netdev - Re: [PATCH net-next 6/6] ethtool: clarify the ethtool FEC interface

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Message-ID: <YGHI6ucPwFZDQE06@lunn.ch>
Date:   Mon, 29 Mar 2021 14:32:42 +0200
From:   Andrew Lunn <andrew@...n.ch>
To:     Edward Cree <ecree.xilinx@...il.com>
Cc:     Jakub Kicinski <kuba@...nel.org>, davem@...emloft.net,
        netdev@...r.kernel.org, michael.chan@...adcom.com,
        damian.dybek@...el.com, paul.greenwalt@...el.com,
        rajur@...lsio.com, jaroslawx.gawin@...el.com, vkochan@...vell.com,
        alobakin@...me, snelson@...sando.io, shayagr@...zon.com,
        ayal@...dia.com, shenjian15@...wei.com, saeedm@...dia.com,
        mkubecek@...e.cz, roopa@...dia.com
Subject: Re: [PATCH net-next 6/6] ethtool: clarify the ethtool FEC interface

On Mon, Mar 29, 2021 at 12:56:30PM +0100, Edward Cree wrote:
> On 25/03/2021 01:12, Jakub Kicinski wrote:
> > Drivers should reject mixing %ETHTOOL_FEC_AUTO_BIT with other
> > + * FEC modes, because it's unclear whether in this case other modes constrain
> > + * AUTO or are independent choices.
> 
> Does this mean you want me to spin a patch to sfc to reject this?
> Currently for us e.g. AUTO|RS means use RS if the cable and link partner
>  both support it, otherwise let firmware choose (presumably between BASER
>  and OFF) based on cable/module & link partner caps and/or parallel detect.
> We took this approach because our requirements writers believed that
>  customers would have a need for this setting; they called it "prefer FEC",
>  and I think the idea was to use FEC if possible (even on cables where the
>  IEEE-recommended default is no FEC, such as CA-25G-N 3m DAC) but allow
>  fallback to no FEC if e.g. link partner doesn't advertise FEC in AN.
> Similarly, AUTO|BASER ("prefer BASE-R FEC") might be desired by a user who
>  wants to use BASE-R if possible to minimise latency, but fall back to RS
>  FEC if the cable or link partner insists on it (eg CA-25G-L 5m DAC).
> Whether we were right and all this is actually useful, I couldn't say.

Jacub was talking about adding a netlink API as the next step. You
should feed this in as a requirement for that. Being able to express
preferences in the API in an explicitly documented way.

It there any other existing ethtool setting which could be used as a
model? EEE, master/slave? I would class pause as an anti model, that
is frequently done wrong :-(

       Andrew