net: pcs: xpcs: fix DW_VR_MII_DIG_CTRL1_2G5_EN bit being set for 1G SGMII w/o inband

On a port with SGMII fixed-link at SPEED_1000, DW_VR_MII_DIG_CTRL1 gets
set to 0x2404. This is incorrect, because bit 2 (DW_VR_MII_DIG_CTRL1_2G5_EN)
is set.

It comes from the previous write to DW_VR_MII_AN_CTRL, because the "val"
variable is reused and is dirty. Actually, its value is 0x4, aka
FIELD_PREP(DW_VR_MII_PCS_MODE_MASK, DW_VR_MII_PCS_MODE_C37_SGMII).

Resolve the issue by clearing "val" to 0 when writing to a new register.
After the fix, the register value is 0x2400.

Prior to the blamed commit, when the read-modify-write was open-coded,
the code saved the content of the DW_VR_MII_DIG_CTRL1 register in the
"ret" variable.

Fixes: ce8d6081fcf4 ("net: pcs: xpcs: add _modify() accessors")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/20250114164721.2879380-1-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/drivers/net/pcs/pcs-xpcs.c b/drivers/net/pcs/pcs-xpcs.c
index 7246a910728d135f7871f023216e2310c129a42e..85cbf144ca445622cdc0c6733005b59d79e61540 100644 (file)
--- a/drivers/net/pcs/pcs-xpcs.c
+++ b/drivers/net/pcs/pcs-xpcs.c
@@ -684,6 +684,7 @@ static int xpcs_config_aneg_c37_sgmii(struct dw_xpcs *xpcs,
        if (ret < 0)
                return ret;
 
+       val = 0;
        mask = DW_VR_MII_DIG_CTRL1_MAC_AUTO_SW;
        if (neg_mode == PHYLINK_PCS_NEG_INBAND_ENABLED)
                val = DW_VR_MII_DIG_CTRL1_MAC_AUTO_SW;