linux-kernel - Re: EXT: Re: [PATCH] drivers: iio: adc: ltc2497: fix LSB shift

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Message-ID: <20230130065359.GA21081@zoo6.em.health.ge.com>
Date:   Mon, 30 Jan 2023 08:53:59 +0200
From:   Ian Ray <ian.ray@...com>
To:     Jonathan Cameron <jic23@...nel.org>
Cc:     Jonathan Cameron <Jonathan.Cameron@...wei.com>, lars@...afoo.de,
        Michael.Hennerich@...log.com, linux-kernel@...r.kernel.org,
        linux-iio@...r.kernel.org
Subject: Re: EXT: Re: [PATCH] drivers: iio: adc: ltc2497: fix LSB shift

On Sat, Jan 28, 2023 at 06:00:32PM +0000, Jonathan Cameron wrote:
> 
> On Fri, 27 Jan 2023 17:47:56 +0000
> Jonathan Cameron <Jonathan.Cameron@...wei.com> wrote:
> 
> > On Fri, 27 Jan 2023 14:57:14 +0200
> > Ian Ray <ian.ray@...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)

Makes sense.


> 
> > 
> > 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@...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.

lgtm

Many thanks!


> 
> 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);  
> > 
> 
>