linux-kernel - [RFC PATCH net-next v3 11/15] net: dsa: mt7530: skip locking if MDIO bus isn't present

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Date:   Wed, 29 Mar 2023 16:59:47 +0100
From:   Daniel Golle <daniel@...rotopia.org>
To:     netdev@...r.kernel.org, linux-mediatek@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Andrew Lunn <andrew@...n.ch>,
        Florian Fainelli <f.fainelli@...il.com>,
        Vladimir Oltean <olteanv@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>,
        Sean Wang <sean.wang@...iatek.com>,
        Landen Chao <Landen.Chao@...iatek.com>,
        DENG Qingfang <dqfext@...il.com>,
        Philipp Zabel <p.zabel@...gutronix.de>
Cc:     Sam Shih <Sam.Shih@...iatek.com>,
        Lorenzo Bianconi <lorenzo@...nel.org>,
        John Crispin <john@...ozen.org>, Felix Fietkau <nbd@....name>
Subject: [RFC PATCH net-next v3 11/15] 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@...rotopia.org>
---
 drivers/net/dsa/mt7530.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/net/dsa/mt7530.c b/drivers/net/dsa/mt7530.c
index ce2665abaaf57..c6fad2d156160 100644
--- a/drivers/net/dsa/mt7530.c
+++ b/drivers/net/dsa/mt7530.c
@@ -145,13 +145,15 @@ core_write_mmd_indirect(struct mt7530_priv *priv, int prtad,
 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
-- 
2.39.2