Re: [PATCH] iio: xilinx-ams: Don't include ams_ctrl_channels in scan_mask

[Jonathan Cameron <jic23@kernel.org>]

On Mon, 11 Mar 2024 12:28:00 -0400
Sean Anderson <sean.anderson@linux.dev> wrote:

> ams_enable_channel_sequence constructs a "scan_mask" for all the PS and
> PL channels. This works out fine, since scan_index for these channels is
> less than 64. However, it also includes the ams_ctrl_channels, where
> scan_index is greater than 64, triggering undefined behavior. Since we
> don't need these channels anyway, just exclude them.
> 
> Fixes: d5c70627a794 ("iio: adc: Add Xilinx AMS driver")
> Signed-off-by: Sean Anderson <sean.anderson@linux.dev>

Hi Sean,

I'd ideally like to understand why we have channels with such large
scan indexes.  Those values should only be used for buffered capture.
It feels like they are being abused here.  Can we set them to -1 instead
and check based on that?
For a channel, a scan index of -1 means it can't be captured via the buffered
interfaces but only accessed via sysfs reads.
I think that's what we have here?

I just feel like if we leave these as things stand, we will get bitten
by similar bugs in the future.  At least with -1 it should be obvious why!

Jonathan


> ---
> 
>  drivers/iio/adc/xilinx-ams.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/iio/adc/xilinx-ams.c b/drivers/iio/adc/xilinx-ams.c
> index a55396c1f8b2..4de7ce598e4d 100644
> --- a/drivers/iio/adc/xilinx-ams.c
> +++ b/drivers/iio/adc/xilinx-ams.c
> @@ -414,8 +414,12 @@ static void ams_enable_channel_sequence(struct iio_dev *indio_dev)
>  
>  	/* Run calibration of PS & PL as part of the sequence */
>  	scan_mask = BIT(0) | BIT(AMS_PS_SEQ_MAX);
> -	for (i = 0; i < indio_dev->num_channels; i++)
> -		scan_mask |= BIT_ULL(indio_dev->channels[i].scan_index);
> +	for (i = 0; i < indio_dev->num_channels; i++) {
> +		const struct iio_chan_spec *chan = &indio_dev->channels[i];
> +
> +		if (chan->scan_index < AMS_CTRL_SEQ_BASE)
> +			scan_mask |= BIT_ULL(chan->scan_index);
> +	}
>  
>  	if (ams->ps_base) {
>  		/* put sysmon in a soft reset to change the sequence */


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