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

[Florian Fainelli <f.fainelli@gmail.com>]

On 2/9/21 12:37 PM, Vladimir Oltean wrote:
> On Tue, Feb 09, 2021 at 05:19:32PM +0200, Vladimir Oltean wrote:
>> From: Vladimir Oltean <vladimir.oltean@nxp.com>
>>
>> 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@nxp.com>
>> ---
> 
> 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

Tough one, from a driver writer perspective the fewer callbacks to wire
up the better, but from a framework perspective it is certainly easier
to audit drivers if there is a callback for a narrow and specific use
case. My vote goes for the single callback, that would lead to an easier
patch set to review IMHO.
-- 
Florian