linux-kernel - Re: [PATCH net-next v1 2/9] net: dsa: tag

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Date:   Sat, 3 Apr 2021 16:46:06 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Oleksij Rempel <o.rempel@...gutronix.de>
Cc:     Andrew Lunn <andrew@...n.ch>,
        Florian Fainelli <f.fainelli@...il.com>,
        netdev@...r.kernel.org, Russell King <linux@...linux.org.uk>,
        "David S. Miller" <davem@...emloft.net>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Jakub Kicinski <kuba@...nel.org>, linux-mips@...r.kernel.org,
        Vivien Didelot <vivien.didelot@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next v1 2/9] net: dsa: tag_ar9331: detect IGMP and
 MLD packets

On Sat, Apr 03, 2021 at 03:26:36PM +0200, Oleksij Rempel wrote:
> On Sat, Apr 03, 2021 at 04:03:18PM +0300, Vladimir Oltean wrote:
> > Hi Oleksij,
> > 
> > On Sat, Apr 03, 2021 at 01:48:41PM +0200, Oleksij Rempel wrote:
> > > The ar9331 switch is not forwarding IGMP and MLD packets if IGMP
> > > snooping is enabled. This patch is trying to mimic the HW heuristic to take
> > > same decisions as this switch would do to be able to tell the linux
> > > bridge if some packet was prabably forwarded or not.
> > > 
> > > Signed-off-by: Oleksij Rempel <o.rempel@...gutronix.de>
> > > ---
> > 
> > I am not familiar with IGMP/MLD, therefore I don't really understand
> > what problem you are trying to solve.
> > 
> > Your switch has packet traps for IGMP and MLD, ok. So it doesn't forward
> > them. Must the IGMP/MLD packets be forwarded by an IGMP/MLD snooping
> > bridge? Which ones and under what circumstances?
> 
> I'll better refer to the rfc:
> https://tools.ietf.org/html/rfc4541

Ok, the question might have been a little bit dumb.
I found this PDF:
https://www.alliedtelesis.com/sites/default/files/documents/how-alliedware/howto_config_igmp1.pdf
and it explains that:
- a snooper floods the Membership Query messages from the network's
  querier towards all ports that are not blocked by STP
- a snooper forwards all Membership Report messages from a client
  towards the All Groups port (which is how it reaches the querier).

I'm asking this because I just want to understand what the bridge code
does. Does the code path for IGMP_HOST_MEMBERSHIP_REPORT (for example)
for a snooper go through should_deliver -> nbp_switchdev_allowed_egress,
which is what you are affecting here?