PTP on Raspberrypi3 with Raspbian

This guide explains how to enable support for Precision Time Protocol (PTP) to a Raspberrypi3 running the Raspbian operating system.

This notes were tested with Raspbian Stretch Lite (2018-10-09), running kernel 4.14.71. The following type of tasks are required to have a working PTP client on the Raspberry:

• linux kernel must be configured, built and deployed to the target
• additional packages must be installed and configured

Preliminary operations

• Download Raspbian
• flash it on an SD card
• boot the OS

The following commands should be run from a terminal on the RaspberryPi target: a local or SSH connection for the Raspbian default user pi is assumed from now on. Note that SSH server is not enabled by default on Raspbian distribution.

(optionally) upgrade the distribution

sudo apt update
sudo apt upgrade

Install ptp related packages

sudo apt update
sudo apt install ethtool linuxptp

Enable software timestamping in ptp4l configuration

Raspberry Pi ethernet phy does not support hardware timestamping: hence software emulation must be enabled in ptp4l configuration. It can be done by patching ptp4l configuration file as follows:

sed -i -e 's/time_stamping.*$/time_stamping\t\tsoftware/' /etc/linuxptp/ptp4l.conf

Less recent kernels required patching the ethernet driver to overcome a limitation with frame timestamping. With those kernels, ptp4l fails to start while complaining for missing support for timestamping. If you are running older kernels, please either consider upgrading or see here for instructions on patching the kernel and fix the timestamping feature.

Build a new kernel with PTP support enabled

In order to build the kernel with required patches and configuration options, kernel sources must be fetched and some required build tools must be installed.

git clone --depth=1 https://github.com/raspberrypi/linux
sudo apt install git bc libncurses5-dev
cd linux
KERNEL=kernel7
make bcm2709_defconfig
make -j4 zImage modules dtbs

This will build the kernel in its default configuration and will take a long time. If anything fails, up to here please see the kernel building raspberrypi.org official page.

Apply required configuration changes

Some extra configuration options must be applied to the default bcm2709_defconfig. This can be done as follows. Run these commands from within the linux kernel sources directory.

echo 'CONFIG_NETWORK_PHY_TIMESTAMPING=y' >> .config
echo 'CONFIG_PTP_1588_CLOCK=y' >> .config

The above commands work if you already built the default kernel before applying the pathes. Otherwise you can enable them manually with any default kernel configuration target like make menuconfig.

Build kernel with PTP enabling changes

It is now possible to build the kernel with the changes we made to support PTP:

make olddefconfig
make -j4 zImage modules dtbs

The new kernel image, modules and dtbs must be installed over the current ones. Backing up current files is suggested in case something goes wrong and you want to rollback.

sudo make modules_install
sudo cp arch/arm/boot/dts/*.dtb /boot/
sudo cp arch/arm/boot/dts/overlays/*.dtb* /boot/overlays/
sudo cp arch/arm/boot/dts/overlays/README /boot/overlays/
sudo cp arch/arm/boot/zImage /boot/$KERNEL.img

The RaspberryPi is now ready to run ptp4l and can be rebooted for changes to take effect.

If you want to cross-compile the kernel on a faster host instead of running the process on the Raspberrypi itself, please refer to the general kernel cross-compile instruction here.

Troubleshooting

• As mentioned above, you can use ethtool to verify that software Tx timestamping is available on the ethernet interface. The expected output of the command should be:

pi@raspberrypi:~ $ ethtool -T eth0
Time stamping parameters for eth0:
Capabilities:
        software-transmit     (SOF_TIMESTAMPING_TX_SOFTWARE)
        software-receive      (SOF_TIMESTAMPING_RX_SOFTWARE)
        software-system-clock (SOF_TIMESTAMPING_SOFTWARE)
PTP Hardware Clock: none
Hardware Transmit Timestamp Modes: none
Hardware Receive Filter Modes: none

If you don't see SOF_TIMESTAMPING_TX_SOFTWARE listed, you might be running an older kernel which requires patching the timestamping support (see kernel-patching.md).

• ptp4l is automatically started as a systemd service at boot in Raspbian. You can inspect the service status with systemctl status ptp4l. If in the log the message failed to create a clock appears, it likely means the required kernel configuration bits were not enabled.