[PATCH 6.12.y] net: pcs: xpcs: fix SGMII state reading

[Coia Prant <coiaprant@gmail.com>]

Commit 2a22b7ae2fa3 ("net: pcs: xpcs: adapt Wangxun NICs for SGMII mode")
added a path in xpcs_get_state_c37_sgmii() that reads speed/duplex from
BMCR after AN completes. However, BMCR does not reflect the negotiated
result on the hardware where this has been tested:

- On RK3568 (MAC side SGMII), BMCR returns a fixed hardware reset value
- Wangxun engineer Jiawen Wu confirmed that on their side, "BMCR looks
  like it only wants to be return as 0" [0]

The correct information is available in CL37_ANSGM_STS, which contains
the actual link status and negotiated speed/duplex.

This bug was previously masked by phylink core, which overrides the PCS
link state with the PHY state when a PHY is present:

        /* If we have a phy, the "up" state is the union of both the
         * PHY and the MAC
         */
        if (phy)
                link_state.link &= pl->phy_state.link;

Thus, when the link is down, the PHY's link_down state is applied on top
of whatever the PCS reports, hiding the broken PCS state reading path.

Modify xpcs_get_state_c37_sgmii() to:
1. Read link state from CL37_ANSGM_STS
2. If link is up, report speed/duplex from CL37_ANSGM_STS
3. Remove the broken BMCR reading path entirely

Also properly set state->an_complete to reflect the AN completion status,
and clear CL37_ANCMPLT_INTR when link is down to avoid stale state.

[0] https://lore.kernel.org/all/000c01dd1593$2ac0b0f0$804212d0$@trustnetic.com/

Fixes: 2a22b7ae2fa3 ("net: pcs: xpcs: adapt Wangxun NICs for SGMII mode")
Cc: stable@vger.kernel.org
Tested-by: Jiawen Wu <jiawenwu@trustnetic.com>
Signed-off-by: Coia Prant <coiaprant@gmail.com>
Tested-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Link: https://patch.msgid.link/20260717074324.3250043-2-coiaprant@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
(cherry picked from commit def9a4745e105145133e442dd8a1c126caf0f553)
---
 drivers/net/pcs/pcs-xpcs.c | 32 +++++++-------------------------
 1 file changed, 7 insertions(+), 25 deletions(-)

diff --git a/drivers/net/pcs/pcs-xpcs.c b/drivers/net/pcs/pcs-xpcs.c
index 82463f9d50c8..0152f820c1bb 100644
--- a/drivers/net/pcs/pcs-xpcs.c
+++ b/drivers/net/pcs/pcs-xpcs.c
@@ -973,6 +973,7 @@ static int xpcs_get_state_c37_sgmii(struct dw_xpcs *xpcs,
 
 	/* Reset link_state */
 	state->link = false;
+	state->an_complete = false;
 	state->speed = SPEED_UNKNOWN;
 	state->duplex = DUPLEX_UNKNOWN;
 	state->pause = 0;
@@ -984,6 +985,8 @@ static int xpcs_get_state_c37_sgmii(struct dw_xpcs *xpcs,
 	if (ret < 0)
 		return ret;
 
+	state->an_complete = ret & DW_VR_MII_AN_STS_C37_ANCMPLT_INTR;
+
 	if (ret & DW_VR_MII_C37_ANSGM_SP_LNKSTS) {
 		int speed_value;
 
@@ -1002,34 +1005,13 @@ static int xpcs_get_state_c37_sgmii(struct dw_xpcs *xpcs,
 			state->duplex = DUPLEX_FULL;
 		else
 			state->duplex = DUPLEX_HALF;
-	} else if (ret == DW_VR_MII_AN_STS_C37_ANCMPLT_INTR) {
-		int speed, duplex;
-
-		state->link = true;
-
-		speed = xpcs_read(xpcs, MDIO_MMD_VEND2, MDIO_CTRL1);
-		if (speed < 0)
-			return speed;
-
-		speed &= SGMII_SPEED_SS13 | SGMII_SPEED_SS6;
-		if (speed == SGMII_SPEED_SS6)
-			state->speed = SPEED_1000;
-		else if (speed == SGMII_SPEED_SS13)
-			state->speed = SPEED_100;
-		else if (speed == 0)
-			state->speed = SPEED_10;
-
-		duplex = xpcs_read(xpcs, MDIO_MMD_VEND2, MII_ADVERTISE);
-		if (duplex < 0)
-			return duplex;
 
-		if (duplex & DW_FULL_DUPLEX)
-			state->duplex = DUPLEX_FULL;
-		else if (duplex & DW_HALF_DUPLEX)
-			state->duplex = DUPLEX_HALF;
+		return 0;
+	}
 
+	/* Clear AN complete status or interrupt */
+	if (state->an_complete)
 		xpcs_write(xpcs, MDIO_MMD_VEND2, DW_VR_MII_AN_INTR_STS, 0);
-	}
 
 	return 0;
 }
-- 
2.47.3