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Implement the standard ETHTOOL_PHY_DOWNSHIFT tunable for the LAN88xx PHY. This allows runtime configuration of the auto-downshift feature via ethtool: ethtool --set-phy-tunable eth0 downshift on count 3 The LAN88xx PHY supports downshifting from 1000BASE-T to 100BASE-TX after 2-5 failed auto-negotiation attempts. Valid count values are 2, 3, 4 and 5. This is based on an earlier downstream implementation by Phil Elwell. Signed-off-by: Nicolai Buchwitz <nb@tipi-net.de> Reviewed-by: Andrew Lunn <andrew@lunn.ch> Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk> Link: https://patch.msgid.link/20260401123848.696766-2-nb@tipi-net.de Signed-off-by: Jakub Kicinski <kuba@kernel.org>