Re: [PATCH] net: dsa: tag_brcm: legacy: fix daisy-chained switches

[Florian Fainelli]

On 3/17/23 09:49, Jonas Gorski wrote:
> On Fri, 17 Mar 2023 at 17:32, Andrew Lunn <andrew@xxxxxxx> wrote:
> > On Fri, Mar 17, 2023 at 01:08:15PM +0100, Álvaro Fernández Rojas wrote:
> > > When BCM63xx internal switches are connected to switches with a 4-byte
> > > Broadcom tag, it does not identify the packet as VLAN tagged, so it adds one
> > > based on its PVID (which is likely 0).
> > > Right now, the packet is received by the BCM63xx internal switch and the 6-byte
> > > tag is properly processed. The next step would to decode the corresponding
> > > 4-byte tag. However, the internal switch adds an invalid VLAN tag after the
> > > 6-byte tag and the 4-byte tag handling fails.
> > > In order to fix this we need to remove the invalid VLAN tag after the 6-byte
> > > tag before passing it to the 4-byte tag decoding.
> >
> > Is there an errata for this invalid VLAN tag? Or is the driver simply
> > missing some configuration for it to produce a valid VLAN tag?
> >
> > The description does not convince me you are fixing the correct
> > problem.
>
> This isn't a bug per se, it's just the interaction of a packet going
> through two tagging CPU ports.
>
> My understanding of the behaviour is:
>
> 1. The external switch inserts a 4-byte Broadcom header before the
> VLAN tag, and sends it to the internal switch.
> 2. The internal switch looks at the EtherType, finds it is not a VLAN
> EtherType, so assumes it is untagged, and adds a VLAN tag based on the
> configured PVID (which 0 in the default case).
> 3. The internal switch inserts a legacy 6-byte Broadcom header before
> the VLAN tag when forwarding to its CPU port.
>
> The internal switch does not know how to handle the (non-legacy)
> Broadcom tag, so it does not know that there is a VLAN tag after it.
>
> The internal switch enforces VLAN tags on its CPU port when it is in
> VLAN enabled mode, regardless what the VLAN table's untag bit says.
>
> The result is a bogus VID 0 and priority 0 tag between the two
> Broadcom Headers. The VID would likely change based on the PVID of the
> port of the external switch.

My understanding matches yours, at the very least, we should only strip 
off the VLAN tag == 0, in case we are stacked onto a 4-bytes Broadcom 
tag speaking switch, otherwise it seems to me we are stripping of VLAN 
tags a bait too greedily.
--
Florian