netdev - Re: [PATCH v3 net-next 2/4] net: dsa: Link aggregation support

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Date:   Thu, 03 Dec 2020 22:35:12 +0100
From:   Tobias Waldekranz <tobias@...dekranz.com>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Vladimir Oltean <olteanv@...il.com>, davem@...emloft.net,
        kuba@...nel.org, vivien.didelot@...il.com, f.fainelli@...il.com,
        j.vosburgh@...il.com, vfalico@...il.com, andy@...yhouse.net,
        netdev@...r.kernel.org
Subject: Re: [PATCH v3 net-next 2/4] net: dsa: Link aggregation support

On Thu, Dec 03, 2020 at 22:09, Andrew Lunn <andrew@...n.ch> wrote:
>> One could argue that if Linus had received an error instead, adapted his
>> teamd config and tried again, he would be a happier user and we might
>> not have to compete with his OS.
>> 
>> I am not sure which way is the correct one, but I do not think that it
>> necessarily _always_ correct to silently fallback to a non-offloaded
>> mode.
>
> This is an argument Mellanox makes, where falling back to software
> would be a bad idea given the huge bandwidth of their hardware
> accelerator, and the slow bandwidth of their CPU.

Yeah when you have 100G interfaces the choice becomes easier :)

> If the switch has no hardware support for LAG at all, then falling
> back to software is reasonable. It is less clear when there is some
> support in the switch. If we do reject it, we should try to make use
> of extack to give the user a useful error messages: Not supported, try
> configuration XYZ. But i guess that needs some plumbing, getting
> extack available in the place we make the decision.

I do not think we need to add anything. Looking at mlxsw, the extack is
available in the `struct netdev_notifier_changeupper_info`.

I am leaning towards the behavior you just described:

- If there is no offloading available, accept anything and let the
  software take care of it. The user wants a LAG and that is the best we
  can do.

- If offloading is available, reject anything that can not be
  offloaded. My guess is that _any_ hardware offloaded setup will almost
  always yield a better solution for the user than a software fallback.