Re: [PATCH] drivers: iio: adc: ltc2497: fix LSB shift

[Jonathan Cameron <jic23@kernel.org>]

On Fri, 27 Jan 2023 17:47:56 +0000
Jonathan Cameron <Jonathan.Cameron@Huawei.com> wrote:

> On Fri, 27 Jan 2023 14:57:14 +0200
> Ian Ray <ian.ray@ge.com> wrote:
> 
> > Correct the "sub_lsb" shift for both ltc2497 and ltc2499.  

I dropped the ltc2499 mention here as that one seems to be unaffected
by this patch (beyond as a simplification)

> 
> Hi Iain, Thanks for the patch.
> 
> > 
> > An earlier version of the code shifted by 14 but this was a consequence
> > of reading three bytes into a __be32 buffer and using be32_to_cpu(), so
> > eight extra bits needed to be skipped.  Now we use get_unaligned_be24()
> > and thus the additional skip is wrong.
> > 
> > Fixes 2187cfe ("drivers: iio: adc: ltc2497: LTC2499 support")  
> 
> This isn't in standard Fixes tag format.  If nothing else comes
> up I'll fix up whilst applying.
> 
> > Signed-off-by: Ian Ray <ian.ray@ge.com>

Fix looks good to me.  (I noted my reasoning below on basis someone might
find it useful) 

Applied to the fixes-togreg branch of iio.git with fixes tag fixed and
marked for stable.

Please take a look at:
https://git.kernel.org/pub/scm/linux/kernel/git/jic23/iio.git/commit/?h=fixes-togreg&id=6327a930ab7bfa1ab33bcdffd5f5f4b1e7131504
to check I didn't mess anything up.

Thanks,

Jonathan


> > ---
> >  drivers/iio/adc/ltc2497.c | 6 ++----
> >  1 file changed, 2 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/iio/adc/ltc2497.c b/drivers/iio/adc/ltc2497.c
> > index 17370c5..ec198c6 100644
> > --- a/drivers/iio/adc/ltc2497.c
> > +++ b/drivers/iio/adc/ltc2497.c
> > @@ -28,7 +28,6 @@ struct ltc2497_driverdata {
> >  	struct ltc2497core_driverdata common_ddata;
> >  	struct i2c_client *client;
> >  	u32 recv_size;
> > -	u32 sub_lsb;
> >  	/*
> >  	 * DMA (thus cache coherency maintenance) may require the
> >  	 * transfer buffers to live in their own cache lines.
> > @@ -65,10 +64,10 @@ static int ltc2497_result_and_measure(struct ltc2497core_driverdata *ddata,
> >  		 * equivalent to a sign extension.
> >  		 */
> >  		if (st->recv_size == 3) {
> > -			*val = (get_unaligned_be24(st->data.d8) >> st->sub_lsb)
> > +			*val = (get_unaligned_be24(st->data.d8) >> 6)
Prior to the buggy patch.

1) Read into top 3 bytes of BE32 which corresponds to 8 bits high.
2) Shift by 14.

Before this patch
1) Read only 3 bytes
2) shift by  (31 - (resolution + 1) where resolution is 16 so shift by 14.
Indeed bug.

After this patch.
1) Read 3 bytes
2) shift by 6 

Matches original behaviour - fix is correct.


> >  				- BIT(ddata->chip_info->resolution + 1);
> >  		} else {
> > -			*val = (be32_to_cpu(st->data.d32) >> st->sub_lsb)
> > +			*val = (be32_to_cpu(st->data.d32) >> 6)

Prior to this patch
sub_lsb = 31 - (resolution + 1) where in this case resolution is 24
31 - 25 = 6 so this change is a noop change, so this is good.

> >  				- BIT(ddata->chip_info->resolution + 1);
> >  		}
> >  
> > @@ -122,7 +121,6 @@ static int ltc2497_probe(struct i2c_client *client)
> >  	st->common_ddata.chip_info = chip_info;
> >  
> >  	resolution = chip_info->resolution;
> > -	st->sub_lsb = 31 - (resolution + 1);
> >  	st->recv_size = BITS_TO_BYTES(resolution) + 1;
> >  
> >  	return ltc2497core_probe(dev, indio_dev);  
>