netdev - Re: [PATCH net] net: dsa: tag_rtl4

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Message-ID: <CACRpkdbX4XErV-7UCezobF4jLX-HvjMHE=dnYYLqD5Sb8LkCpw@mail.gmail.com>
Date:   Tue, 31 Aug 2021 20:35:05 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Vladimir Oltean <olteanv@...il.com>
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 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.

> 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.

> > > > -     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.

Yours,
Linus Walleij