Re: [PATCH net-next 1/3] net: ethtool: add support for forward error correction modes

[Jakub Kicinski <kubakici@wp.pl>]

On Thu, 6 Jul 2017 18:53:42 +0000, Casey Leedom wrote:
> However, the first question which pops up is: what happens if a user
> explicitly selects a particular FEC for from the set offered by the
> current Transceiver Module, and then swap out Transceiver Modules to
> one which doesn't support that FEC?  Does the OS/Driver/Firmware
> "forget" the user's FEC setting?  Does it respect it and potentially
> not get a link established?  Do we "temporarily" put the user's FEC
> setting aside?  Or do we have FEC settings per-Transceiver Module
> Type?  Each choice has downsides in terms of "expected behavior" and
> the complexity of the abstraction model offered to the user.
> 
> In our discussions of the above, we decided that we should err in the
> direction of the absolutely simplest abstraction model, even when
> that might result in a failure to establish a link.  Our feeling was
> that doing anything else would result in endless user confusion.
> Basically, if it takes longer than a simple paragraph to explain,
> you're probably doing the wrong thing.  So, we decided that if a user
> sets up a particular FEC setting, we keep that regardless of conflict
> with different Transceiver Modules.  (But flag such issues in the
> System Log in order to try to give the user a chance to understand
> what the new cable they plugged in wasn't working.)

IMHO if something gets replugged all the settings should be reset.
I feel that it's not entirely unlike replugging a USB adapter.  Perhaps
we should introduce some (devlink) notifications for SFP module events
so userspace can apply whatever static user config it has?