Re: [PATCH v2 1/2] i2c: imx: fix locked bus on SMBus block-read of 0 (atomic)

[Stefan Eichenberger <eichest@gmail.com>]

Sorry for the late reply.

On Mon, May 25, 2026 at 06:43:15PM +0200, Vincent Jardin wrote:
> SMBus 3.1 6.5.7 allows a Block Read byte count of 0, but the atomic
> (polling) path rejects it as -EPROTO. Worse, it returns without a
> NACK+STOP: the next receive cycle has already started, so the target
> keeps holding SDA and the bus stays stuck until a power cycle for
> this i2c controller.
> 
> Reading I2DR to obtain the count likewise arms the next byte on the
> count > I2C_SMBUS_BLOCK_MAX path, which also returned -EPROTO directly
> and left the bus held.
> 
> Handle both: NACK the in-flight dummy byte (TXAK) and extend msgs->len so
> the existing last-byte handling emits STOP; the dummy byte is discarded.
> A count of 0 is a valid empty block read; a count above
> I2C_SMBUS_BLOCK_MAX is still reported as -EPROTO, but only after the bus
> has been released.
> 
> The interrupt-driven path has the same flaw from a later commit and is
> fixed separately, as it carries a different Fixes: tag and stable range.
> 
> Fixes: 8e8782c71595 ("i2c: imx: add SMBus block read support")
> Cc: <stable@vger.kernel.org> # v3.16+
> Signed-off-by: Vincent Jardin <vjardin@free.fr>
> ---
>  drivers/i2c/busses/i2c-imx.c | 19 ++++++++++++++++---
>  1 file changed, 16 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/i2c/busses/i2c-imx.c b/drivers/i2c/busses/i2c-imx.c
> index a208fefd3c3b..14107e1ad413 100644
> --- a/drivers/i2c/busses/i2c-imx.c
> +++ b/drivers/i2c/busses/i2c-imx.c
> @@ -1415,6 +1415,7 @@ static int i2c_imx_atomic_read(struct imx_i2c_struct *i2c_imx,
>  	int i, result;
>  	unsigned int temp;
>  	int block_data = msgs->flags & I2C_M_RECV_LEN;
> +	int block_err = 0;
>  
>  	result = i2c_imx_prepare_read(i2c_imx, msgs, false);
>  	if (result)
> @@ -1436,8 +1437,20 @@ static int i2c_imx_atomic_read(struct imx_i2c_struct *i2c_imx,
>  		 */
>  		if ((!i) && block_data) {
>  			len = imx_i2c_read_reg(i2c_imx, IMX_I2C_I2DR);
> -			if ((len == 0) || (len > I2C_SMBUS_BLOCK_MAX))
> -				return -EPROTO;
> +			if ((len == 0) || (len > I2C_SMBUS_BLOCK_MAX)) {
> +				/*
> +				 * SMBus 3.1 6.5.7: support count byte of 0.
> +				 * I2C_SMBUS_BLOCK_MAX case should not hold the SDA either.
> +				 */
> +				if (len > I2C_SMBUS_BLOCK_MAX)
> +					block_err = -EPROTO;
> +				temp = imx_i2c_read_reg(i2c_imx, IMX_I2C_I2CR);
> +				temp |= I2CR_TXAK;
> +				imx_i2c_write_reg(temp, i2c_imx, IMX_I2C_I2CR);
> +				msgs->buf[0] = 0;
> +				msgs->len = 2;
> +				continue;
> +			}
>  			dev_dbg(&i2c_imx->adapter.dev,
>  				"<%s> read length: 0x%X\n",
>  				__func__, len);
> @@ -1485,7 +1498,7 @@ static int i2c_imx_atomic_read(struct imx_i2c_struct *i2c_imx,
>  			"<%s> read byte: B%d=0x%X\n",
>  			__func__, i, msgs->buf[i]);
>  	}
> -	return 0;
> +	return block_err;
>  }

Reviewed-by: Stefan Eichenberger <eichest@gmail.com>