ramips: mt7530 swconfig: fix race condition in register access

The mt7530_{r,w}32 operation over MDIO uses 3 mdiobus operations and
does not hold a lock, which causes a race condition when multiple
threads try to access a register, they may get unexpected results.

To avoid this, handle the MDIO lock manually, and use the unlocked
__mdiobus_{read,write} in the critical section.

This fixes the "Ghost VLAN" artifact[1] in MT7530/7621 when the VLAN
operation and the swconfig LED link status poll race between each other.

[1] https://forum.openwrt.org/t/mysterious-vlan-ids-on-mt7621-device/64495

Signed-off-by: DENG Qingfang <[email protected]>

target/linux/ramips/files/drivers/net/ethernet/ralink/mt7530.c

@@ -290,9 +290,11 @@ mt7530_r32(struct mt7530_priv *priv, u32 reg)
 	if (priv->bus) {
 		u16 high, low;
 
-		mdiobus_write(priv->bus, 0x1f, 0x1f, (reg >> 6) & 0x3ff);
-		low = mdiobus_read(priv->bus, 0x1f, (reg >> 2) & 0xf);
-		high = mdiobus_read(priv->bus, 0x1f, 0x10);
+		mutex_lock(&priv->bus->mdio_lock);
+		__mdiobus_write(priv->bus, 0x1f, 0x1f, (reg >> 6) & 0x3ff);
+		low = __mdiobus_read(priv->bus, 0x1f, (reg >> 2) & 0xf);
+		high = __mdiobus_read(priv->bus, 0x1f, 0x10);
+		mutex_unlock(&priv->bus->mdio_lock);
 
 		return (high << 16) | (low & 0xffff);
 	}
@@ -307,9 +309,11 @@ static void
 mt7530_w32(struct mt7530_priv *priv, u32 reg, u32 val)
 {
 	if (priv->bus) {
-		mdiobus_write(priv->bus, 0x1f, 0x1f, (reg >> 6) & 0x3ff);
-		mdiobus_write(priv->bus, 0x1f, (reg >> 2) & 0xf,  val & 0xffff);
-		mdiobus_write(priv->bus, 0x1f, 0x10, val >> 16);
+		mutex_lock(&priv->bus->mdio_lock);
+		__mdiobus_write(priv->bus, 0x1f, 0x1f, (reg >> 6) & 0x3ff);
+		__mdiobus_write(priv->bus, 0x1f, (reg >> 2) & 0xf,  val & 0xffff);
+		__mdiobus_write(priv->bus, 0x1f, 0x10, val >> 16);
+		mutex_unlock(&priv->bus->mdio_lock);
 		return;
 	}