net: dsa: mxl-gsw1xx: fix SerDes RX polarity

According to MaxLinear engineer Benny Weng the RX lane of the SerDes
port of the GSW1xx switches is inverted in hardware, and the
SGMII_PHY_RX0_CFG2_INVERT bit is set by default in order to compensate
for that. Hence also set the SGMII_PHY_RX0_CFG2_INVERT bit by default in
gsw1xx_pcs_reset().

Fixes: 2233593 ("net: dsa: add driver for MaxLinear GSW1xx switch family")
Reported-by: Rasmus Villemoes <ravi@prevas.dk>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://patch.msgid.link/ca10e9f780c0152ecf9ae8cbac5bf975802e8f99.1764668951.git.daniel@makrotopia.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: dangowrt

drivers/net/dsa/lantiq/mxl-gsw1xx.c

@@ -255,10 +255,16 @@ static int gsw1xx_pcs_reset(struct gsw1xx_priv *priv)
 	      FIELD_PREP(GSW1XX_SGMII_PHY_RX0_CFG2_FILT_CNT,
 			 GSW1XX_SGMII_PHY_RX0_CFG2_FILT_CNT_DEF);
 
-	/* TODO: Take care of inverted RX pair once generic property is
+	/* RX lane seems to be inverted internally, so bit
+	 * GSW1XX_SGMII_PHY_RX0_CFG2_INVERT needs to be set for normal
+	 * (ie. non-inverted) operation.
+	 *
+	 * TODO: Take care of inverted RX pair once generic property is
 	 *       available
 	 */
 
+	val |= GSW1XX_SGMII_PHY_RX0_CFG2_INVERT;
+
 	ret = regmap_write(priv->sgmii, GSW1XX_SGMII_PHY_RX0_CFG2, val);
 	if (ret < 0)
 		return ret;