Message-ID: <20210118130009.GU3565223@nanopsycho.orion>
Date:   Mon, 18 Jan 2021 14:00:09 +0100
From:   Jiri Pirko <jiri@...nulli.us>
To:     Jakub Kicinski <kuba@...nel.org>
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

Fri, Jan 15, 2021 at 08:26:17PM CET, kuba@...nel.org wrote:
>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.

Ah, got you.


>
>> >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.

Hmm, rare in scale is common...


>Reconfiguring routing is not the end of the world.

Well, yes, but you don't really want netdevices to come and go then you
plug in/out cables/modules. That's why we have split implemented as we
do. I don't understand why do you think linecards are different.

Plus, I'm not really sure that our hw can report the type, will check.
One way or another, I think that both configuration flows have valid
usecase. Some user may want pre-configuration, some user may want auto.
Btw, it is possible to implement splitter cable in auto mode as well.


>
>> >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 :)

Well, it can contain much more. It can contain a smartnic/fpga/whatever
for example. Not sure we can find something that fits to all cases.
I was thinking about it in the past, I think that the linecard is quite
appropriate. It connects with lines/lanes, and it does something,
either phy/gearbox, or just interconnects the lanes using smartnic/fpga
for example.

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