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

[Stefan Eichenberger]

On Mon, May 25, 2026 at 06:43:16PM +0200, Vincent Jardin wrote:
> SMBus 3.1 6.5.7 allows a Block Read byte count of 0, but the
> interrupt-driven block-read state machine 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 of this i2c controller.
> 
> Accept count=0: NACK the in-flight dummy byte (TXAK) and set msg->len to
> 2 so i2c_imx_isr_read_continue() emits STOP via its normal last-byte
> path. The dummy byte is discarded; block-read callers only consume
> buf[0..count-1].
> 
> Reading I2DR has likewise already armed the next byte on the
> count > I2C_SMBUS_BLOCK_MAX error path, so NACK it (TXAK) before aborting
> with -EPROTO; otherwise the failing transfer's STOP cannot complete and
> the bus stays held.
> 
> The atomic path regressed earlier (v3.16) and is fixed separately; this
> patch covers only the v6.13 state-machine rework.
> 
> Fixes: 5f5c2d4579ca ("i2c: imx: prevent rescheduling in non dma mode")
> Cc: <stable@xxxxxxxxxxxxxxx> # v6.13+
> Signed-off-by: Vincent Jardin <vjardin@xxxxxxx>
> ---
>  drivers/i2c/busses/i2c-imx.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/drivers/i2c/busses/i2c-imx.c b/drivers/i2c/busses/i2c-imx.c
> index 14107e1ad413..8db8d2e10f5c 100644
> --- a/drivers/i2c/busses/i2c-imx.c
> +++ b/drivers/i2c/busses/i2c-imx.c
> @@ -1061,11 +1061,28 @@ static inline enum imx_i2c_state i2c_imx_isr_read_continue(struct imx_i2c_struct
>  static inline void i2c_imx_isr_read_block_data_len(struct imx_i2c_struct *i2c_imx)
>  {
>  	u8 len = imx_i2c_read_reg(i2c_imx, IMX_I2C_I2DR);
> +	unsigned int temp;
>  
>  	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.
> +		 * So NACK it (TXAK) to not hold the bus.
> +		 */
> +		temp = imx_i2c_read_reg(i2c_imx, IMX_I2C_I2CR);
> +		temp |= I2CR_TXAK;
> +		imx_i2c_write_reg(temp, i2c_imx, IMX_I2C_I2CR);
> +
> +		if (len == 0) {
> +			i2c_imx->msg->buf[i2c_imx->msg_buf_idx++] = 0;
> +			i2c_imx->msg->len = 2;
> +			return;
> +		}
> +
>  		i2c_imx->isr_result = -EPROTO;
>  		i2c_imx->state = IMX_I2C_STATE_FAILED;
>  		wake_up(&i2c_imx->queue);
> +		return;
>  	}
>  	i2c_imx->msg->len += len;
>  	i2c_imx->msg->buf[i2c_imx->msg_buf_idx++] = len;
> 
> -- 
> 2.43.0
> 

Reviewed-by: Stefan Eichenberger <eichest@xxxxxxxxx>