netdev - Re: [patch net-next RFC 00/10] introduce line card support for modular switch

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Date:   Fri, 15 Jan 2021 11:26:17 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Jiri Pirko <jiri@...nulli.us>
Cc:     netdev@...r.kernel.org, davem@...emloft.net,
        jacob.e.keller@...el.com, roopa@...dia.com, mlxsw@...dia.com
Subject: Re: [patch net-next RFC 00/10] introduce line card support for
 modular switch

On Fri, 15 Jan 2021 15:39:06 +0100 Jiri Pirko wrote:
> >I'm not a SFP experts so maybe someone will correct me but AFAIU
> >the QSFP (for optics) is the same regardless of breakout. It's the
> >passive optical strands that are either bundled or not. So there is 
> >no way for the system to detect the cable type (AFAIK).  
> 
> For SFP module, you are able to detect those.

Not sure you understand what I'm saying. Maybe you're thinking about
DACs? This is a optical cable for breakout:

https://www.fs.com/products/68048.html

There is no electronics in it to "detect" things AFAIU. Same QSFP can
be used with this cable or a non-breakout.

> >Or to put it differently IMO the netdev should be provisioned if the
> >system has a port into which user can plug in a cable. When there is   
> 
> Not really. For slit cables, the ports are provisioned not matter which
> cable is connected, slitter 1->2/1->4 or 1->1 cable.
> 
> 
> >a line card-sized hole in the chassis, I'd be surprised to see ports.
> >
> >That said I never worked with real world routers so maybe that's what
> >they do. Maybe some with a Cisco router in the basement can tell us? :)  
> 
> The need for provision/pre-configure splitter/linecard is that the
> ports/netdevices do not disapper/reappear when you replace
> splitter/linecard. Consider a faulty linecard with one port burned. You
> just want to replace it with new one. And in that case, you really don't
> want kernel to remove netdevices and possibly mess up routing for
> example.

Having a single burned port sounds like a relatively rare scenario.
Reconfiguring routing is not the end of the world.

> >If the device really needs this configuration / can't detect things
> >automatically, then we gotta do something like what you have.
> >The only question is do we still want to call it a line card.
> >Sounds more like a front panel module. At Netronome we called 
> >those phymods.  
> 
> Sure, the name is up to the discussion. We call it "linecard"
> internally. I don't care about the name.

Yeah, let's call it something more appropriate to indicate its
breakout/retimer/gearbox nature, and we'll be good :)