Date:   Tue, 31 Aug 2021 22:05:59 +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 <netdev@...r.kernel.org>,
        DENG Qingfang <dqfext@...il.com>,
        Mauri Sandberg <sandberg@...lfence.com>
Subject: Re: [PATCH net] net: dsa: tag_rtl4_a: Fix egress tags

On Tue, Aug 31, 2021 at 08:35:05PM +0200, Linus Walleij wrote:
> On Tue, Aug 31, 2021 at 12:20 AM Vladimir Oltean <olteanv@...il.com> wrote:
> 
> > > > Does it get broadcast, or forwarded by MAC DA/VLAN ID as you'd expect
> > > > for a regular data packet?
> > >
> > > It gets broadcast :/
> >
> > Okay, so a packet sent to a port mask of zero behaves just the same as a
> > packet sent to a port mask of all ones is what you're saying?
> > Sounds a bit... implausible?
> >
> > When I phrased the question whether it gets "forwarded by MAC DA/VLAN ID",
> > obviously this includes the possibility of _flooding_, if the MAC
> > DA/VLAN ID is unknown to the FDB. The behavior of flooding a packet due
> > to unknown destination can be practically indistinguishable from a
> > "broadcast" (the latter having the sense that "you've told the switch to
> > broadcast this packet to all ports", at least this is what is implied by
> > the context of your commit message).
> >
> > The point is that if the destination is not unknown, the packet is not
> > flooded (or "broadcast" as you say). So "broadcast" would be effectively
> > a mischaracterization of the behavior.
> 
> Oh OK sorry what I mean is that the packet appears on all ports of
> the switch. Not sent to the broadcast address.

Yes, but why (due to which hardware decision does this behavior take place)?

I was not hung up on the "broadcast" word. That was used a bit imprecisely,
but I got over that. I was curious as to _why_ would the packets be
delivered to all ports of the switch. After all, you told the switch to
send the packet to _no_ port :-/

The reason why I'm so interested about this is because other switches
(mt7530) treat a destination port mask of 0x0 as "look up the FDB"
(reported by Qingfang here):
https://patchwork.kernel.org/project/netdevbpf/patch/20210825083832.2425886-3-dqfext@gmail.com/#24407683

This means it would be possible to implement the bridge TX forwarding
offload feature:
https://patchwork.kernel.org/project/netdevbpf/cover/20210722155542.2897921-1-vladimir.oltean@nxp.com/

I just wanted to know what type of packets were you testing with. If you
were testing with a unidirectional stream (where the switch has no
opportunity to learn the destination MAC on a particular port), then it
is much more likely that what's happening in your case is that the
packets were flooded, and not simply "broadcast". Pick a different MAC
DA, which _is_ learned in the FDB, and the packets would not be
"broadcast" (actually flooded) at all.

This is still my hypothesis about what was going on.

> > Just want to make sure that the switch does indeed "broadcast" packets
> > with a destination port mask of zero. Also curious if by "all ports",
> > the CPU port itself is also included (effectively looping back the packet)?
> 
> It does not seem to appear at the CPU port. It appear on ports
> 0..4.

Which again would be consistent with my theory.

> > > > > -     out = (RTL4_A_PROTOCOL_RTL8366RB << 12) | (2 << 8);
> > > >
> > > > What was 2 << 8? This patch changes that part.
> > >
> > > It was a bit set in the ingress packets, we don't really know
> > > what egress tag bits there are so first I just copied this
> > > and since it turns out the bits in the lower order are not
> > > correct I dropped this too and it works fine.
> > >
> > > Do you want me to clarify in the commit message and
> > > resend?
> >
> > Well, it is definitely not a logical part of the change. Also, a bug fix
> > patch that goes to stable kernels seems like the last place to me where
> > you'd want to change something that you don't really know what it does...
> > In net-next, this extra change is more than welcome. Possibly has
> > something to do with hardware address learning on the CPU port, but this
> > is just a very wild guess based on some other Realtek tagging protocol
> > drivers I've looked at recently. Anyway, more than likely not just a
> > random number with no effect.
> 
> Yeah but I don't know anything else about it than that it appear
> in the ingress packets which are known to have a different
> format than the egress packets, the assumption that they were
> the same format was wrong ... so it seems best to drop it as well.
> But if you insist I can defer that to a separate patch for next.
> I just can't see that it has any effect at all.

I was about to say that you can do as you wish, but I see you've resent
already. In general it is good to keep a change to the point, and
documented well and clear.

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