[PATCH net v2 2/5] net: dsa: mt7530: check bus->read() errors in the MDIO regmap backend

[Daniel Golle <daniel@makrotopia.org>]

bus->read() returns a negative errno on failure, but
mt7530_regmap_read() assigns it to a u16, truncating e.g. -ETIMEDOUT
into 0xff92, and returns success. The garbage word is then consumed as
register data, and read-modify-write cycles write it back to the
switch. Check both reads and propagate their errors.

The same defect existed in mt7530_mii_read() since the driver was
introduced and moved into the regmap backend unchanged. core_rmw(),
which accesses the MMD core registers directly rather than through the
regmap, has the identical unchecked bus->read(); fix it the same way.

Fixes: b8f126a8d543 ("net-next: dsa: add dsa support for Mediatek MT7530 switch")
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
---
v2:
 * also check the identical unchecked bus->read() in core_rmw()
   (found by Sashiko AI review)
 * split the mtk_pcs_lynxi_get_state() fix into its own patch

 drivers/net/dsa/mt7530-mdio.c | 11 +++++++++--
 drivers/net/dsa/mt7530.c      |  6 +++++-
 2 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/drivers/net/dsa/mt7530-mdio.c b/drivers/net/dsa/mt7530-mdio.c
index 11ea924a9f35..784dd58a7158 100644
--- a/drivers/net/dsa/mt7530-mdio.c
+++ b/drivers/net/dsa/mt7530-mdio.c
@@ -55,8 +55,15 @@ mt7530_regmap_read(void *context, unsigned int reg, unsigned int *val)
 	if (ret < 0)
 		return ret;
 
-	lo = bus->read(bus, priv->mdiodev->addr, r);
-	hi = bus->read(bus, priv->mdiodev->addr, 0x10);
+	ret = bus->read(bus, priv->mdiodev->addr, r);
+	if (ret < 0)
+		return ret;
+	lo = ret;
+
+	ret = bus->read(bus, priv->mdiodev->addr, 0x10);
+	if (ret < 0)
+		return ret;
+	hi = ret;
 
 	*val = (hi << 16) | (lo & 0xffff);
 
diff --git a/drivers/net/dsa/mt7530.c b/drivers/net/dsa/mt7530.c
index 3c2a3029b10c..f349b54c2d9e 100644
--- a/drivers/net/dsa/mt7530.c
+++ b/drivers/net/dsa/mt7530.c
@@ -124,8 +124,12 @@ core_rmw(struct mt7530_priv *priv, u32 reg, u32 mask, u32 set)
 		goto err;
 
 	/* Read the content of the MMD's selected register */
-	val = bus->read(bus, MT753X_CTRL_PHY_ADDR(priv->mdiodev->addr),
+	ret = bus->read(bus, MT753X_CTRL_PHY_ADDR(priv->mdiodev->addr),
 			MII_MMD_DATA);
+	if (ret < 0)
+		goto err;
+	val = ret;
+
 	val &= ~mask;
 	val |= set;
 	/* Write the data into MMD's selected register */
-- 
2.55.0