RX packet drops on Raspberry Pi 5 not reflected in ethtool or softnet_stat

[richaso]

Describe the bug

RX dropped packets on Raspberry Pi 5 (RP1 Ethernet) not accounted for by ethtool or softnet_stat

Summary

Using a Raspberry Pi 5 (Model B Rev 1.0) with Raspberry Pi OS and the onboard RP1 Ethernet (driver: macb), I'm experiencing a consistent increase in RX packet drops as reported by ip -s link show eth0. However, these drops are not reflected in ethtool -S eth0 or /proc/net/softnet_stat.

This strongly suggests the packet loss is occurring between the NIC and the kernel network stack, likely due to RP1 driver (or DMA/IRQ handling) issues.

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Symptoms

1. RX drops seen with ip -s link show eth0:

   RX:  bytes packets errors dropped missed mcast
        4972630 27502   0      617     0     4248
   

2. No drops shown with ethtool -S eth0:

   q0_rx_dropped: 0
   rx_resource_errors: 0
   rx_overruns: 0

3. No softnet backlog congestion:

   grep . /proc/net/softnet_stat
   # all second columns (softnet drops) are 0

4. IRQs are handled only by CPU0 by default:

   cat /proc/interrupts | grep eth
   104:  44535  0  0  0  rp1_irq_chip  6 Level  eth0
   

5. Installing and enabling irqbalance does not stop the RX drops.

6. Behavior persists even using USB 3.0 Ethernet adapters (tested with AX88179).

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Diagnosis

• The kernel is counting dropped RX packets.
• The NIC driver (macb) and DMA interface do not acknowledge drops.
• The loss appears to happen in the path between NIC → driver → netdev → socket.
• The issue could be related to the RP1 integration or IRQ/DMA handling.
• Affects onboard Ethernet on Pi 5, not specific to cables or switch.

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Request

Please advise on:

• Whether this is a known issue with the RP1 or the macb driver
• Any recommended kernel module parameters or overlays
• Whether there are upcoming fixes in rpi-firmware or kernel updates
• How to help trace or test this further

Happy to provide more debug logs if needed.

Steps to reproduce the behaviour

Reproducing the issue

1. Boot Raspberry Pi 5 with default Raspberry Pi OS.
2. Connect to gigabit network.
3. Run any kind of sustained traffic (ping, DNS, iperf3).
4. Monitor ip -s link show eth0 and observe dropped increasing.
5. Check ethtool -S eth0 and /proc/net/softnet_stat: no errors reported.

Device (s)

Raspberry Pi 5

System

• Model: Raspberry Pi 5 Model B Rev 1.0
• OS: Raspberry Pi OS (64-bit, aarch64)
• Kernel:

  uname -a
  Linux pihole6 6.12.25+rpt-rpi-2712 #1 SMP PREEMPT Debian 1:6.12.25-1+rpt1 (2025-04-30) aarch64 GNU/Linux

• Ethernet driver:

  ethtool -i eth0
  driver: macb
  version: 6.12.25+rpt-rpi-2712

The issue is not related to physical network components. The following were tested and ruled out:

• Multiple high-quality Ethernet cables (Cat6 and Cat7)
• Different switch ports and switches (1 Gbps)
• Direct connection to a known-good Asus ET12 router
• USB 3.0 Ethernet adapters (with AX88179 chipset)
• The problem persists regardless of the physical interface used

This confirms that the RX packet drops are not caused by link instability, faulty cables, or external hardware.

[pelwell]

Duplicate of #6855?

[richaso]

Duplicate of #6855?

I've reviewed related issues (e.g., #6855), but my case appears to be different:

• In my case, RX drops are persistent and consistently reproducible, not intermittent.
• USB 3.0 Ethernet adapters (e.g. AX88179) also show drops, unlike in Onboard Ethernet on Raspberry Pi 5 experiences packet loss in LAN environment #6855.