linux-kernel - Re: [PATCH v2] net: sfp: add SMBus I2C block support

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Message-ID: <f976d637-defd-4e32-8a82-6705e69246d2@gmail.com>
Date: Mon, 5 Jan 2026 17:19:27 +0100
From: Jonas Jelonek <jelonek.jonas@...il.com>
To: "Russell King (Oracle)" <linux@...linux.org.uk>
Cc: Andrew Lunn <andrew@...n.ch>, Heiner Kallweit <hkallweit1@...il.com>,
 "David S . Miller" <davem@...emloft.net>, Eric Dumazet
 <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org, Bjørn Mork <bjorn@...k.no>,
 Maxime Chevallier <maxime.chevallier@...tlin.com>
Subject: Re: [PATCH v2] net: sfp: add SMBus I2C block support

Hi Russell,

On 05.01.26 17:16, Russell King (Oracle) wrote:
> On Mon, Jan 05, 2026 at 03:46:53PM +0000, Jonas Jelonek wrote:
>> @@ -765,26 +794,70 @@ static int sfp_smbus_byte_write(struct sfp *sfp, bool a2, u8 dev_addr,
>>  		dev_addr++;
>>  	}
>>  
>> +	return data - (u8 *)buf;
> A separate fix is being submitted for this.
>
>> +}
>> +
>> +static int sfp_smbus_block_write(struct sfp *sfp, bool a2, u8 dev_addr,
>> +				 void *buf, size_t len)
>> +{
>> +	size_t block_size = sfp->i2c_block_size;
>> +	union i2c_smbus_data smbus_data;
>> +	u8 bus_addr = a2 ? 0x51 : 0x50;
>> +	u8 *data = buf;
>> +	u8 this_len;
>> +	int ret;
>> +
>> +	while (len) {
>> +		this_len = min(len, block_size);
>> +
>> +		smbus_data.block[0] = this_len;
>> +		memcpy(&smbus_data.block[1], data, this_len);
>> +		ret = i2c_smbus_xfer(sfp->i2c, bus_addr, 0,
>> +				     I2C_SMBUS_WRITE, dev_addr,
>> +				     I2C_SMBUS_I2C_BLOCK_DATA, &smbus_data);
>> +		if (ret)
>> +			return ret;
>> +
>> +		len -= this_len;
>> +		data += this_len;
>> +		dev_addr += this_len;
>> +	}
>> +
>>  	return 0;
> This is wrong. As already said, the I2C accessors return the number of
> bytes successfully transferred. Zero means no bytes were transferred,
> which is an error.
>
> All callers to sfp_write() validate that the expected number of bytes
> were written. Thus, returning zero will cause failures.
>

yes. I totally messed up v2 but already pushed out a v3 that fixes that, and
has the separate fix as a prerequisite.

Sorry for the confusion.

Best,
Jonas