openwrt/target/linux/generic/backport-5.15/790-v6.4-0012-net-dsa-mt7530-skip-locking-if-MDIO-bus-isn-t-presen.patch

From 54d4147a121cec5004a673a58572da346e4458f8 Mon Sep 17 00:00:00 2001
From: Daniel Golle <daniel@makrotopia.org>
Date: Mon, 3 Apr 2023 02:19:28 +0100
Subject: [PATCH 15/16] net: dsa: mt7530: skip locking if MDIO bus isn't
 present

As MT7530 and MT7531 internally use 32-bit wide registers, each access
to any register of the switch requires several operations on the MDIO
bus. Hence if there is congruent access, e.g. due to PCS or PHY
polling, this can mess up and interfere with another ongoing register
access sequence.

However, the MDIO bus mutex is only relevant for MDIO-connected
switches. Prepare switches which have there registers directly mapped
into the SoCs register space via MMIO which do not require such
locking. There we can simply use regmap's default locking mechanism.

Hence guard mutex operations to only be performed in case of MDIO
connected switches.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>
---
 drivers/net/dsa/mt7530.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

--- a/drivers/net/dsa/mt7530.c
+++ b/drivers/net/dsa/mt7530.c
@@ -144,13 +144,15 @@ err:
 static void
 mt7530_mutex_lock(struct mt7530_priv *priv)
 {
-	mutex_lock_nested(&priv->bus->mdio_lock, MDIO_MUTEX_NESTED);
+	if (priv->bus)
+		mutex_lock_nested(&priv->bus->mdio_lock, MDIO_MUTEX_NESTED);
 }
 
 static void
 mt7530_mutex_unlock(struct mt7530_priv *priv)
 {
-	mutex_unlock(&priv->bus->mdio_lock);
+	if (priv->bus)
+		mutex_unlock(&priv->bus->mdio_lock);
 }
 
 static void