[RFC PATCH net-next] net: mvneta: deny disabling autoneg for 802.3z modes

The documentation for Armada 38x says:

  Bit 2 Field InBandAnEn In-band Auto-Negotiation enable. ...
  When <PortType> = 1 (1000BASE-X) this field must be set to 1.

We presently ignore whether userspace requests autonegotiation or not
through the ethtool ksettings interface. However, we have some network
interfaces that wish to do this. To offer a consistent API across
network interfaces, deny the ability to disable autonegotiation on
mvneta hardware when in 1000BASE-X and 2500BASE-X.

This means the only way to switch between 2500BASE-X and 1000BASE-X
on SFPs that support this will be:

 # ethtool -s ethX advertise 0x20000002000 # 1000BASE-X Pause
 # ethtool -s ethX advertise 0xa000        # 2500BASE-X Pause

Signed-off-by: Russell King (Oracle) <redacted>
---
net-next is currently closed, but I'd like to collect acks for this
patch. Thanks.

 drivers/net/ethernet/marvell/mvneta.c | 20 ++++++++++++++------
 1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/drivers/net/ethernet/marvell/mvneta.c b/drivers/net/ethernet/marvell/mvneta.c
index 361bc4fbe20b..0a35d216b742 100644
--- a/drivers/net/ethernet/marvell/mvneta.c
+++ b/drivers/net/ethernet/marvell/mvneta.c

@@ -3832,12 +3832,20 @@ static void mvneta_validate(struct phylink_config *config,
 	struct mvneta_port *pp = netdev_priv(ndev);
 	__ETHTOOL_DECLARE_LINK_MODE_MASK(mask) = { 0, };
 
-	/* We only support QSGMII, SGMII, 802.3z and RGMII modes */
-	if (state->interface != PHY_INTERFACE_MODE_NA &&
-	    state->interface != PHY_INTERFACE_MODE_QSGMII &&
-	    state->interface != PHY_INTERFACE_MODE_SGMII &&
-	    !phy_interface_mode_is_8023z(state->interface) &&
-	    !phy_interface_mode_is_rgmii(state->interface)) {
+	/* We only support QSGMII, SGMII, 802.3z and RGMII modes.
+	 * When in 802.3z mode, we must have AN enabled:
+	 * "Bit 2 Field InBandAnEn In-band Auto-Negotiation enable. ...
+	 * When <PortType> = 1 (1000BASE-X) this field must be set to 1."
+	 */
+	if (phy_interface_mode_is_8023z(state->interface)) {
+		if (!phylink_test(state->advertising, Autoneg)) {
+			bitmap_zero(supported, __ETHTOOL_LINK_MODE_MASK_NBITS);
+			return;
+		}
+	} else if (state->interface != PHY_INTERFACE_MODE_NA &&
+		   state->interface != PHY_INTERFACE_MODE_QSGMII &&
+		   state->interface != PHY_INTERFACE_MODE_SGMII &&
+		   !phy_interface_mode_is_rgmii(state->interface)) {
 		bitmap_zero(supported, __ETHTOOL_LINK_MODE_MASK_NBITS);
 		return;
 	}