net: stmmac: work around sporadic tx issue on link-up

This is a follow-up to the discussion in [0]. It seems to me that
at least the IP version used on Amlogic SoC's sometimes has a problem
if register MAC_CTRL_REG is written whilst the chip is still processing
a previous write. But that's just a guess.
Adding a delay between two writes to this register helps, but we can
also simply omit the offending second write. This patch uses the second
approach and is based on a suggestion from Qi Duan.
Benefit of this approach is that we can save few register writes, also
on not affected chip versions.

[0] https://www.spinics.net/lists/netdev/msg831526.html

Fixes: bfab27a146 ("stmmac: add the experimental PCI support")
Suggested-by: Qi Duan <qi.duan@amlogic.com>
Suggested-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Link: https://lore.kernel.org/r/e99857ce-bd90-5093-ca8c-8cd480b5a0a2@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/stmicro/stmmac/dwmac_lib.c

@@ -258,14 +258,18 @@ EXPORT_SYMBOL_GPL(stmmac_set_mac_addr);
 /* Enable disable MAC RX/TX */
 void stmmac_set_mac(void __iomem *ioaddr, bool enable)
 {
-	u32 value = readl(ioaddr + MAC_CTRL_REG);
+	u32 old_val, value;
+
+	old_val = readl(ioaddr + MAC_CTRL_REG);
+	value = old_val;
 
 	if (enable)
 		value |= MAC_ENABLE_RX | MAC_ENABLE_TX;
 	else
 		value &= ~(MAC_ENABLE_TX | MAC_ENABLE_RX);
 
-	writel(value, ioaddr + MAC_CTRL_REG);
+	if (value != old_val)
+		writel(value, ioaddr + MAC_CTRL_REG);
 }
 
 void stmmac_get_mac_addr(void __iomem *ioaddr, unsigned char *addr,

drivers/net/ethernet/stmicro/stmmac/stmmac_main.c

@@ -986,10 +986,10 @@ static void stmmac_mac_link_up(struct phylink_config *config,
 			       bool tx_pause, bool rx_pause)
 {
 	struct stmmac_priv *priv = netdev_priv(to_net_dev(config->dev));
-	u32 ctrl;
+	u32 old_ctrl, ctrl;
 
-	ctrl = readl(priv->ioaddr + MAC_CTRL_REG);
-	ctrl &= ~priv->hw->link.speed_mask;
+	old_ctrl = readl(priv->ioaddr + MAC_CTRL_REG);
+	ctrl = old_ctrl & ~priv->hw->link.speed_mask;
 
 	if (interface == PHY_INTERFACE_MODE_USXGMII) {
 		switch (speed) {
@@ -1064,7 +1064,8 @@ static void stmmac_mac_link_up(struct phylink_config *config,
 	if (tx_pause && rx_pause)
 		stmmac_mac_flow_ctrl(priv, duplex);
 
-	writel(ctrl, priv->ioaddr + MAC_CTRL_REG);
+	if (ctrl != old_ctrl)
+		writel(ctrl, priv->ioaddr + MAC_CTRL_REG);
 
 	stmmac_mac_set(priv, priv->ioaddr, true);
 	if (phy && priv->dma_cap.eee) {