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From:   Vladimir Oltean <vladimir.oltean@nxp.com>
To:     Florian Fainelli <f.fainelli@gmail.com>
CC:     Maxime Chevallier <maxime.chevallier@bootlin.com>,
        "davem@davemloft.net" <davem@davemloft.net>,
        Rob Herring <robh+dt@kernel.org>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
        "thomas.petazzoni@bootlin.com" <thomas.petazzoni@bootlin.com>,
        Andrew Lunn <andrew@lunn.ch>,
        Heiner Kallweit <hkallweit1@gmail.com>,
        Russell King <linux@armlinux.org.uk>,
        "linux-arm-kernel@lists.infradead.org" 
        <linux-arm-kernel@lists.infradead.org>,
        Luka Perkov <luka.perkov@sartura.hr>,
        Robert Marko <robert.marko@sartura.hr>
Subject: Re: [PATCH net-next v2 2/5] net: dsa: add out-of-band tagging
 protocol
Thread-Topic: [PATCH net-next v2 2/5] net: dsa: add out-of-band tagging
 protocol
Thread-Index: AQHYZ6RHhg5iGg0Rm0uC0ArSy8F0MK0e99AAgAOw5ACAA6gZgIAAJsyAgAAGiQA=
Date:   Thu, 19 May 2022 17:34:37 +0000
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On Thu, May 19, 2022 at 10:11:13AM -0700, Florian Fainelli wrote:
> unless we somehow manage to put it in the linear portion of
> the SKB to avoid using any control buffer or extension.

But how? Essentially the DSA master has to look at a packet and
determine whether it came from DSA based on something which non-DSA
code could not have done. In fact, I'm looking at the calls to
skb_reset_mac_{header,len} from net/core/skbuff.c, specifically at VLAN
and MPLS, and I believe (but haven't tested) that pushing such headers
would also alter skb->mac_len to some value !=3D ETH_HLEN. So simply
having the DSA master check whether DSA was there by checking whether
skb->mac_len is ETH_HLEN + DSA tag len could easily confuse DSA with
some other protocol of same header size.=