Re: [PATCH net v2 2/2] net: stmmac: Consider Tx VLAN offload tag length for maxSDU

["G Thomas, Rohan" <rohan.g.thomas@altera.com>]

Hi Jakub,

On 9/26/2025 7:22 AM, Jakub Kicinski wrote:
> On Thu, 25 Sep 2025 16:33:21 +0530 G Thomas, Rohan wrote:
>> While testing 802.1AD with XGMAC hardware using a simple ping test, I
>> observed an unexpected behavior: the hardware appears to insert an
>> additional 802.1Q CTAG with VLAN ID 0. Despite this, the ping test
>> functions correctly.
>>
>> Here’s a snapshot from the pcap captured at the remote end. Outer VLAN
>> tag used is 100 and inner VLAN tag used is 200.
>>
>> Frame 1: 110 bytes on wire (880 bits), 110 bytes captured (880 bits)
>> Ethernet II, Src: <src> (<src>), Dst: <dst> (<dst>)
>> IEEE 802.1ad, ID: 100
>> 802.1Q Virtual LAN, PRI: 0, DEI: 0, ID: 0(unexpected)
>> 802.1Q Virtual LAN, PRI: 0, DEI: 0, ID: 200
>> Internet Protocol Version 4, Src: 192.168.4.10, Dst: 192.168.4.11
>> Internet Control Message Protocol
> 
> And the packet arrives at the driver with only the .1Q ID 200 pushed?
>

Yes, the packet arrives the driver with only 802.1Q ID.

[  210.192912] stmmaceth 10830000.ethernet eth0: >>> frame to be 
transmitted:
[  210.192917] len = 46 byte, buf addr: 0x0000000067c78222
[  210.192923] 00000000: xx xx xx xx xx xx xx xx xx xx xx xx 81 00 00 c8
[  210.192928] 00000010: 08 06 00 01 08 00 06 04 00 02 46 9b 06 1b 5b b6
[  210.192931] 00000020: c0 a8 04 0a c8 a3 62 0e d7 04 c0 a8 04 0b
> Indeed, that looks like a problem with the driver+HW interaction.
> IDK what the right terminology is but IIRC VLAN 0 is not a real VLAN,
> just an ID reserved for frames that don't have a VLAN ID but want to
> use the priority field. Which explains why it "works", receiver just
> ignores that tag. But it's definitely not correct because switches
> on the network will no see the real C-TAG after the S-TAG is stripped.

Yes, we are trying to figure out the right configuration for the driver
so that the right tag is inserted by the driver for double and single
VLANs. Based on the register configuration options for MAC_VLAN_Incl and
MAC_Inner_VLAN_Incl registers and descriptor configuration options
available, the hardware may not support simultaneous offloading of STAG
for 802.1AD double-tagged packets and CTAG for 802.1Q single-tagged
packets. If that is the case disable STAG insertion offloading may be
the right approach.

Best Regards,
Rohan