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

[Jiri Pirko <jiri@resnulli.us>]

Thu, Jan 14, 2021 at 03:27:16AM CET, kuba@kernel.org 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.

Well, for split cable, you need to be able to say:
port 2, split into 4. And you will have 4 netdevices. These netdevices
you can use to put into bridge, configure mtu, speeds, routes, etc.
These will exist no matter if the splitter cable is actually inserted or
not.

With linecards, this is very similar. By provisioning, you also create
certain number of ports, according to the linecard that you plan to
insert. And similarly to the splitter, the netdevices are created.

You may combine the linecard/splitter config when splitter cable is
connected to a linecard port. Then you provision a linecard,
port is going to appear and you will split this port.


>
>> 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.

No. It is basically a phy gearbox. The mac is not there. The interface
between asic and linecard are lanes. The linecards is basically an
attachable phy.


>
>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..