netdev - Re: [PATCH v2 net-next 06/10] net: dsa: Pass VLAN MSTI migration notifications to driver

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Date:   Wed, 9 Mar 2022 19:03:50 +0200
From:   Vladimir Oltean <olteanv@...il.com>
To:     Tobias Waldekranz <tobias@...dekranz.com>
Cc:     davem@...emloft.net, kuba@...nel.org, Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Jiri Pirko <jiri@...nulli.us>,
        Ivan Vecera <ivecera@...hat.com>,
        Roopa Prabhu <roopa@...dia.com>,
        Nikolay Aleksandrov <razor@...ckwall.org>,
        Russell King <linux@...linux.org.uk>,
        Petr Machata <petrm@...dia.com>,
        Cooper Lees <me@...perlees.com>,
        Ido Schimmel <idosch@...dia.com>,
        Matt Johnston <matt@...econstruct.com.au>,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        bridge@...ts.linux-foundation.org
Subject: Re: [PATCH v2 net-next 06/10] net: dsa: Pass VLAN MSTI migration
 notifications to driver

On Wed, Mar 09, 2022 at 04:47:02PM +0100, Tobias Waldekranz wrote:
> >> +int dsa_port_vlan_msti(struct dsa_port *dp, const struct switchdev_attr *attr)
> >> +{
> >> +	struct dsa_switch *ds = dp->ds;
> >> +
> >> +	if (!ds->ops->vlan_msti_set)
> >> +		return -EOPNOTSUPP;
> >> +
> >> +	return ds->ops->vlan_msti_set(ds, attr);
> >
> > I guess this doesn't need to be a cross-chip notifier event for all
> > switches, because replication to all bridge ports is handled by
> > switchdev_handle_port_attr_set(). Ok. But isn't it called too many times
> > per switch?
> 
> It is certainly called more times than necessary. But I'm not aware of
> any way to limit it. Just as with other bridge-global settings like
> ageing timeout, the bridge will just replicate the event to each port,
> not knowing whether some of them belong to the same underlying ASIC or
> not.
> 
> We could leverage hwdoms in the bridge to figure that out, but then:

Hmm, uncalled for. Also, not sure how it helps (it just plain doesn't
work, as you've pointed out below yourself).

> 
> - Drivers that do not implement forward offloading would miss out on
>   this optimization. Unfortunate but not a big deal.
> - Since DSA presents multi-chip trees as a single switchdev, the DSA
>   layer would have to replicate the event out to each device. Doable,
>   but feels like a series of its own.

I've mentally walked through the alternatives and I don't see a practical
alternative than letting the driver cut out the duplicate calls.

Maybe it's worth raising awareness by adding a comment above the
dsa_switch_ops :: vlan_msti_set definition that drivers should be
prepared to handle such calls.

Case in point, in mv88e6xxx_vlan_msti_set() you could avoid some useless
MDIO transactions (a call to mv88e6xxx_vtu_loadpurge) with a simple
"if (vlan.sid != new_sid)" check. Basically just go through a refcount
bump followed by an immediate drop.