Re: [PATCH net-next v7 2/2] net: sfp: extend SMBus support

[Maxime Chevallier <maxime.chevallier@bootlin.com>]

Hi Jonas,

On 5/7/26 11:33, Jonas Jelonek wrote:
> Commit 7662abf4db94 ("net: phy: sfp: Add support for SMBus module access")
> added SMBus access for SFP modules, but limited it to single-byte
> transfers. As a side effect, hwmon is disabled (16-bit reads cannot be
> guaranteed atomic) and a warning is printed.
> 
> Many SMBus-only I2C controllers in the wild support more than just
> byte access, and SFP cages are often wired to such controllers
> rather than to a full-featured I2C controller -- e.g. the SMBus
> controllers in the Realtek longan and mango SoCs, which advertise
> word access and I2C block reads. Today, they cannot drive an SFP at
> all without falling back to the byte-only path.
> 
> Extend sfp_smbus_read()/sfp_smbus_write() so that, in addition to
> the existing byte access, they also use SMBus word access and SMBus
> I2C block access whenever the adapter advertises them. Both
> directions are handled in a single read and a single write helper
> that pick the largest supported transfer per chunk and fall back as
> needed.
> 
> I2C-block is preferred unconditionally when available: the protocol
> carries any length 1..32, so it can serve every chunk -- including
> the 1- and 2-byte tails -- without help from word or byte access.
> Note that this requires I2C_FUNC_SMBUS_I2C_BLOCK, which reads a
> caller-specified number of bytes. This deviates from the official
> SMBus Block Read (length is supplied by the slave) but is widely
> supported by Linux I2C controllers/drivers.
> 
> Capability matrix this implementation supports:
> 
>    - BYTE only:                  works (unchanged behaviour); 1-byte
>                                  xfers, hwmon disabled.
>    - BYTE + WORD:                word for >=2-byte chunks, byte for
>                                  trailing odd byte.
>    - I2C_BLOCK present (with or
>      without BYTE/WORD):         block as the universal transport for
>                                  every chunk.
>    - WORD only (no BYTE/BLOCK):  accepted with WARN_ONCE. Even-length
>                                  transfers work; odd-length transfers
>                                  (e.g. the 3-byte cotsworks fixup
>                                  write) hit the BYTE branch which the
>                                  adapter does not implement, so the
>                                  xfer returns an error and the
>                                  operation is aborted. No mainline
>                                  I2C driver was found to advertise
>                                  WORD without BYTE; the warning lets
>                                  us learn about it if it ever shows
>                                  up.
> 
> Adapters with asymmetric R/W capabilities (e.g. only READ_I2C_BLOCK
> but not WRITE_I2C_BLOCK) remain functionally correct -- the
> per-iteration fallback uses the direction-specific bits -- but the
> shared i2c_max_block_size is sized by the all-bits-set check, so a
> transfer in the better-supported direction is not upgraded. None of
> the mainline I2C bus drivers surveyed during review advertise such
> asymmetry; promoting i2c_max_block_size to per-direction sizes can
> be revisited if needed.
> 
> Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>

I gave this a test and it seems to work fine with all modules I have :)

I tested on both boards that have single-byte smbus, and ones with a 
real i2c controller. I had to hack around in the i2c controller to 
pretend it only supported smbus, that did the trick.

I'm unsure if you're going to followup with the sashiko review on this 
particular patch, it does seem to raise a valid point. I'll followup 
with tested-by/reviewed-by on the next iteration :)

Thanks for this work :)

