[PATCH net] net: phy: skip EEE advertisement write when autoneg is disabled

["Nerijus Bendžiūnas" <nerijus.bendziunas@gmail.com>]

genphy_c45_an_config_eee_aneg() writes the EEE advertisement to the
auto-negotiation device's MMD register space (MDIO_MMD_AN, register
MDIO_AN_EEE_ADV).  These registers are only consumed by the link
partner during auto-negotiation, so writing them while autoneg is
disabled is semantically a no-op.

On at least the Broadcom BCM54213PE PHY, however, the indirect MMD
write triggered from this path while the link is currently forced
(autoneg off) disturbs the receive datapath: the kernel still
reports the link as UP at the configured speed/duplex, but no
frames are received until the link is renegotiated.  Concretely,
running

    ethtool -s eth0 speed 100 duplex full autoneg off
    ethtool --set-eee eth0 eee off

leaves eth0 with TX working and RX completely silent on a
Raspberry Pi 4 / CM4 board (bcmgenet + BCM54213PE in rgmii-rxid).
Switching back to autoneg recovers the link.

Prior to commit f26a29a038ee ("net: phy: ensure that genphy_c45_an_config_eee_aneg() sees new value of phydev->eee_cfg.eee_enabled"),
the disable path was effectively a no-op because the helper read
the stale eee_cfg.eee_enabled, so the underlying PHY behavior never
surfaced.

Bisected on rpi-6.12.y between commits 83943264 (good) and
effcbc88 (bad) to f26a29a038ee.

Fixes: f26a29a038ee ("net: phy: ensure that genphy_c45_an_config_eee_aneg() sees new value of phydev->eee_cfg.eee_enabled")
Cc: stable@vger.kernel.org
Signed-off-by: Nerijus Bendžiūnas <nerijus.bendziunas@gmail.com>
---
 drivers/net/phy/phy-c45.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/net/phy/phy-c45.c b/drivers/net/phy/phy-c45.c
index d48aa7231b37..65de6a5fa204 100644
--- a/drivers/net/phy/phy-c45.c
+++ b/drivers/net/phy/phy-c45.c
@@ -940,6 +940,10 @@ EXPORT_SYMBOL_GPL(genphy_c45_read_eee_abilities);
  */
 int genphy_c45_an_config_eee_aneg(struct phy_device *phydev)
 {
+	/* MMD AN advertisements are only consumed during autoneg. */
+	if (phydev->autoneg == AUTONEG_DISABLE)
+		return 0;
+
 	if (!phydev->eee_cfg.eee_enabled) {
 		__ETHTOOL_DECLARE_LINK_MODE_MASK(adv) = {};
 
-- 
2.54.0