netdev - Re: [PATCH] net: dsa: tag_rtl4_a: Support also egress tags

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Date:   Wed, 17 Feb 2021 13:41:17 +0200
From:   Vladimir Oltean <olteanv@...il.com>
To:     DENG Qingfang <dqfext@...il.com>
Cc:     Andrew Lunn <andrew@...n.ch>,
        Linus Walleij <linus.walleij@...aro.org>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>,
        Mauri Sandberg <sandberg@...lfence.com>
Subject: Re: [PATCH] net: dsa: tag_rtl4_a: Support also egress tags

On Wed, Feb 17, 2021 at 01:37:21PM +0800, DENG Qingfang wrote:
> On Wed, Feb 17, 2021 at 11:29 AM Andrew Lunn <andrew@...n.ch> wrote:
> >
> > > +     /* Pad out to at least 60 bytes */
> > > +     if (unlikely(eth_skb_pad(skb)))
> > > +             return NULL;
> >
> > The core will do the padding for you. Turn on .tail_tag in
> > dsa_device_ops.
> >
> 
> Please don't. .tail_tag does far more than just padding.

Qingfang is right, this is an EtherType tagging protocol not a tail tagger.

Note that there was an interesting discussion with Florian and Kuba
around the Broadcom switches that require padding for short frames too:
https://patchwork.kernel.org/project/netdevbpf/patch/20201101191620.589272-10-vladimir.oltean@nxp.com/