The expected on-wire format of an SMBus Block Write is

  S Addr Wr [A] Comm [A] Count [A] Data [A] Data [A] ... [A] Data [A] P

Everything starting from the Count byte is provided by the I2C subsystem in
the array data->block. But the driver was skipping the Count byte
(data->block[0]) when sending it to the RTL93xx I2C controller.

Only the actual data could be seen on the wire:

  S Addr Wr [A] Comm [A] Data [A] Data [A] ... [A] Data [A] P

This wire format is not SMBus Block Write compatible but matches the format
of an I2C Block Write. Simply adding the count byte to the buffer for the
I2C controller enough to fix the transmission.

Cc: <stable@vger.kernel.org>
Fixes: c366be720235 ("i2c: Add driver for the RTL9300 I2C controller")
Signed-off-by: Sven Eckelmann <sven@narfation.org>
---
 drivers/i2c/busses/i2c-rtl9300.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/i2c/busses/i2c-rtl9300.c b/drivers/i2c/busses/i2c-rtl9300.c
index a10e5e6e00075fabb8906d56f09f5b9141fbc06e..ed82e6c21aaf46000a263740123ffba833578cc2 100644
--- a/drivers/i2c/busses/i2c-rtl9300.c
+++ b/drivers/i2c/busses/i2c-rtl9300.c
@@ -288,7 +288,7 @@ static int rtl9300_i2c_smbus_xfer(struct i2c_adapter *adap, u16 addr, unsigned s
 		if (ret)
 			goto out_unlock;
 		if (read_write == I2C_SMBUS_WRITE) {
-			ret = rtl9300_i2c_write(i2c, &data->block[1], data->block[0]);
+			ret = rtl9300_i2c_write(i2c, &data->block[0], data->block[0] + 1);
 			if (ret)
 				goto out_unlock;
 		}

-- 
2.47.2

On Saturday, 2 August 2025 12:32:02 CEST Sven Eckelmann wrote:
[...]
> diff --git a/drivers/i2c/busses/i2c-rtl9300.c b/drivers/i2c/busses/i2c-rtl9300.c
> index a10e5e6e00075fabb8906d56f09f5b9141fbc06e..ed82e6c21aaf46000a263740123ffba833578cc2 100644
> --- a/drivers/i2c/busses/i2c-rtl9300.c
> +++ b/drivers/i2c/busses/i2c-rtl9300.c
> @@ -288,7 +288,7 @@ static int rtl9300_i2c_smbus_xfer(struct i2c_adapter *adap, u16 addr, unsigned s
>  		if (ret)
>  			goto out_unlock;
>  		if (read_write == I2C_SMBUS_WRITE) {
> -			ret = rtl9300_i2c_write(i2c, &data->block[1], data->block[0]);
> +			ret = rtl9300_i2c_write(i2c, &data->block[0], data->block[0] + 1);
>  			if (ret)
>  				goto out_unlock;
>  		}
> 
> 

Yes, there is as always this "2 seconds" after the send "wtf. were did the 
line go" situation:

diff --git a/drivers/i2c/busses/i2c-rtl9300.c b/drivers/i2c/busses/i2c-rtl9300.c
index a10e5e6e00075fabb8906d56f09f5b9141fbc06e..4d109ebd02f0cad86f50e4e148966b3fa68b0196 100644
--- a/drivers/i2c/busses/i2c-rtl9300.c
+++ b/drivers/i2c/busses/i2c-rtl9300.c
@@ -284,11 +284,11 @@ static int rtl9300_i2c_smbus_xfer(struct i2c_adapter *adap, u16 addr, unsigned s
 		ret = rtl9300_i2c_reg_addr_set(i2c, command, 1);
 		if (ret)
 			goto out_unlock;
-		ret = rtl9300_i2c_config_xfer(i2c, chan, addr, data->block[0]);
+		ret = rtl9300_i2c_config_xfer(i2c, chan, addr, data->block[0] + 1);
 		if (ret)
 			goto out_unlock;
 		if (read_write == I2C_SMBUS_WRITE) {
-			ret = rtl9300_i2c_write(i2c, &data->block[1], data->block[0]);
+			ret = rtl9300_i2c_write(i2c, &data->block[0], data->block[0] + 1);
 			if (ret)
 				goto out_unlock;
 		}

v2 will be send tomorrow. Sorry about the noise.

Kind regards,
	Sven