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

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Message-ID: <13348f01-2c68-c0a6-3bd8-a111fb0e565b@intel.com>
Date:   Thu, 14 Jan 2021 14:58:33 -0800
From:   Jacob Keller <jacob.e.keller@...el.com>
To:     Jakub Kicinski <kuba@...nel.org>, Jiri Pirko <jiri@...nulli.us>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, roopa@...dia.com,
        mlxsw@...dia.com
Subject: Re: [patch net-next RFC 00/10] introduce line card support for
 modular switch



On 1/13/2021 6:27 PM, Jakub Kicinski wrote:
> On Wed, 13 Jan 2021 13:12:12 +0100 Jiri Pirko wrote:
>> This patchset introduces support for modular switch systems.
>> NVIDIA Mellanox SN4800 is an example of such. It contains 8 slots
>> to accomodate line cards. Available line cards include:
>> 16X 100GbE (QSFP28)
>> 8X 200GbE (QSFP56)
>> 4X 400GbE (QSFP-DD)
>>
>> Similar to split cabels, it is essencial for the correctness of
>> configuration and funcionality to treat the line card entities
>> in the same way, no matter the line card is inserted or not.
>> Meaning, the netdevice of a line card port cannot just disappear
>> when line card is removed. Also, system admin needs to be able
>> to apply configuration on netdevices belonging to line card port
>> even before the linecard gets inserted.
> 
> I don't understand why that would be. Please provide reasoning, 
> e.g. what the FW/HW limitation is.
> 

I agree, I wouldn't imagine that plugging or unplugging line cards is
expected to be done on a regular basis?

>> To resolve this, a concept of "provisioning" is introduced.
>> The user may "provision" certain slot with a line card type.
>> Driver then creates all instances (devlink ports, netdevices, etc)
>> related to this line card type. The carrier of netdevices stays down.
>> Once the line card is inserted and activated, the carrier of the
>> related netdevices goes up.
> 
> Dunno what "line card" means for Mellovidia but I don't think 
> the analogy of port splitting works. To my knowledge traditional
> line cards often carry processors w/ full MACs etc. so I'd say 
> plugging in a line card is much more like plugging in a new NIC.
> 

Even if they didn't...

> There is no way to tell a breakout cable from normal one, so the
> system has no chance to magically configure itself. Besides SFP
> is just plugging a cable, not a module of the system.. 
> 
If you're able to tell what is plugged in, why would we want to force
user to provision ahead of time? Wouldn't it make more sense to just
instantiate them as the card is plugged in? I guess it might be useful
to allow programming the netdevices before the cable is actually
inserted... I guess I don't see why that is valuable.

It would be sort of like if you provision a PCI slot before a device is
plugged into it..