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

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Message-ID: <20210119115115.GY3565223@nanopsycho.orion>
Date:   Tue, 19 Jan 2021 12:51:15 +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

Mon, Jan 18, 2021 at 06:59:28PM CET, kuba@...nel.org wrote:
>On Mon, 18 Jan 2021 14:00:09 +0100 Jiri Pirko wrote:
>> >> >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...
>
>Sure but at a scale of million switches it doesn't matter if a couple
>are re-configuring their routing.
>
>> >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.
>
>If I have an unused port it will still show up as a netdev.
>If I have an unused phymod slot w/ a slot cover in it, why would there
>be a netdev? Our definition of a physical port is something like "a
>socket for a networking cable on the outside of the device". With your
>code I can "provision" a phymod and there is no whole to plug in a
>cable. If we follow the same logic, if I have a server with PCIe
>hotplug, why can't I "provision" some netdevs for a NIC that I will
>plug in later?
>
>> Plus, I'm not really sure that our hw can report the type, will check.
>
>I think that's key.

So, it can't. The driver is only aware of "activation" of the linecard
being successful or not.


>
>> 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.
>
>Auto as in iterate over possible configs until link up? That's nasty.
>
>> >> >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.
>
>If it has a FPGA / NPU in it, it's definitely auto-discoverable. 
>I don't understand why you think that it's okay to "provision" NICs
>which aren't there but only for this particular use case.