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

From: Andrew Lunn <[email-masked]>
Date: Thu, 3 Nov 2016 00:05:17 +0100

> 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