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

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Date:   Thu, 6 Jul 2017 12:02:14 -0700
From:   Jakub Kicinski <kubakici@...pl>
To:     Casey Leedom <leedom@...lsio.com>
Cc:     Dustin Byford <dustin@...ulusnetworks.com>,
        Andrew Lunn <andrew@...n.ch>,
        Roopa Prabhu <roopa@...ulusnetworks.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "linville@...driver.com" <linville@...driver.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "vidya.chowdary@...il.com" <vidya.chowdary@...il.com>,
        "olson@...ulusnetworks.com" <olson@...ulusnetworks.com>,
        Manoj Malviya <manojmalviya@...lsio.com>,
        Santosh Rastapur <santosh@...lsio.com>,
        "yuval.mintz@...gic.com" <yuval.mintz@...gic.com>,
        "odedw@...lanox.com" <odedw@...lanox.com>,
        "ariela@...lanox.com" <ariela@...lanox.com>,
        "galp@...lanox.com" <galp@...lanox.com>,
        "jeffrey.t.kirsher@...el.com" <jeffrey.t.kirsher@...el.com>
Subject: Re: [PATCH net-next 1/3] net: ethtool: add support for forward
 error correction modes

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?