Date:   Tue, 19 Feb 2019 18:08:11 +0000
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Vivien Didelot <vivien.didelot@...il.com>
Cc:     Andrew Lunn <andrew@...n.ch>,
        Florian Fainelli <f.fainelli@...il.com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH net-next v2 2/3] net: dsa: mv88e6xxx: add support for
 bridge flags

On Tue, Feb 19, 2019 at 12:38:28PM -0500, Vivien Didelot wrote:
> Hi Russell,
> 
> On Tue, 19 Feb 2019 17:14:14 +0000, Russell King - ARM Linux admin <linux@...linux.org.uk> wrote:
> > > > > > +static unsigned long mv88e6xxx_bridge_flags_support(struct dsa_switch *ds)
> > > > > > +{
> > > > > > +	struct mv88e6xxx_chip *chip = ds->priv;
> > > > > > +	unsigned long support = 0;
> > > > > > +
> > > > > > +	if (chip->info->ops->port_set_egress_floods)
> > > > > > +		support |= BR_FLOOD | BR_MCAST_FLOOD;
> > > > > > +
> > > > > > +	return support;
> > > > > > +}
> > > > > 
> > > > > I think that it isn't necessary to propagate the notion of bridge flags down
> > > > > to the DSA drivers. It might be just enough to add:
> > > > > 
> > > > >     port_egress_flood(dsa_switch *ds, int port, bool uc, bool mc)
> > > > > 
> > > > > to dsa_switch_ops and set BR_FLOOD | BR_MCAST_FLOOD from the DSA core,
> > > > > if the targeted driver has ds->ops->port_set_egress_flood. What do you think?
> > > > 
> > > > There are two other flags that I haven't covered which the bridge code
> > > > expects to be offloaded, and those are the broadcast flood flag and
> > > > the learning flag.
> > > 
> > > I see. What does the bridge code do if these flags are set? Does it expect
> > > the underlying devices to handle ff:ff:ff:ff:ff:ff magically or does it
> > > program this entry into the bridged ports?
> > 
> > The bridge code defaults to all four flags set.  See new_nbp() in
> > net/bridge/br_if.c:
> > 
> > 	p->flags = BR_LEARNING | BR_FLOOD | BR_MCAST_FLOOD | BR_BCAST_FLOOD;
> > 
> > bridge(8) doesn't touch BR_BCAST_FLOOD, but it is made available to
> > userspace via netlink and IFLA_BRPORT_BCAST_FLOOD.  Hence, there's
> > no man page documentation for that flag.
> > 
> > According to br_flood() in net/bridge/br_forward.c, it controls
> > whether broadcast frames are flooded to all ports or not.  Changing
> > this flag is merely handled just like the multicast/unicast flooding
> > flags - a call is made to set the offloaded flags, and if it isn't
> > returned as being supported, a warning is printed.  No attempt is
> > made to create or change a forwarding entry for the broadcast MAC
> > address.
> 
> OK, thanks for the details. The programming of the broadcast MAC address
> must be handled in the core then, I will move this from mv88e6xxx up to the
> DSA layer later, but that's totally orthogonal here.
> 
> > 
> > bridge(8) does document BR_LEARNING via IFLA_BRPORT_LEARNING:
> > 
> >        learning on or learning off
> >               Controls whether a given port will learn MAC addresses from
> >               received traffic or not. If learning if off, the bridge will end
> >               up flooding any traffic for which it has no FDB entry. By
> >               default this flag is on.
> > 
> > > In the latter case we have almost nothing to do. In the former case, we can
> > > make the core call dsa_port_mdb_add on setup and when a VLAN is added.
> > > 
> > > mv88e6xxx tries to be smart and is already doing that and I'm really not a fan.
> > > 
> > > If tomorrow there's a switch capable of simply toggling a bit to do that,
> > > we can add a new ops and skip the port_mdb_add call in the core.
> > > 
> > > > I know that the Marvell switches don't have a bit to control the
> > > > broadcast flooding, that appears to be controlled via a static entry
> > > > in the ATU which would have to be modified as the broadcast flood flag
> > > > is manipulated.  I don't know how that is handled in other bridges.
> > > > 
> > > > Do we want to include the broadcast flood in the above prototype?
> > > > If we go for this, how do we detect which options a switch supports?
> > > 
> > > If the necessary dsa_switch_ops routine is correctly prototyped, having it
> > > implemented by a driver or not should be enough to inform the core that the
> > > related feature(s) is/are supported by the switch.
> > > 
> > > I'll try to give a bit more context on why I'd prefer this approach, hoping
> > > it makes sense: a switch driver does not need to understand bridge flags
> > > per-se, the core should give enough abstraction to this layer (and any other
> > > net-specifics). The core just needs to know if a driver can program this or
> > > that. More importantly, it can easily become messy to maintain switch-cases
> > > of arbitrary flags in all drivers and the core.
> > 
> > So, should we go the other way and have:
> > 
> > 	int (*port_learning)(struct dsa_switch *ds, int port, bool enable);
> > 	int (*port_egress_flood_uc)(struct dsa_switch *ds, int port, bool enable);
> > 	int (*port_egress_flood_mc)(struct dsa_switch *ds, int port, bool enable);
> > 	int (*port_egress_flood_bc)(struct dsa_switch *ds, int port, bool enable);
> > 
> > rather than trying to combine uc/mc into one?  It would mean that we'd
> > be performing more bus reads/writes, but I guess that doesn't matter
> > for these configuration parameters.
> 
> I like this very much. As long as these flags can be programmed in switch
> devices, these ops totally make sense.

Having these as separate functions means that we would then need
additional complexity in mv88e6xxx to store the per-port flooding state,
so we can do this:

        reg &= ~MV88E6352_PORT_CTL0_EGRESS_FLOODS_MASK;

        if (unicast && multicast)
                reg |= MV88E6352_PORT_CTL0_EGRESS_FLOODS_ALL_UNKNOWN_DA;
        else if (unicast)
                reg |= MV88E6352_PORT_CTL0_EGRESS_FLOODS_NO_UNKNOWN_MC_DA;
        else if (multicast)
                reg |= MV88E6352_PORT_CTL0_EGRESS_FLOODS_NO_UNKNOWN_UC_DA;
        else
                reg |= MV88E6352_PORT_CTL0_EGRESS_FLOODS_NO_UNKNOWN_DA;

for some of the switches.  It looks to me like mv88e6xxx would prefer
having at least both the unicast and multicast flags together.

Even without that, it means more code in mv88e6xxx to wrap each of
these calls between the DSA ops and the chip specific ops...

-- 
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