Re: Crosschip bridge functionality

[Andrew Lunn <andrew@lunn.ch>]

On Fri, Dec 23, 2022 at 11:37:47AM -0800, Colin Foster wrote:
> Hello,
> 
> I've been looking into what it would take to add the Distributed aspect
> to the Felix driver, and I have some general questions about the theory
> of operation and if there are any limitations I don't foresee. It might
> be a fair bit of work for me to get hardware to even test, so avoiding
> dead ends early would be really nice!
> 
> Also it seems like all the existing Felix-like hardware is all
> integrated into a SOC, so there's really no other potential users at
> this time.
> 
> For a distributed setup, it looks like I'd just need to create
> felix_crosschip_bridge_{join,leave} routines, and use the mv88e6xxx as a
> template. These routines would create internal VLANs where, assuming
> they use a tagging protocol that the switch can offload (your
> documentation specifically mentions Marvell-tagged frames for this
> reason, seemingly) everything should be fully offloaded to the switches.
> 
> What's the catch?

I actually think you need silicon support for this. Earlier versions
of the Marvell Switches are missing some functionality, which results
in VLANs leaking in distributed setups. I think the switches also
share information between themselves, over the DSA ports, i.e. the
ports between switches.

I've no idea if you can replicate the Marvell DSA concept with VLANs.
The Marvell header has D in DSA as a core concept. The SoC can request
a frame is sent out a specific port of a specific switch. And each
switch has a routing table which indicates what egress port to use to
go towards a specific switch. Frames received at the SoC indicate both
the ingress port and the ingress switch, etc.

> In the Marvell case, is there any gotcha where "under these scenarios,
> the controlling CPU needs to process packets at line rate"?

None that i know of. But i'm sure Marvell put a reasonable amount of
thought into how to make a distributed switch. There is at least one
patent covering the concept. It could be that a VLAN based
re-implemention could have such problems. 

	Andrew