When accessing an MDIO register using single-byte smbus accesses, we have to
perform 2 consecutive operations targeting the same address,
first accessing the MSB then the LSB of the 16 bit register:

  read_1_byte(addr); <- returns MSB of register at address 'addr'
  read_1_byte(addr); <- returns LSB

Some PHY devices present in SFP such as the Broadcom 5461 don't like
seeing foreign i2c transactions in-between these 2 smbus accesses, and
will return the MSB a second time when trying to read the LSB :

  read_1_byte(addr); <- returns MSB

  	i2c_transaction_for_other_device_on_the_bus();

  read_1_byte(addr); <- returns MSB again

Given the already fragile nature of accessing PHYs/SFPs with single-byte
smbus accesses, it's safe to say that this Broadcom PHY may not be the
only one acting like this.

Let's therefore hold the i2c bus lock while performing our smbus
transactions to avoid interleaved accesses.

Fixes: d4bd3aca33c2 ("net: mdio: mdio-i2c: Add support for single-byte SMBus operations")
Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
---
 drivers/net/mdio/mdio-i2c.c | 39 ++++++++++++++++++++++++-------------
 1 file changed, 25 insertions(+), 14 deletions(-)

diff --git a/drivers/net/mdio/mdio-i2c.c b/drivers/net/mdio/mdio-i2c.c
index 53e96bfab542..ed20352a589a 100644
--- a/drivers/net/mdio/mdio-i2c.c
+++ b/drivers/net/mdio/mdio-i2c.c
@@ -116,17 +116,23 @@ static int smbus_byte_mii_read_default_c22(struct mii_bus *bus, int phy_id,
 	if (!i2c_mii_valid_phy_id(phy_id))
 		return 0;
 
-	ret = i2c_smbus_xfer(i2c, i2c_mii_phy_addr(phy_id), 0,
-			     I2C_SMBUS_READ, reg,
-			     I2C_SMBUS_BYTE_DATA, &smbus_data);
+	i2c_lock_bus(i2c, I2C_LOCK_SEGMENT);
+
+	ret = __i2c_smbus_xfer(i2c, i2c_mii_phy_addr(phy_id), 0,
+			       I2C_SMBUS_READ, reg,
+			       I2C_SMBUS_BYTE_DATA, &smbus_data);
 	if (ret < 0)
-		return ret;
+		goto unlock;
 
 	val = (smbus_data.byte & 0xff) << 8;
 
-	ret = i2c_smbus_xfer(i2c, i2c_mii_phy_addr(phy_id), 0,
-			     I2C_SMBUS_READ, reg,
-			     I2C_SMBUS_BYTE_DATA, &smbus_data);
+	ret = __i2c_smbus_xfer(i2c, i2c_mii_phy_addr(phy_id), 0,
+			       I2C_SMBUS_READ, reg,
+			       I2C_SMBUS_BYTE_DATA, &smbus_data);
+
+unlock:
+	i2c_unlock_bus(i2c, I2C_LOCK_SEGMENT);
+
 	if (ret < 0)
 		return ret;
 
@@ -147,17 +153,22 @@ static int smbus_byte_mii_write_default_c22(struct mii_bus *bus, int phy_id,
 
 	smbus_data.byte = (val & 0xff00) >> 8;
 
-	ret = i2c_smbus_xfer(i2c, i2c_mii_phy_addr(phy_id), 0,
-			     I2C_SMBUS_WRITE, reg,
-			     I2C_SMBUS_BYTE_DATA, &smbus_data);
+	i2c_lock_bus(i2c, I2C_LOCK_SEGMENT);
+
+	ret = __i2c_smbus_xfer(i2c, i2c_mii_phy_addr(phy_id), 0,
+			       I2C_SMBUS_WRITE, reg,
+			       I2C_SMBUS_BYTE_DATA, &smbus_data);
 	if (ret < 0)
-		return ret;
+		goto unlock;
 
 	smbus_data.byte = val & 0xff;
 
-	ret = i2c_smbus_xfer(i2c, i2c_mii_phy_addr(phy_id), 0,
-			     I2C_SMBUS_WRITE, reg,
-			     I2C_SMBUS_BYTE_DATA, &smbus_data);
+	ret = __i2c_smbus_xfer(i2c, i2c_mii_phy_addr(phy_id), 0,
+			       I2C_SMBUS_WRITE, reg,
+			       I2C_SMBUS_BYTE_DATA, &smbus_data);
+
+unlock:
+	i2c_unlock_bus(i2c, I2C_LOCK_SEGMENT);
 
 	return ret < 0 ? ret : 0;
 }
-- 
2.49.0

On Fri, Oct 03, 2025 at 09:03:06AM +0200, Maxime Chevallier wrote:
> When accessing an MDIO register using single-byte smbus accesses, we have to
> perform 2 consecutive operations targeting the same address,
> first accessing the MSB then the LSB of the 16 bit register:
> 
>   read_1_byte(addr); <- returns MSB of register at address 'addr'
>   read_1_byte(addr); <- returns LSB
> 
> Some PHY devices present in SFP such as the Broadcom 5461 don't like
> seeing foreign i2c transactions in-between these 2 smbus accesses, and
> will return the MSB a second time when trying to read the LSB :
> 
>   read_1_byte(addr); <- returns MSB
> 
>   	i2c_transaction_for_other_device_on_the_bus();
> 
>   read_1_byte(addr); <- returns MSB again
> 
> Given the already fragile nature of accessing PHYs/SFPs with single-byte
> smbus accesses, it's safe to say that this Broadcom PHY may not be the
> only one acting like this.
> 
> Let's therefore hold the i2c bus lock while performing our smbus
> transactions to avoid interleaved accesses.
> 
> Fixes: d4bd3aca33c2 ("net: mdio: mdio-i2c: Add support for single-byte SMBus operations")
> Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>

Reviewed-by: Andrew Lunn <andrew@lunn.ch>

    Andrew

On Fri,  3 Oct 2025 09:03:06 +0200
Maxime Chevallier <maxime.chevallier@bootlin.com> wrote:

> When accessing an MDIO register using single-byte smbus accesses, we have to
> perform 2 consecutive operations targeting the same address,
> first accessing the MSB then the LSB of the 16 bit register:
> 
>   read_1_byte(addr); <- returns MSB of register at address 'addr'
>   read_1_byte(addr); <- returns LSB
> 
> Some PHY devices present in SFP such as the Broadcom 5461 don't like
> seeing foreign i2c transactions in-between these 2 smbus accesses, and
> will return the MSB a second time when trying to read the LSB :
> 
>   read_1_byte(addr); <- returns MSB
> 
>   	i2c_transaction_for_other_device_on_the_bus();
> 
>   read_1_byte(addr); <- returns MSB again
> 
> Given the already fragile nature of accessing PHYs/SFPs with single-byte
> smbus accesses, it's safe to say that this Broadcom PHY may not be the
> only one acting like this.
> 
> Let's therefore hold the i2c bus lock while performing our smbus
> transactions to avoid interleaved accesses.
> 
> Fixes: d4bd3aca33c2 ("net: mdio: mdio-i2c: Add support for single-byte SMBus
> operations") Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>

Reviewed-by: Kory Maincent <kory.maincent@bootlin.com>

Thank you!

-- 
Köry Maincent, Bootlin
Embedded Linux and kernel engineering
https://bootlin.com