamping and
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is exercised regularly with `ptp4l`.
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The **Marvell EspressoBIN** (Armada 3720 with 88E6341 switch, same `mv88e6xxx`
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driver) also works well for PTP and is considerably cheaper — it is not as
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regularly tested but included in the `aarch64*_defconfig` and is a good choice
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if you already have one.
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The BPI-R3 and BPI-R4 are the most accessible boards, but the MediaTek MT7531
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switch driver does not yet support hardware PTP timestamps; run `ethtool -T`
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to verify before committing to a setup.
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> QEMU (the x86_64 virtual target) runs `ptp4l` and exercises the full
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> configuration and state machine path, which is useful during development.
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> But the virtual NIC only provides software timestamps, so do not expect
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> nanosecond offsets there.
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{: .prompt-info }
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### Ordinary Clock — Time-Receiver
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The simplest setup: one device tracks time from another. Configure the
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**time-receiver** first.
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```console
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admin@receiver:/> configure
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admin@receiver:/config/> edit ptp instance 0
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admin@receiver:/config/ptp/instance/0/> set default-ds domain-number 0
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admin@receiver:/config/ptp/instance/0/> set default-ds time-receiver-only true
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admin@receiver:/config/ptp/instance/0/> edit port 1
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admin@receiver:/config/ptp/…/0/port/1/> set underlying-interface eth0
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admin@receiver:/config/ptp/…/0/port/1/> leave
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```
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### Ordinary Clock — Grandmaster
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On the **grandmaster** side, lower `priority1` values win the clock election.
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Setting it to `1` ensures this device always wins when reachable.
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```console
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admin@grandmaster:/> configure
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admin@grandmaster:/config/> edit ptp instance 0
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admin@grandmaster:/config/ptp/instance/0/> set default-ds domain-number 0
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admin@grandmaster:/config/ptp/instance/0/> set default-ds priority1 1
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admin@grandmaster:/config/ptp/instance/0/> set default-ds priority2 1
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admin@grandmaster:/config/ptp/instance/0/> edit port 1
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admin@grandmaster:/config/ptp/…/0/port/1/> set underlying-interface eth0
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admin@grandmaster:/config/ptp/…/0/port/1/> leave
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```
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Connect `eth0` on both boards with a direct cable or through a PTP-transparent
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switch, then give it a few seconds to converge.
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### Verifying Synchronization
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On the time-receiver, `show ptp` displays the live offset and path delay:
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```console
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admin@receiver:/> show ptp
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PTP Instance 0 Ordinary Clock · domain 0
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────────────────────────────────────────────────────────────────────
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Clock identity : AA-BB-CC-FF-FE-11-22-33
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Grandmaster : DD-EE-FF-FF-FE-44-55-66
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Priority1/Priority2 : 128 / 128
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GM Priority1/Priority2 : 1 / 1
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Clock class : cc-time-receiver-only
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GM clock class : cc-primary-sync
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Time source : internal-oscillator
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PTP timescale : yes
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UTC offset : 37 s
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Time traceable : yes
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Freq. traceable : yes
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Offset from GM : -42 ns
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Mean path delay : 1250 ns
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Steps removed : 1
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────────────────────────────────────────────────────────────────────
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Ports
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PORT INTERFACE STATE DELAY LINK DELAY (ns)
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1 eth0 time-receiver E2E 0
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────────────────────────────────────────────────────────────────────
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Message Statistics (▼ rx ▲ tx)
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PORT INTERFACE SYNC ▼ SYNC ▲ ANN ▼ ANN ▲ PD ▼ PD ▲
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1 eth0 42 0 15 0 0 0
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```
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Port state is colour-coded in the live output — green for `time-transmitter`
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and `time-receiver`, yellow for transient states, red for faults.
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A port in `time-receiver` state with an offset in the tens-of-nanoseconds
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range confirms that hardware timestamping is working. If the port stays in
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`listening`, check the cable. If the offset is in the milliseconds, re-check
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`ethtool -T eth0` — you may be on software timestamping only.
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### gPTP / IEEE 802.1AS
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For TSN networks, swap the profile on both devices to `ieee802-dot1as`. The
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profile sets Layer 2 transport and P2P delay measurement implicitly, so no
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per-port configuration is needed. Time-receiver side:
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```console
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admin@receiver:/config/> edit ptp instance 0
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admin@receiver:/config/ptp/instance/0/> set default-ds profile ieee802-dot1as
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admin@receiver:/config/ptp/instance/0/> set default-ds domain-number 0
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admin@receiver:/config/ptp/instance/0/> set default-ds time-receiver-only true
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admin@receiver:/config/ptp/instance/0/> edit port 1
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admin@receiver:/config/ptp/…/0/port/1/> set underlying-interface eth0
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admin@receiver:/config/ptp/…/0/port/1/> leave
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```
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Set `profile ieee802-dot1as` with `priority1 1` on the grandmaster side the
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same way as the IEEE 1588 example above.
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### Saving the Configuration
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Remember to save your changes for next boot:
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```console
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admin@example:/> copy running-config startup-config
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```
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> For boundary clocks, transparent clocks, multiple simultaneous instances,
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> port interval tuning, and monitoring with Wireshark, see the
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> [User's Guide][1].
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{: .prompt-info }
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[1]: https://www.kernelkit.org/infix/latest/ptp/
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[2]: /posts/router-boards/

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