On Sat, 6 Feb 2021 19:48:26 +0100 Wilfried Wessner <wilfried.wessner@xxxxxxxxx> wrote: > On Sat, Feb 6, 2021 at 6:01 PM Jonathan Cameron <jic23@xxxxxxxxxx> wrote: > > > > On Sat, 6 Feb 2021 15:52:58 +0100 > > Wilfried Wessner <wilfried.wessner@xxxxxxxxx> 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? > > The issue was found in combination of an AD7682 ADC with an ARM based iMX7-CPU. > The SPI line was analyzed with a logic analyzer and a discrepancy > between applied > voltage level and the ADC reported value in user space was observed. > Digging into > the driver code revealed the error. Thanks for the info. > > > > > Also, needs a fixes tag so we can work out what kernels to back port it to. > > Not sure about that, but I found the issue in: > > commit 61556703b610a104de324e4f061dc6cf7b218b46 (HEAD -> master, > origin/master, origin/HEAD) > Merge: 3afe9076a7c1 7f3414226b58 > Author: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> > Date: Wed Feb 3 11:56:58 2021 -0800 > > Merge tag 'for-linus-5.11-rc7' of > git://git.kernel.org/pub/scm/linux/kernel/git/rw/uml > Ok, that doesn't tell us anything much so here we need to look back and find where the bug was introduced by looking at the code. git blame drivers/iio/adc/ad7949.c 7f40e0614317f (Charles-Antoine Couret 2018-10-22 23:02:42 +0200 94) int mask = GENMASK(ad7949_adc->resolution, 0); git log 7f40e0614317f commit 7f40e0614317f20ac07b5aa5cec2eb43737e28d6 Author: Charles-Antoine Couret <charles-antoine.couret@xxxxxxxxxxxxx> Date: Mon Oct 22 23:02:42 2018 +0200 iio:adc:ad7949: Add AD7949 ADC driver family Compatible with AD7682 and AD7689 chips. It is a Analog Devices ADC driver 14/16 bits 4/8 channels with SPI protocol Datasheet of the device: http://www.analog.com/media/en/technical-documentation/data-sheets/AD7949.pdf Signed-off-by: Charles-Antoine Couret <charles-antoine.couret@xxxxxxxxxxxxx> Signed-off-by: Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx> So it goes all the way back to the beginning. The fixes tag will therefore need to be Fixes: 7f40e0614317f ("iio:adc:ad7949: Add AD7949 ADC driver family") Please add that Fixes tag to your v3 with a short statement of how you identified the issue. Thanks, Jonathan > > > > +CC Charle-Antoine Couret as the original driver author. > > done. > I wrote also an email to Charles-Antoine with the proposed fix, his comment was: > ------ > [Wilfried:] > >> since the GENMASK macro uses zero-based indexing? > >> > [Charles-Antoine:] > >You're right, it's a mistake. > >It wouldn't be a problem in many cases but it's better to be compliant. > [Wilfried:] > >> Could you pls. comment on that? > >> > [Charles-Antoine:] > >Good for me. > ------ > > > > > > Thanks, > > > > Jonathan > > > > > > > > Signed-off-by: Wilfried Wessner <wilfried.wessner@xxxxxxxxx> > > > > > > --- > > > 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[] = { > > > { > >