netdev - Re: [PATCH net-next 4/8] net: dsa: rtl8366rb: Always treat VLAN 0 as untagged

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Message-ID: <20210913154536.v7rc7ln7ctcuqxl7@skbuf>
Date:   Mon, 13 Sep 2021 18:45:36 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Linus Walleij <linus.walleij@...aro.org>
Cc:     Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        Mauri Sandberg <sandberg@...lfence.com>,
        Alvin Šipraga <alsi@...g-olufsen.dk>,
        DENG Qingfang <dqfext@...il.com>
Subject: Re: [PATCH net-next 4/8] net: dsa: rtl8366rb: Always treat VLAN 0 as
 untagged

On Mon, Sep 13, 2021 at 04:42:56PM +0200, Linus Walleij wrote:
> VLAN 0 shall always be treated as untagged, as per example
> from other drivers (I guess from the spec).
> 
> Cc: Vladimir Oltean <olteanv@...il.com>
> Cc: Mauri Sandberg <sandberg@...lfence.com>
> Cc: Alvin Šipraga <alsi@...g-olufsen.dk>
> Cc: Florian Fainelli <f.fainelli@...il.com>
> Cc: DENG Qingfang <dqfext@...il.com>
> Signed-off-by: Linus Walleij <linus.walleij@...aro.org>
> ---
> ChangeLog v1->v4:
> - New patch after noting that other drivers always sets VLAN 0
>   as untagged.
> ---

"Other drivers" are not always a good example.

Technically speaking, IEEE 802.1Q-2018 wants switches to _preserve_ the
VID 0 found inside packets when forwarding them, but treat them the same
as untagged packets otherwise (aka classify them to the port's PVID, and
forward them according to the forwarding domain of the $(PVID) VLAN in
that bridge).

"Preserve" the VID 0 tag means "mark it as egress-tagged", so the
opposite of the change you are making.

Now, I know all too well it is not always possible to satisfy that
expectation, and we have had some back-and-forth on other drivers about
this, and ended up accepting the fact that the processing of VID 0 is
more or less broken. User space deals with that the best it can
(read as: sometimes it can't):
https://sourceforge.net/p/linuxptp/mailman/message/37318312/

But the justification given here to make VID 0 egress-untagged is pretty
weak as it is.