The PHY built in to the Realtek RTL8366S switch controller was
previously supported by genphy_driver. This PHY does not implement MMD
operations. Since commit 9b01c885be36 ("net: phy: c22: migrate to
genphy_c45_write_eee_adv()"), MMD register reads have been made during
phy_probe to determine EEE support. For genphy_driver, these reads are
transformed into 802.3 annex 22D clause 45-over-clause 22
mmd_phy_indirect operations that perform MII register writes to
MII_MMD_CTRL and MII_MMD_DATA. This overwrites those two MII registers,
which on this PHY are reserved and have another function, rendering the
PHY unusable while so configured.

Proper support for this PHY is restored by providing a phy_driver that
declares MMD operations as unsupported by using the helper functions
provided for that purpose, while remaining otherwise identical to
genphy_driver.

Fixes: 9b01c885be36 ("net: phy: c22: migrate to genphy_c45_write_eee_adv()")
Reported-by: Russell Senior <russell@personaltelco.net>
Closes: https://github.com/openwrt/openwrt/issues/15981
Link: https://github.com/openwrt/openwrt/issues/15739
Signed-off-by: Mark Mentovai <mark@mentovai.com>
---
 drivers/net/phy/realtek.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/net/phy/realtek.c b/drivers/net/phy/realtek.c
index bed839237fb5..87865918dab6 100644
--- a/drivers/net/phy/realtek.c
+++ b/drivers/net/phy/realtek.c
@@ -1465,6 +1465,13 @@ static struct phy_driver realtek_drvs[] = {
 		.handle_interrupt = genphy_handle_interrupt_no_ack,
 		.suspend	= genphy_suspend,
 		.resume		= genphy_resume,
+	}, {
+		PHY_ID_MATCH_EXACT(0x001cc960),
+		.name		= "RTL8366S Gigabit Ethernet",
+		.suspend	= genphy_suspend,
+		.resume		= genphy_resume,
+		.read_mmd	= genphy_read_mmd_unsupported,
+		.write_mmd	= genphy_write_mmd_unsupported,
 	},
 };
 
-- 
2.45.2

Hello Mark,

On Thu, 25 Jul 2024 16:41:44 -0400
Mark Mentovai <mark@mentovai.com> wrote:

> The PHY built in to the Realtek RTL8366S switch controller was
> previously supported by genphy_driver. This PHY does not implement MMD
> operations. Since commit 9b01c885be36 ("net: phy: c22: migrate to
> genphy_c45_write_eee_adv()"), MMD register reads have been made during
> phy_probe to determine EEE support. For genphy_driver, these reads are
> transformed into 802.3 annex 22D clause 45-over-clause 22
> mmd_phy_indirect operations that perform MII register writes to
> MII_MMD_CTRL and MII_MMD_DATA. This overwrites those two MII registers,
> which on this PHY are reserved and have another function, rendering the
> PHY unusable while so configured.
> 
> Proper support for this PHY is restored by providing a phy_driver that
> declares MMD operations as unsupported by using the helper functions
> provided for that purpose, while remaining otherwise identical to
> genphy_driver.
> 
> Fixes: 9b01c885be36 ("net: phy: c22: migrate to genphy_c45_write_eee_adv()")
> Reported-by: Russell Senior <russell@personaltelco.net>
> Closes: https://github.com/openwrt/openwrt/issues/15981
> Link: https://github.com/openwrt/openwrt/issues/15739
> Signed-off-by: Mark Mentovai <mark@mentovai.com>

This looks correct to me.

Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>

Thanks,

Maxime