netdev - Re: [PATCH net-next 07/11] net: dsa: mv88e6xxx: add port link setter

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Date:   Thu, 3 Nov 2016 00:05:17 +0100
From:   Andrew Lunn <andrew@...n.ch>
To:     Vivien Didelot <vivien.didelot@...oirfairelinux.com>
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel@...oirfairelinux.com,
        "David S. Miller" <davem@...emloft.net>,
        Florian Fainelli <f.fainelli@...il.com>
Subject: Re: [PATCH net-next 07/11] net: dsa: mv88e6xxx: add port link setter

> Do you expect to return an error if adjust_link is called with
> phydev->duplex == DUPLEX_UNKNOWN, or, do you expect to fallback to
> unforced duplex when setting such value?

ethtool(1) itself does not allow you to specify "unknown". It only
allows "full" or "half". So passing DUPLEX_UNKNOWN means using the API
directly. The core ethtool code does not sanity check the request, so
will pass on DUPLEX_UNKNOWN to the drivers.

A quick search of the drivers, 99% seem to ignore DUPLEX_UNKNOWN. The
1% is bnx2x, which has:

        /* If received a request for an unknown duplex, assume full*/
        if (cmd->duplex == DUPLEX_UNKNOWN)
                cmd->duplex = DUPLEX_FULL;

I personally would return -EINVAL, since it is unclear what
DUPLEX_UNKNOWN means. It could be argued that falling back to Half is
correct, since failed autoneg generally results in 10/Half. Every
Ethernet can do that, where as a device needs to be 25 years or
younger to support Full :-)

	  Andrew