netdev - Re: [PATCH 1/1] net: dsa: fix tag

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Message-ID: <20241025141810.b37jxsaz2jjhxhvb@skbuf>
Date: Fri, 25 Oct 2024 17:18:10 +0300
From: Vladimir Oltean <vladimir.oltean@....com>
To: Hervé Gourmelon <herve.gourmelon@...nops.com>
Cc: Andrew Lunn <andrew@...n.ch>, Florian Fainelli <f.fainelli@...il.com>,
	Vivien Didelot <vivien.didelot@...il.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Tobias Waldekranz <tobias@...dekranz.com>
Subject: Re: [PATCH 1/1] net: dsa: fix tag_dsa.c for untagged VLANs

On Fri, Oct 25, 2024 at 01:46:48PM +0000, Hervé Gourmelon wrote:
> Hello,
> 
> Trying to set up an untagged VLAN bridge on a DSA architecture of 
> mv88e6xxx switches, I realized that whenever I tried to emit a 
> 'From_CPU' or 'Forward' DSA packet, it would always egress with an 
> unwanted 802.1Q header on the bridge port.

What does the link partner see? The packet with the 8021.Q tag or
without it?

The packet will always exit the Linux bridge layer as VLAN-tagged,
because skb->offload_fwd_mark will be set*. It will appear with the VLAN
tag in tcpdump, but it should not appear with the VLAN tag on the wire
or on the other side, if the VLAN on the bridge port is egress-untagged.
If you only see this in tcpdump, it is expected behavior and not a problem.

*in turn, that is because we set tx_fwd_offload to true, and the bridge
layer entrusts DSA that it will send the packet into the right VLAN.
Hence the unconditional presence of the tag, and the reliance upon the
port egress setting to strip it in hardware where needed.