phylink: add helper for configuring 2500BaseX modes

Add a helper for MAC drivers to use in their validate callback to deal
with 2500BaseX vs 1000BaseX modes, where the hardware supports both
but it is not possible to automatically select between them.

This helper defaults to 1000BaseX, as that is the 802.3 standard, and
will allow users to select 2500BaseX either by forcing the speed if
AN is disabled, or by changing the advertising mask if AN is enabled.
Disabling AN is not recommended as it is only the speed that we're
interested in controlling, not the duplex or pause mode parameters.
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/phy/phylink.c

@@ -1688,4 +1688,34 @@ static const struct sfp_upstream_ops sfp_phylink_ops = {
 	.disconnect_phy = phylink_sfp_disconnect_phy,
 };
 
+/* Helpers for MAC drivers */
+
+/**
+ * phylink_helper_basex_speed() - 1000BaseX/2500BaseX helper
+ * @state: a pointer to a &struct phylink_link_state
+ *
+ * Inspect the interface mode, advertising mask or forced speed and
+ * decide whether to run at 2.5Gbit or 1Gbit appropriately, switching
+ * the interface mode to suit.  @state->interface is appropriately
+ * updated, and the advertising mask has the "other" baseX_Full flag
+ * cleared.
+ */
+void phylink_helper_basex_speed(struct phylink_link_state *state)
+{
+	if (phy_interface_mode_is_8023z(state->interface)) {
+		bool want_2500 = state->an_enabled ?
+			phylink_test(state->advertising, 2500baseX_Full) :
+			state->speed == SPEED_2500;
+
+		if (want_2500) {
+			phylink_clear(state->advertising, 1000baseX_Full);
+			state->interface = PHY_INTERFACE_MODE_2500BASEX;
+		} else {
+			phylink_clear(state->advertising, 2500baseX_Full);
+			state->interface = PHY_INTERFACE_MODE_1000BASEX;
+		}
+	}
+}
+EXPORT_SYMBOL_GPL(phylink_helper_basex_speed);
+
 MODULE_LICENSE("GPL");

include/linux/phylink.h

@@ -234,5 +234,6 @@ int phylink_mii_ioctl(struct phylink *, struct ifreq *, int);
 #define phylink_test(bm, mode)	__phylink_do_bit(test_bit, bm, mode)
 
 void phylink_set_port_modes(unsigned long *bits);
+void phylink_helper_basex_speed(struct phylink_link_state *state);
 
 #endif