On Tue, 29 Aug 2023 16:38:42 +0100 Russell King (Oracle) wrote:

So technically it's possible. However, there is no notification to
userspace when such a change may occur. There's also the issue that
userspace may be in the process of issuing ethtool commands that are
affecting one of the PHYs. While holding the rtnl lock will block
those calls, a change between the PHY and e.g. a PHY on the SFP
would cause the ethtool command to target a different PHY from what
was the original target.

To solve that sanely, every PHY-based ethtool probably needs a way
to specify which PHY the command is intended for, but then there's
the question of how userspace users react to that - because it's
likely more than just modifying the ethtool utility, ethtool
commands are probably used from many programs.

Would it simplify anything if we only did the selection from ndo_open?
We can send a notification to user space that the SFP got plugged in,
but its up to user space to down / up the interface to use it?