Re: [PATCH v2 net-next 07/11] net: dsa: kill .port_egress_floods overengineering

[Vladimir Oltean]

On Tue, Feb 09, 2021 at 05:19:32PM +0200, Vladimir Oltean wrote:
> From: Vladimir Oltean <vladimir.oltean@xxxxxxx>
>
> The bridge offloads the port flags through a single bit mask using
> switchdev, which among others, contains learning and flooding settings.
>
> The commit 57652796aa97 ("net: dsa: add support for bridge flags")
> missed one crucial aspect of the SWITCHDEV_ATTR_ID_PORT_BRIDGE_FLAGS API
> when designing the API one level lower, towards the drivers.
> This is that the bitmask of passed brport flags never has more than one
> bit set at a time. On the other hand, the prototype passed to the driver
> is .port_egress_floods(int port, bool unicast, bool multicast), which
> configures two flags at a time.
>
> DSA currently checks if .port_egress_floods is implemented, and if it
> is, reports both BR_FLOOD and BR_MCAST_FLOOD as supported. So the driver
> has no choice if it wants to inform the bridge that, for example, it
> can't configure unicast flooding independently of multicast flooding -
> the DSA mid layer is standing in the way. Or the other way around: a new
> driver wants to start configuring BR_BCAST_FLOOD separately, but what do
> we do with the rest, which only support unicast and multicast flooding?
> Do we report broadcast flooding configuration as supported for those
> too, and silently do nothing?
>
> Secondly, currently DSA deems the driver too dumb to deserve knowing that
> a SWITCHDEV_ATTR_ID_BRIDGE_MROUTER attribute was offloaded, because it
> just calls .port_egress_floods for the CPU port. When we'll add support
> for the plain SWITCHDEV_ATTR_ID_PORT_MROUTER, that will become a real
> problem because the flood settings will need to be held statefully in
> the DSA middle layer, otherwise changing the mrouter port attribute will
> impact the flooding attribute. And that's _assuming_ that the underlying
> hardware doesn't have anything else to do when a multicast router
> attaches to a port than flood unknown traffic to it. If it does, there
> will need to be a dedicated .port_set_mrouter anyway.
>
> Lastly, we have DSA drivers that have a backlink into a pure switchdev
> driver (felix -> ocelot). It seems reasonable that the other switchdev
> drivers should not have to suffer from the oddities of DSA overengineering,
> so keeping DSA a pass-through layer makes more sense there.
>
> To simplify the brport flags situation we just delete .port_egress_floods
> and we introduce a simple .port_bridge_flags which is passed to the
> driver. Also, the logic from dsa_port_mrouter is removed and a
> .port_set_mrouter is created.
>
> Functionally speaking, we simply move the calls to .port_egress_floods
> one step lower, in the two drivers that implement it: mv88e6xxx and b53,
> so things should work just as before.
>
> Signed-off-by: Vladimir Oltean <vladimir.oltean@xxxxxxx>
> ---

Florian, Andrew, what are your opinions on this patch? I guess what I
dislike the most about .port_egress_floods is that it combines the
unicast and multicast settings in the same callback, for no good
apparent reason. So that, at the very least, needs to change.
What do you prefer between having:
	.port_set_unicast_floods
	.port_set_multicast_floods
	.port_set_broadcast_floods
	.port_set_learning
and a single:
	.port_bridge_flags
?