linux-kernel - Re: [PATCH RFC net-next 07/10] net: dsa: mv88e6xxx: Track bridge mdb objects

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Date: Thu, 4 Apr 2024 16:43:38 -0400
From: Joseph Huang <joseph.huang.2024@...il.com>
To: Vladimir Oltean <olteanv@...il.com>,
 Joseph Huang <Joseph.Huang@...min.com>
Cc: netdev@...r.kernel.org, Andrew Lunn <andrew@...n.ch>,
 Florian Fainelli <f.fainelli@...il.com>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 Roopa Prabhu <roopa@...dia.com>, Nikolay Aleksandrov <razor@...ckwall.org>,
 Linus Lüssing <linus.luessing@...3.blue>,
 linux-kernel@...r.kernel.org, bridge@...ts.linux.dev
Subject: Re: [PATCH RFC net-next 07/10] net: dsa: mv88e6xxx: Track bridge mdb
 objects

Hi Vladimir,

On 4/2/2024 8:23 AM, Vladimir Oltean wrote:
> Can you comment on the feasibility/infeasibility of Tobias' proposal of:
> "The bridge could just provide some MDB iterator to save us from having
> to cache all the configured groups."?
> https://lore.kernel.org/netdev/87sg31n04a.fsf@waldekranz.com/
> 
> What is done here will have to be scaled to many drivers - potentially
> all existing DSA ones, as far as I'm aware.
> 

I thought about implementing an MDB iterator as suggested by Tobias, but 
I'm a bit concerned about the coherence of these MDB objects. In theory, 
when the device driver is trying to act on an event, the source of the 
trigger may have changed its state in the bridge already. If, upon 
receiving an event in the device driver, we iterate over what the bridge 
has at that instant, the differences between the worlds as seen by the 
bridge and the device driver might lead to some unexpected results. 
However, if we cache the MDB objects in the device driver, at least the 
order in which the events took place will be coherent and at any give 
time the state of the MDB objects in the device driver can be guaranteed 
to be sane. This is also the approach the prestera device driver took.

Thanks,
Joseph