The ethernet driver for RaspberryPi (smsc95xx) which comes with older (i.e.
4.9.y) kernel sources was missing support for software Tx timestamping feature.
This fix has already been available in mainline linux sources for a while. If
you use a more recent kernel and need to check if this patch is required or
not, you can use the ethtool command to verify whether
SOF_TIMESTAMPING_TX_SOFTWARE is listed or not. If it's in the list, you can
skip to the next section, otherwise patching is required.
Required patch can be found in
patches/0001-smsc95xx-use-generic-ethtool_op_get_ts_info-callback.patch. You can
apply them with the patch command or with a git workflow in a separate branch
as follow. Run these commands from within the linux kernel sources directory.
git checkout -b ptp-patches
git am <path-to-this-repo>/patches/0001-smsc95xx-use-generic-ethtool_op_get_ts_info-callback
Then proceed with the building instruction at README.md