From 4e432e530db0056450fbc4a3cee793f16adc39a7 Mon Sep 17 00:00:00 2001
From: Daniel Golle <daniel@makrotopia.org>
Date: Tue, 8 Oct 2024 23:58:41 +0100
Subject: [PATCH] net: phy: populate host_interfaces when attaching PHY

Use bitmask of interfaces supported by the MAC for the PHY to choose
from if the declared interface mode is among those using a single pair
of SerDes lanes.
This will allow 2500Base-T PHYs to switch to SGMII on most hosts, which
results in half-duplex being supported in case the MAC supports them.
Without this change, 2500Base-T PHYs will always operate in 2500Base-X
mode with rate-matching, which is not only wasteful in terms of energy
consumption, but also limits the supported interface modes to
full-duplex only.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
---
 drivers/net/phy/phylink.c | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

--- a/drivers/net/phy/phylink.c
+++ b/drivers/net/phy/phylink.c
@@ -2278,7 +2278,7 @@ int phylink_fwnode_phy_connect(struct ph
 {
 	struct fwnode_handle *phy_fwnode;
 	struct phy_device *phy_dev;
-	int ret;
+	int i, ret;
 
 	/* Fixed links and 802.3z are handled without needing a PHY */
 	if (pl->cfg_link_an_mode == MLO_AN_FIXED ||
@@ -2308,6 +2308,25 @@ int phylink_fwnode_phy_connect(struct ph
 	if (pl->config->mac_requires_rxc)
 		flags |= PHY_F_RXC_ALWAYS_ON;
 
+	/* Assume single-lane SerDes interface modes share the same
+	 * lanes and allow the PHY to switch to slower also supported modes
+	 */
+	for (i = ARRAY_SIZE(phylink_sfp_interface_preference) - 1; i >= 0; i--) {
+		/* skip unsupported modes */
+		if (!test_bit(phylink_sfp_interface_preference[i], pl->config->supported_interfaces))
+			continue;
+
+		__set_bit(phylink_sfp_interface_preference[i], phy_dev->host_interfaces);
+
+		/* skip all faster modes */
+		if (phylink_sfp_interface_preference[i] == pl->link_interface)
+			break;
+	}
+
+	if (test_bit(pl->link_interface, phylink_sfp_interfaces))
+		phy_interface_and(phy_dev->host_interfaces, phylink_sfp_interfaces,
+				  pl->config->supported_interfaces);
+
 	ret = phy_attach_direct(pl->netdev, phy_dev, flags,
 				pl->link_interface);
 	phy_device_free(phy_dev);
