net: ethernet: mtk_eth_soc: only write values if needed

Only restart auto-negotiation and write link timer if actually
necessary. This prevents losing the link in case of minor
changes.

Fixes: 7e53837269 ("net: ethernet: mediatek: Re-add support SGMII")
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Tested-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/mediatek/mtk_sgmii.c

@@ -38,20 +38,16 @@ static int mtk_pcs_config(struct phylink_pcs *pcs, unsigned int mode,
 			  const unsigned long *advertising,
 			  bool permit_pause_to_mac)
 {
+	bool mode_changed = false, changed, use_an;
 	struct mtk_pcs *mpcs = pcs_to_mtk_pcs(pcs);
 	unsigned int rgc3, sgm_mode, bmcr;
 	int advertise, link_timer;
-	bool changed, use_an;
 
 	advertise = phylink_mii_c22_pcs_encode_advertisement(interface,
 							     advertising);
 	if (advertise < 0)
 		return advertise;
 
-	link_timer = phylink_get_link_timer_ns(interface);
-	if (link_timer < 0)
-		return link_timer;
-
 	/* Clearing IF_MODE_BIT0 switches the PCS to BASE-X mode, and
 	 * we assume that fixes it's speed at bitrate = line rate (in
 	 * other words, 1000Mbps or 2500Mbps).
@@ -77,13 +73,16 @@ static int mtk_pcs_config(struct phylink_pcs *pcs, unsigned int mode,
 	}
 
 	if (use_an) {
-		/* FIXME: Do we need to set AN_RESTART here? */
-		bmcr = SGMII_AN_RESTART | SGMII_AN_ENABLE;
+		bmcr = SGMII_AN_ENABLE;
 	} else {
 		bmcr = 0;
 	}
 
 	if (mpcs->interface != interface) {
+		link_timer = phylink_get_link_timer_ns(interface);
+		if (link_timer < 0)
+			return link_timer;
+
 		/* PHYA power down */
 		regmap_update_bits(mpcs->regmap, SGMSYS_QPHY_PWR_STATE_CTRL,
 				   SGMII_PHYA_PWD, SGMII_PHYA_PWD);
@@ -101,16 +100,17 @@ static int mtk_pcs_config(struct phylink_pcs *pcs, unsigned int mode,
 		regmap_update_bits(mpcs->regmap, mpcs->ana_rgc3,
 				   RG_PHY_SPEED_3_125G, rgc3);
 
+		/* Setup the link timer */
+		regmap_write(mpcs->regmap, SGMSYS_PCS_LINK_TIMER, link_timer / 2 / 8);
+
 		mpcs->interface = interface;
+		mode_changed = true;
 	}
 
 	/* Update the advertisement, noting whether it has changed */
 	regmap_update_bits_check(mpcs->regmap, SGMSYS_PCS_ADVERTISE,
 				 SGMII_ADVERTISE, advertise, &changed);
 
-	/* Setup the link timer and QPHY power up inside SGMIISYS */
-	regmap_write(mpcs->regmap, SGMSYS_PCS_LINK_TIMER, link_timer / 2 / 8);
-
 	/* Update the sgmsys mode register */
 	regmap_update_bits(mpcs->regmap, SGMSYS_SGMII_MODE,
 			   SGMII_REMOTE_FAULT_DIS | SGMII_SPEED_DUPLEX_AN |
@@ -118,7 +118,7 @@ static int mtk_pcs_config(struct phylink_pcs *pcs, unsigned int mode,
 
 	/* Update the BMCR */
 	regmap_update_bits(mpcs->regmap, SGMSYS_PCS_CONTROL_1,
-			   SGMII_AN_RESTART | SGMII_AN_ENABLE, bmcr);
+			   SGMII_AN_ENABLE, bmcr);
 
 	/* Release PHYA power down state
 	 * Only removing bit SGMII_PHYA_PWD isn't enough.
@@ -132,7 +132,7 @@ static int mtk_pcs_config(struct phylink_pcs *pcs, unsigned int mode,
 	usleep_range(50, 100);
 	regmap_write(mpcs->regmap, SGMSYS_QPHY_PWR_STATE_CTRL, 0);
 
-	return changed;
+	return changed || mode_changed;
 }
 
 static void mtk_pcs_restart_an(struct phylink_pcs *pcs)