RE: [PATCH net-next 0/9] net: add phylink managed EEE support

quoted

Since merging these patches, phylink_connect_phy() can no longer be
invoked correctly in ngbe_open(). The error is returned from the function
phy_eee_rx_clock_stop(). Since EEE is not supported on our NGBE hardware.

That would mean phy_modify_mmd() is failing, but the question is why
that is. Please investigate. Thanks.

Yes, phy_modify_mmd() returns -EOPNOTSUPP. Since .read/write_mmd are
implemented in the PHY driver, but it's not supported to read/write the
register field (devnum=MDIO_MMD_PCS, regnum= MDIO_CTRL1).

So the error occurs on  __phy_read_mmd():
	if (phydev->drv && phydev->drv->read_mmd)
		return phydev->drv->read_mmd(phydev, devad, regnum);