ixgbe: Document how to enable NBASE-T support

Commit a296d665ea ("ixgbe: Add ethtool support to enable 2.5 and 5.0
Gbps support") introduced suppression of the advertisement of NBASE-T
speeds by default, according to Todd Fujinaka to accommodate customers
with network switches which could not cope with advertised NBASE-T
speeds, as posted in the E1000-devel mailing list:

https://sourceforge.net/p/e1000/mailman/message/37106269/

However, the suppression was not documented at all, nor was how to
enable NBASE-T support.

Properly document the NBASE-T suppression and how to enable NBASE-T
support.

Fixes: a296d665ea ("ixgbe: Add ethtool support to enable 2.5 and 5.0 Gbps support")
Reported-by: Robert Schlabbach <robert_s@gmx.net>
Signed-off-by: Robert Schlabbach <robert_s@gmx.net>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>

Documentation/networking/device_drivers/ethernet/intel/ixgbe.rst

@@ -440,6 +440,22 @@ NOTE: For 82599-based network connections, if you are enabling jumbo frames in
 a virtual function (VF), jumbo frames must first be enabled in the physical
 function (PF). The VF MTU setting cannot be larger than the PF MTU.
 
+NBASE-T Support
+---------------
+The ixgbe driver supports NBASE-T on some devices. However, the advertisement
+of NBASE-T speeds is suppressed by default, to accommodate broken network
+switches which cannot cope with advertised NBASE-T speeds. Use the ethtool
+command to enable advertising NBASE-T speeds on devices which support it::
+
+  ethtool -s eth? advertise 0x1800000001028
+
+On Linux systems with INTERFACES(5), this can be specified as a pre-up command
+in /etc/network/interfaces so that the interface is always brought up with
+NBASE-T support, e.g.::
+
+  iface eth? inet dhcp
+       pre-up ethtool -s eth? advertise 0x1800000001028 || true
+
 Generic Receive Offload, aka GRO
 --------------------------------
 The driver supports the in-kernel software implementation of GRO. GRO has

drivers/net/ethernet/intel/ixgbe/ixgbe_main.c

@@ -5531,6 +5531,10 @@ static int ixgbe_non_sfp_link_config(struct ixgbe_hw *hw)
 	if (!speed && hw->mac.ops.get_link_capabilities) {
 		ret = hw->mac.ops.get_link_capabilities(hw, &speed,
 							&autoneg);
+		/* remove NBASE-T speeds from default autonegotiation
+		 * to accommodate broken network switches in the field
+		 * which cannot cope with advertised NBASE-T speeds
+		 */
 		speed &= ~(IXGBE_LINK_SPEED_5GB_FULL |
 			   IXGBE_LINK_SPEED_2_5GB_FULL);
 	}