Pi 5 Ethernet drops packets at 10Mb/half duplex

[pelwell]

Describe the bug

It has been observed that the Pi 5/RP1 Ethernet interface drops packets when configured to 10Mb/s, half duplex. The evidence suggests that outgoing packets are transmitted correctly, but that the interface goes through periods of dropping received packets. In this time, none of the statistics on the EMAC change - it is as if nothing had arrived.

Steps to reproduce the behaviour

1. Add a point-to-point cable from a Pi 5 to another device - I used a laptop running Windows 11.
2. Configure both ends to use 10Mb/half, with static IP addresses on the same subnet. For Windows you may need to enable responses to ICMP packets in the firewall.
3. On the Pi, run sudo ping -f <peer>, where <peer> is the IP address of the other end of the link.
4. Observe the large quantity of lost packets, but with periods of successful pings.

Device (s)

Raspberry Pi 5

System

Verified in Linux 6.12.55, but probably much more widespread.

Logs

No response

Additional context

No response

[pelwell]

N.B. Introducing any switch into the system will avoid the problem, since it decouples the link requirements of the two ends, allowing the Pi 5 to run at 10Mb/full duplex or better even if the peer requires 10Mb/half duplex.

[pelwell]

10ms seems to be a magic packet interval - less than that and the link has the relative stability of the flood ping, sometimes not losing a single packet for many 10s of seconds. More than that and huge numbers of packets are dropped.

Also, Pi 4 (which uses the same PHY) is fine at 10Mb/half.

[pelwell]

Disabling power-down on the PHY (removing the brcm,powerdown-enable DT property) doesn't seem to make any difference.