[PATCH] i2c: imx: fix emulated smbus block read

[Stefan Eichenberger]

Hi Lukasz,

On Tue, May 20, 2025 at 02:22:52PM +0200, Lukasz Kucharczyk wrote:
> Acknowledge the byte count submitted by the target.
> When I2C_SMBUS_BLOCK_DATA read operation is executed by
> i2c_smbus_xfer_emulated(), the length of the second (read) message is set
> to 1. Length of the block is supposed to be obtained from the target by the
> underlying bus driver.
> The i2c_imx_isr_read() function should emit the acknowledge on i2c bus
> after reading the first byte (i.e., byte count) while processing such
> message (as defined in Section 6.5.7 of System Management Bus
> Specification [1]). Without this acknowledge, the target does not submit
> subsequent bytes and the controller only reads 0xff's.
> 
> In addition, store the length of block data obtained from the target in
> the buffer provided by i2c_smbus_xfer_emulated() - otherwise the first
> byte of actual data is erroneously interpreted as length of the data
> block.
> 
> [1] https://smbus.org/specs/SMBus_3_3_20240512.pdf
> 
> Fixes: 5f5c2d4579ca ("i2c: imx: prevent rescheduling in non dma mode")
> Signed-off-by: Lukasz Kucharczyk <lukasz.kucharczyk at leica-geosystems.com>
> ---
>  drivers/i2c/busses/i2c-imx.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/i2c/busses/i2c-imx.c b/drivers/i2c/busses/i2c-imx.c
> index ee0d25b498cb..4bf550a3b98d 100644
> --- a/drivers/i2c/busses/i2c-imx.c
> +++ b/drivers/i2c/busses/i2c-imx.c
> @@ -1008,7 +1008,7 @@ static inline int i2c_imx_isr_read(struct imx_i2c_struct *i2c_imx)
>  	/* setup bus to read data */
>  	temp = imx_i2c_read_reg(i2c_imx, IMX_I2C_I2CR);
>  	temp &= ~I2CR_MTX;
> -	if (i2c_imx->msg->len - 1)
> +	if ((i2c_imx->msg->len - 1) || (i2c_imx->msg->flags & I2C_M_RECV_LEN))
>  		temp &= ~I2CR_TXAK;
>  
>  	imx_i2c_write_reg(temp, i2c_imx, IMX_I2C_I2CR);
> @@ -1063,6 +1063,7 @@ static inline void i2c_imx_isr_read_block_data_len(struct imx_i2c_struct *i2c_im
>  		wake_up(&i2c_imx->queue);
>  	}
>  	i2c_imx->msg->len += len;
> +	i2c_imx->msg->buf[i2c_imx->msg_buf_idx++] = len;
>  }
>  
>  static irqreturn_t i2c_imx_master_isr(struct imx_i2c_struct *i2c_imx, unsigned int status)

This makes sense, we never tested the actual SMBus emulation. Thanks a lot for the fix.

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