Re: [PATCH v2] iio: ad7949: fix wrong ADC result due to incorrect bit mask

[Jonathan Cameron <jic23@kernel.org>]

On Sat, 6 Feb 2021 15:52:58 +0100
Wilfried Wessner <wilfried.wessner@gmail.com> wrote:

> Fixes a wrong bit mask used for the ADC's result, which was caused by an
> improper usage of the GENMASK() macro. The bits higher than ADC's 
> resolution are undefined and if not masked out correctly, a wrong result 
> can be given. The GENMASK() macro indexing is zero based, so the mask has 
> to go from [resolution - 1 , 0].

Hi Wilfried,

Welcome to IIO and kernel in general!

It's useful to add to the description if the error was found by inspection / script
or by observing an actual error on hardware?

Also, needs a fixes tag so we can work out what kernels to back port it to.

+CC Charle-Antoine Couret as the original driver author.

Thanks,

Jonathan

> 
> Signed-off-by: Wilfried Wessner <wilfried.wessner@gmail.com>
> 
> ---
>  drivers/iio/adc/ad7949.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/iio/adc/ad7949.c b/drivers/iio/adc/ad7949.c
> index 5d597e5050f6..1b4b3203e428 100644
> --- a/drivers/iio/adc/ad7949.c
> +++ b/drivers/iio/adc/ad7949.c
> @@ -91,7 +91,7 @@ static int ad7949_spi_read_channel(struct ad7949_adc_chip *ad7949_adc, int *val,
>  	int ret;
>  	int i;
>  	int bits_per_word = ad7949_adc->resolution;
> -	int mask = GENMASK(ad7949_adc->resolution, 0);
> +	int mask = GENMASK(ad7949_adc->resolution - 1, 0);
>  	struct spi_message msg;
>  	struct spi_transfer tx[] = {
>  		{