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

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Date:   Mon, 29 Mar 2021 12:56:30 +0100
From:   Edward Cree <ecree.xilinx@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>, davem@...emloft.net
Cc:     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, andrew@...n.ch, roopa@...dia.com
Subject: Re: [PATCH net-next 6/6] ethtool: clarify the ethtool FEC interface

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.

-ed