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

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Message-ID: <20210113182716.2b2aa8fa@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Wed, 13 Jan 2021 18:27:16 -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 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.

> 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.

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..