[PATCH net v2 2/6] net: stmmac: Enable double VLAN processing only when supported

[Joseph Steel]

On Fri, Aug 21, 2026 at 05:09:55PM +0000, Ovidiu Panait wrote:
> stmmac_vlan_update() turns on double/S-VLAN processing whenever an
> 802.1ad VLAN is registered, without checking whether the MAC actually
> supports double VLAN processing. That capability is reported in
> dma_cap.dvlan.

This is not true. S-VLAN is never reported by the dma_cap.dvlan
feature flag. Double VLAN tagging and S-Tag VLAN features are fully
independent.

-Joseph

> 
> This was found while investigating a separate bug in the double VLAN RX
> stripping path. The outer 802.1ad tags were unexpectedly stripped by the
> MAC (because the ESVL bit was set).
> 
> Check dma_cap.dvlan before enabling EDVLP/ESVL/DOVLTC bits, so that double
> VLAN processing is enabled only on supported hardware. Also, advertise
> NETIF_F_HW_VLAN_STAG_RX and NETIF_F_HW_VLAN_STAG_FILTER only when
> dma_cap.dvlan is set.
>