[PATCH net v3] net: phy: mediatek-ge: disable EEE on the MT7530 PHY

The MT7530 internal GE PHY advertises EEE by hardware default, but its
EEE support is defective: with EEE advertised, some link partners fail
to establish a stable link. On a 2-pair (4-wire) cable where both ends
advertise gigabit, 1000BASE-T training cannot succeed, and instead of
falling back to 100 Mbps the port loops, so no link or DHCP lease is
ever obtained. MediaTek confirms the hardware is the root cause (Landen
Chao, 2021): "EEE of the 10-year-old MT7530 internal gephy has many IOT
problems, so it is recommended to disable its EEE."

mtk_gephy_config_init() used to clear the EEE advertisement early, but
commit af3b4b0e59de ("net: phy: mediatek-ge: do not disable EEE
advertisement") removed that on the rationale that the DSA subdriver
already performs an early disable. That holds for MT7531, whose
mt7531_setup() clears MDIO_AN_EEE_ADV on each switch PHY, but not for
the MT7530 PHY: neither the MT7621 integrated switch nor the dedicated
MT7530 IC ever had such a loop, so removing it left those boards
without any working early EEE disable and the link flapping came back.

Since the broken hardware is the PHY, fix it in the PHY driver so it
covers all users of this PHY, integrated in a switch or standalone:

  - clear MDIO_AN_EEE_ADV in probe(), before anything can negotiate
    EEE with the link partner;
  - call phy_disable_eee() so phylib does not write the advertisement
    back on later renegotiations and userspace cannot re-enable EEE.

Auto-negotiation then falls back to a stable 100 Mbps link instead of
looping at gigabit. Tested on ASUS RT-AX53U (MT7621): with a 2-pair
cable on the WAN port, a single clean 100 Mbps link comes up and a
DHCP lease is obtained, where the unpatched driver loops.

Fixes: af3b4b0e59de ("net: phy: mediatek-ge: do not disable EEE advertisement")
Suggested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Vladislav Karmanov <redacted>
---
Changes in v3:
- Move the fix from the DSA driver to the MT7530 PHY driver: the
  broken hardware is the PHY, so the workaround belongs there and
  covers all users of the PHY, switch-integrated or standalone
  (Andrew Lunn).
- Clear MDIO_AN_EEE_ADV from probe() rather than config_init() so the
  advertisement is off before anything can negotiate EEE, and call
  phy_disable_eee() so neither phylib nor userspace can re-enable it
  (Andrew Lunn).
- Remove the eee-broken-* device tree properties discussion from the
  commit message; the properties are not needed once the PHY driver
  disables broken EEE itself (Andrew Lunn).

v1: https://lore.kernel.org/netdev/20260818182829.1580811-1-vladislav.karmanov.dev@gmail.com/ (local)
v2: https://lore.kernel.org/netdev/20260820202844.1821687-1-vladislav.karmanov.dev@gmail.com/ (local)

 drivers/net/phy/mediatek/mtk-ge.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/drivers/net/phy/mediatek/mtk-ge.c b/drivers/net/phy/mediatek/mtk-ge.c
index 73d9b72..35600c7 100644
--- a/drivers/net/phy/mediatek/mtk-ge.c
+++ b/drivers/net/phy/mediatek/mtk-ge.c

@@ -62,6 +62,27 @@ static void mtk_gephy_config_init(struct phy_device *phydev)
 		       FIELD_PREP(MTK_MCC_NEARECHO_OFFSET_MASK, 0x3));
 }
 
+static int mt7530_phy_probe(struct phy_device *phydev)
+{
+	int ret;
+
+	/* The MT7530 internal GE PHY has broken EEE: with EEE advertised,
+	 * some link partners fail to establish a stable link (on a 2-pair
+	 * cable, 1000BASE-T training fails and the port loops instead of
+	 * falling back). MediaTek recommends disabling EEE on this PHY.
+	 * Clear the EEE advertisement early and mark EEE disabled, so
+	 * that neither phylib nor userspace can re-enable it.
+	 */
+
+	ret = phy_write_mmd(phydev, MDIO_MMD_AN, MDIO_AN_EEE_ADV, 0);
+	if (ret)
+		return ret;
+
+	phy_disable_eee(phydev);
+
+	return 0;
+}
+
 static int mt7530_phy_config_init(struct phy_device *phydev)
 {
 	mtk_gephy_config_init(phydev);

@@ -100,6 +121,7 @@ static struct phy_driver mtk_gephy_driver[] = {
 	{
 		PHY_ID_MATCH_EXACT(MTK_GPHY_ID_MT7530),
 		.name		= "MediaTek MT7530 PHY",
+		.probe		= mt7530_phy_probe,
 		.config_init	= mt7530_phy_config_init,
 		/* Interrupts are handled by the switch, not the PHY
 		 * itself.

-- 
2.43.0