ethtool: reset #lanes when lanes is omitted

If the number of lanes was forced and then subsequently the user
omits this parameter, the ksettings->lanes is reset. The driver
should then reset the number of lanes to the device's default
for the specified speed.

However, although the ksettings->lanes is set to 0, the mod variable
is not set to true to indicate the driver and userspace should be
notified of the changes.

The consequence is that the same ethtool operation will produce
different results based on the initial state.

If the initial state is:
$ ethtool swp1 | grep -A 3 'Speed: '
        Speed: 500000Mb/s
        Lanes: 2
        Duplex: Full
        Auto-negotiation: on

then executing 'ethtool -s swp1 speed 50000 autoneg off' will yield:
$ ethtool swp1 | grep -A 3 'Speed: '
        Speed: 500000Mb/s
        Lanes: 2
        Duplex: Full
        Auto-negotiation: off

While if the initial state is:
$ ethtool swp1 | grep -A 3 'Speed: '
        Speed: 500000Mb/s
        Lanes: 1
        Duplex: Full
        Auto-negotiation: off

executing the same 'ethtool -s swp1 speed 50000 autoneg off' results in:
$ ethtool swp1 | grep -A 3 'Speed: '
        Speed: 500000Mb/s
        Lanes: 1
        Duplex: Full
        Auto-negotiation: off

This patch fixes this behavior. Omitting lanes will always results in
the driver choosing the default lane width for the chosen speed. In this
scenario, regardless of the initial state, the end state will be, e.g.,

$ ethtool swp1 | grep -A 3 'Speed: '
        Speed: 500000Mb/s
        Lanes: 2
        Duplex: Full
        Auto-negotiation: off

Fixes: 012ce4d ("ethtool: Extend link modes settings uAPI with lanes")
Signed-off-by: Andy Roulin <[email protected]>
Reviewed-by: Danielle Ratson <[email protected]>
Reviewed-by: Ido Schimmel <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: aroulin

net/ethtool/linkmodes.c

@@ -270,11 +270,12 @@ static int ethnl_update_linkmodes(struct genl_info *info, struct nlattr **tb,
 					    "lanes configuration not supported by device");
 			return -EOPNOTSUPP;
 		}
-	} else if (!lsettings->autoneg) {
-		/* If autoneg is off and lanes parameter is not passed from user,
-		 * set the lanes parameter to 0.
+	} else if (!lsettings->autoneg && ksettings->lanes) {
+		/* If autoneg is off and lanes parameter is not passed from user but
+		 * it was defined previously then set the lanes parameter to 0.
 		 */
 		ksettings->lanes = 0;
+		*mod = true;
 	}
 
 	ret = ethnl_update_bitset(ksettings->link_modes.advertising,