Reader sent Geerling's CM4 PTP write-up, which shows two CM4s syncing to 10-15ns.
Our headline overclaimed ('impossible on a Pi 4') and our mechanism was wrong (we
blamed the bcmgenet MAC for not exposing a PHC).
The real story is better: the CM4 (BCM54210PE) and the 4B (BCM54213PE) report the
IDENTICAL PHY ID 0x600d84a2, so the kernel cannot tell them apart by asking the
chip. bcm-phy-ptp.c disambiguates by MDIO bus address instead — addr 0 = CM4,
proceed; addr 1 = 4B, return NULL. Ours is addr 1.
It is not a kernel config gap and not our RT patch: CONFIG_BCM_NET_PHYPTP=y and
CONFIG_NETWORK_PHY_TIMESTAMPING=y are both set. The driver loads, looks at the
board, and declines.
Correction is visible on the page rather than quietly patched. Credit to Geerling.
Ryan Malloy
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