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

[Florian Fainelli]

On 3/17/23 14:51, Álvaro Fernández Rojas wrote:
> El vie, 17 mar 2023 a las 17:55, Florian Fainelli
> (<f.fainelli@xxxxxxxxx>) escribió:
> > 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.
>
> Maybe I'm wrong here, but we're only removing the VLAN tag for a
> specific case in which we shouldn't have any kind of VLAN tag, right?
>
> For example, let's say we have an internal switch with the following ports:
> - 0: LAN 1
> - 4: RGMII -> External switch
> - 8: CPU -> enetsw controller
>
> And the external switch has the following ports:
> - 0: LAN 2
> - 1: LAN 3
> ...
> - 8: CPU -> Internal switch RGMII
>
> A. If we get a packet from LAN 1, it will only have the 6-bytes tag
> (and optionally the VLAN tag).
> When dsa_master_find_slave() is called, the net_device returned won't
> have any kind of DSA protocol and therefore netdev_uses_dsa() will
> return FALSE.
>
> B. However, when a packet is received from LAN 2/3, the first tag
> processed will be the 6-byte tag (corresponding to the internal
> switch).
> The 6-byte tag will identify this as coming from port 4 of the
> internal switch (RGMII) and therefore dsa_master_find_slave() will
> return the extsw interface which will have the DSA protocol of the
> 4-byte tag and netdev_uses_dsa() will return TRUE.
>
> Only for the second case the invalid VLAN tag will be removed and
> since extsw (RGMI) will never have VLANs enabled, I don't see the
> problem that you suggest about removing the VLAN tags too greedily.
>
> Am I wrong here?

I totally missed the netdev_uses_dsa() check you added on the looked up 
net_device, your explanation makes sense to me, thanks!
--
Florian