linux-kernel - RE: [PATCH net] net: renesas: rswitch: fix forwarding offload statemachine

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Message-ID:
 <TY4PR01MB142820FC29E751D8C7F0B7C348266A@TY4PR01MB14282.jpnprd01.prod.outlook.com>
Date: Fri, 6 Feb 2026 05:41:46 +0000
From: Michael Dege <michael.dege@...esas.com>
To: Nikita Yushchenko <nikita.yoush@...entembedded.com>, Yoshihiro Shimoda
	<yoshihiro.shimoda.uh@...esas.com>, Andrew Lunn <andrew+netdev@...n.ch>,
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>
CC: "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-renesas-soc@...r.kernel.org" <linux-renesas-soc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Christian
 Mardmoeller <christian.mardmoeller@...esas.com>, Dennis Ostermann
	<dennis.ostermann@...esas.com>
Subject: RE: [PATCH net] net: renesas: rswitch: fix forwarding offload
 statemachine

Hello Nikita,

> -----Original Message-----
> From: Nikita Yushchenko <nikita.yoush@...entembedded.com>
> Sent: Thursday, February 5, 2026 3:45 PM
> To: Michael Dege <michael.dege@...esas.com>; Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>;
> Andrew Lunn <andrew+netdev@...n.ch>; David S. Miller <davem@...emloft.net>; Eric Dumazet
> <edumazet@...gle.com>; Jakub Kicinski <kuba@...nel.org>; Paolo Abeni <pabeni@...hat.com>
> Cc: netdev@...r.kernel.org; linux-renesas-soc@...r.kernel.org; linux-kernel@...r.kernel.org
> Subject: Re: [PATCH net] net: renesas: rswitch: fix forwarding offload statemachine
> 
> 
> 
> WBR,
> Nikita Yushchenko,
> System Software Engineer @ Cogent Embedded
> 
> 05.02.2026 15:41, Nikita Yushchenko wrote:
> >>> The driver was originally designed to enable hardware forwarding
> >>> when not less than two ports are in forwarding state. When only one port has hw forwarding, there
> is no destination to forward.
> >>>
> >>> Nikita
> >>>
> >>
> >> The current driver allows Linux to use the bridge port as local port
> >> to the bridge. The offloading Also supports switching traffic to
> >> Linux through the bridge port. Therefore, the offloading shouldn't Be dropped if only one external
> port is up on the bridge.
> >
> > "Offloading" means - forward a frame from one hw port to other hw port
> > without inserting it into CPU queue. Offloaded frame is never visible to software bridge.
> >
> > There is code that allows offload only if the linux bridge device used
> > to connect rswitch ports does not have anything else.  If it has
> > something else, offloading is disabled (because there is no way to know when a frame can be
> processed within rswitch hw without sending it to cpu).
> 
> A frame being received by bridge device itself is not a subject for offload, ever.

Unfortunately, your argumentation is very _academic_. There is _no_practical_reason_, not to 
forward the traffic to the SW bridge via the HW bridge, even if only one link is currently up. 
Your suggestion to switch to port forwarding if only one external link is up, makes the 
overall HW switch handling overly complicated. Especially when VLANs come into the picture 
and this patch is leading towards the introduction of VLANs.

If you look at the behavior of the SW bridge, it does not change its operating state even if 
all links but one go down. So why should we go through the hassle to _temporarily_ switch the 
remaining port to port forwarding?

Another feature, "exception path for MAC learning", which will be coming for the VLAN 
Introduction probably makes the is whole offloading and learning handling obsolete. Then we 
can just set the forwarding rule in the HW bridge once a port is added to the bridge.

Best regards,

Michael