Re: [PATCH] net: dsa: tag_rtl8_4: push VLAN tag into payload before DSA CPU tag

On Tue, Sep 08, 2026 at 09:30:41AM +0530, Amitesh Singh wrote:

When a socket buffer has a hardware-accelerated VLAN tag (skb->vlan_tci
set), the upstream NIC (e.g. imx-dwmac with tx-vlan-offload fixed:on)
inserts the 802.1Q header after the DSA CPU tag, producing:

  [8100 VID][8899 CPU tag]

on the wire instead of the correct ordering:

  [8899 CPU tag][8100 VID]

The switch reads 0x8100 as the EtherType, does not recognise a valid
CPU tag, and fails to strip it on egress. The raw 0x8899 tag then leaks
to the peer port, breaking any protocol (e.g. batman-adv over a VLAN
subinterface) that relies on seeing clean 802.1Q frames.

Fix this by calling __vlan_hwaccel_push_inside() to move the VLAN tag
into the skb payload before prepending the RTL8_4 DSA CPU tag in
rtl8_4_tag_xmit(). The helper frees the skb internally on allocation
failure, so returning NULL directly is correct and consistent with how
tag_sja1105.c handles the same pattern.

Please take a step back. Rather than have two drivers do this, then a
third, a fourth, ... should the core be doing this?

       Andrew