Maxime

> ---
>   drivers/net/phy/sfp.c | 134 +++++++++++++++++++++++++++++++++---------
>   1 file changed, 107 insertions(+), 27 deletions(-)
> 
> diff --git a/drivers/net/phy/sfp.c b/drivers/net/phy/sfp.c
> index e58e29a1e8d2..16d41d7ee632 100644
> --- a/drivers/net/phy/sfp.c
> +++ b/drivers/net/phy/sfp.c
> @@ -14,6 +14,7 @@
>   #include <linux/platform_device.h>
>   #include <linux/rtnetlink.h>
>   #include <linux/slab.h>
> +#include <linux/unaligned.h>
>   #include <linux/workqueue.h>
>   
>   #include "sfp.h"
> @@ -756,50 +757,110 @@ static int sfp_i2c_write(struct sfp *sfp, bool a2, u8 dev_addr, void *buf,
>   	return ret == ARRAY_SIZE(msgs) ? len : 0;
>   }
>   
> -static int sfp_smbus_byte_read(struct sfp *sfp, bool a2, u8 dev_addr,
> -			       void *buf, size_t len)
> +static int sfp_smbus_read(struct sfp *sfp, bool a2, u8 dev_addr, void *buf,
> +			  size_t len)
>   {
>   	union i2c_smbus_data smbus_data;
>   	u8 bus_addr = a2 ? 0x51 : 0x50;
> +	size_t this_len, transferred;
> +	u32 functionality;
>   	u8 *data = buf;
>   	int ret;
>   
> -	while (len) {
> -		ret = i2c_smbus_xfer(sfp->i2c, bus_addr, 0,
> -				     I2C_SMBUS_READ, dev_addr,
> -				     I2C_SMBUS_BYTE_DATA, &smbus_data);
> -		if (ret < 0)
> -			return ret;
> +	functionality = i2c_get_functionality(sfp->i2c);
>   
> -		*data = smbus_data.byte;
> +	while (len) {
> +		this_len = min(len, sfp->i2c_block_size);
> +
> +		if (functionality & I2C_FUNC_SMBUS_READ_I2C_BLOCK) {
> +			smbus_data.block[0] = this_len;
> +			ret = i2c_smbus_xfer(sfp->i2c, bus_addr, 0,
> +					     I2C_SMBUS_READ, dev_addr,
> +					     I2C_SMBUS_I2C_BLOCK_DATA, &smbus_data);
> +			if (ret < 0)
> +				return ret;
> +
> +			memcpy(data, &smbus_data.block[1], this_len);
> +			transferred = this_len;
> +		} else if (this_len >= 2 &&
> +			   (functionality & I2C_FUNC_SMBUS_READ_WORD_DATA)) {
> +			ret = i2c_smbus_xfer(sfp->i2c, bus_addr, 0,
> +					     I2C_SMBUS_READ, dev_addr,
> +					     I2C_SMBUS_WORD_DATA, &smbus_data);
> +			if (ret < 0)
> +				return ret;
> +
> +			put_unaligned_le16(smbus_data.word, data);
> +			transferred = 2;
> +		} else {
> +			ret = i2c_smbus_xfer(sfp->i2c, bus_addr, 0,
> +					     I2C_SMBUS_READ, dev_addr,
> +					     I2C_SMBUS_BYTE_DATA, &smbus_data);
> +			if (ret < 0)
> +				return ret;
> +
> +			*data = smbus_data.byte;
> +			transferred = 1;
> +		}
>   
> -		len--;
> -		data++;
> -		dev_addr++;
> +		data += transferred;
> +		len -= transferred;
> +		dev_addr += transferred;
>   	}
>   
>   	return data - (u8 *)buf;
>   }
>   
> -static int sfp_smbus_byte_write(struct sfp *sfp, bool a2, u8 dev_addr,
> -				void *buf, size_t len)
> +static int sfp_smbus_write(struct sfp *sfp, bool a2, u8 dev_addr, void *buf,
> +			   size_t len)
>   {
>   	union i2c_smbus_data smbus_data;
>   	u8 bus_addr = a2 ? 0x51 : 0x50;
