Some pine64 models have a problem with the Gigabit ethernet dropping packets frequently. The Pine64 engineering team's supplied fix is to reconfigure the RTL8211E used to force off TXD/RXD (RXD is defaulting to on, in my checks) and turn on some bits in the configuration register for this PHY that are undocumented.
See: this dual BSD/GPL licensed driver change: https://github.com/longsleep/linux-pine64/commit/ffe3ca5be7682bbeb0fdadede29acd4a3c888015
As an aside- I wasn't sure how to handle the specifics of when to apply it, so I opted for doing an FDT check in rgephy_attach and simply enabling a 'quirk' if we're on an affected model and configured for RGMII (uses RTL8211E specifically). I don't know if this 'quirks' flag is overkill since this will probably be our only quirk.