> +	size_t this_len, transferred;
> +	u32 functionality;
>   	u8 *data = buf;
>   	int ret;
>   
> +	functionality = i2c_get_functionality(sfp->i2c);
> +
>   	while (len) {
> -		smbus_data.byte = *data;
> -		ret = i2c_smbus_xfer(sfp->i2c, bus_addr, 0,
> -				     I2C_SMBUS_WRITE, dev_addr,
> -				     I2C_SMBUS_BYTE_DATA, &smbus_data);
> -		if (ret)
> -			return ret;
> +		this_len = min(len, sfp->i2c_block_size);
> +
> +		if (functionality & I2C_FUNC_SMBUS_WRITE_I2C_BLOCK) {
> +			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 < 0)
> +				return ret;
> +
> +			transferred = this_len;
> +		} else if (this_len >= 2 &&
> +			   (functionality & I2C_FUNC_SMBUS_WRITE_WORD_DATA)) {
> +			smbus_data.word = get_unaligned_le16(data);
> +			ret = i2c_smbus_xfer(sfp->i2c, bus_addr, 0,
> +					     I2C_SMBUS_WRITE, dev_addr,
> +					     I2C_SMBUS_WORD_DATA, &smbus_data);
> +			if (ret < 0)
> +				return ret;
> +
> +			transferred = 2;
> +		} else {
> +			smbus_data.byte = *data;
> +			ret = i2c_smbus_xfer(sfp->i2c, bus_addr, 0,
> +					     I2C_SMBUS_WRITE, dev_addr,
> +					     I2C_SMBUS_BYTE_DATA, &smbus_data);
> +			if (ret < 0)
> +				return ret;
> +
> +			transferred = 1;
> +		}
>   
> -		len--;
> -		data++;
> -		dev_addr++;
> +		data += transferred;
> +		len -= transferred;
> +		dev_addr += transferred;
>   	}
>   
>   	return data - (u8 *)buf;
> @@ -815,10 +876,29 @@ static int sfp_i2c_configure(struct sfp *sfp, struct i2c_adapter *i2c)
>   		sfp->read = sfp_i2c_read;
>   		sfp->write = sfp_i2c_write;
>   		max_block_size = SFP_EEPROM_BLOCK_SIZE;
> -	} else if (i2c_check_functionality(i2c, I2C_FUNC_SMBUS_BYTE_DATA)) {
> -		sfp->read = sfp_smbus_byte_read;
> -		sfp->write = sfp_smbus_byte_write;
> -		max_block_size = 1;
> +	} else if (i2c_check_functionality(i2c, I2C_FUNC_SMBUS_BYTE_DATA) ||
> +		   i2c_check_functionality(i2c, I2C_FUNC_SMBUS_I2C_BLOCK)) {
> +		/* Either protocol alone covers any length: I2C-block carries
> +		 * 1..32 bytes per xfer, byte iterates one byte at a time.
> +		 */
> +		sfp->read = sfp_smbus_read;
> +		sfp->write = sfp_smbus_write;
> +
> +		if (i2c_check_functionality(i2c, I2C_FUNC_SMBUS_I2C_BLOCK))
> +			max_block_size = SFP_EEPROM_BLOCK_SIZE;
> +		else if (i2c_check_functionality(i2c, I2C_FUNC_SMBUS_WORD_DATA))
> +			max_block_size = 2;
> +		else
> +			max_block_size = 1;
> +	} else if (WARN_ONCE(i2c_check_functionality(i2c, I2C_FUNC_SMBUS_WORD_DATA),
> +			     "SMBus word-only adapter; odd-length transfers will fail\n")) {
> +		/* Word-only: even-length xfers work; odd-length xfers fall
> +		 * to BYTE, which the adapter does not advertise and will
> +		 * likely fail.
> +		 */
> +		sfp->read = sfp_smbus_read;
> +		sfp->write = sfp_smbus_write;
> +		max_block_size = 2;
>   	} else {
>   		sfp->i2c = NULL;
>   		return -EINVAL;