linux-kernel - Re: [PATCH v2 3/8] iio: inkern: error out on unsupported offset type

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Date:   Wed, 09 Jun 2021 17:40:47 -0400
From:   "Liam Beguin" <liambeguin@...il.com>
To:     "Jonathan Cameron" <jic23@...nel.org>
Cc:     <peda@...ntia.se>, <lars@...afoo.de>, <pmeerw@...erw.net>,
        <linux-kernel@...r.kernel.org>, <linux-iio@...r.kernel.org>,
        <devicetree@...r.kernel.org>, <robh+dt@...nel.org>
Subject: Re: [PATCH v2 3/8] iio: inkern: error out on unsupported offset
 type

Hi Jonathan,

On Wed Jun 9, 2021 at 4:28 PM EDT, Jonathan Cameron wrote:
> On Mon, 7 Jun 2021 10:47:13 -0400
> Liam Beguin <liambeguin@...il.com> wrote:
>
> > From: Liam Beguin <lvb@...hos.com>
> > 
> > iio_convert_raw_to_processed_unlocked() assumes the offset is an
> > integer.
> > Make that clear to the consumer by returning an error on unsupported
> > offset types without breaking valid implicit truncations.
> > 
> > Signed-off-by: Liam Beguin <lvb@...hos.com>
> > ---
> >  drivers/iio/inkern.c | 34 +++++++++++++++++++++++++++++-----
> >  1 file changed, 29 insertions(+), 5 deletions(-)
> > 
> > diff --git a/drivers/iio/inkern.c b/drivers/iio/inkern.c
> > index b69027690ed5..0b5667f22b1d 100644
> > --- a/drivers/iio/inkern.c
> > +++ b/drivers/iio/inkern.c
> > @@ -578,13 +578,37 @@ EXPORT_SYMBOL_GPL(iio_read_channel_average_raw);
> >  static int iio_convert_raw_to_processed_unlocked(struct iio_channel *chan,
> >  	int raw, int *processed, unsigned int scale)
> >  {
> > -	int scale_type, scale_val, scale_val2, offset;
> > +	int scale_type, scale_val, scale_val2;
> > +	int offset_type, offset_val, offset_val2;
> >  	s64 raw64 = raw;
> > -	int ret;
> >  
> > -	ret = iio_channel_read(chan, &offset, NULL, IIO_CHAN_INFO_OFFSET);
> > -	if (ret >= 0)
> > -		raw64 += offset;
> > +	offset_type = iio_channel_read(chan, &offset_val, &offset_val2,
> > +				       IIO_CHAN_INFO_OFFSET);
> > +	if (offset_type >= 0) {
> > +		switch (offset_type) {
> > +		case IIO_VAL_INT:
> > +			break;
> > +		case IIO_VAL_INT_PLUS_MICRO:
> > +			if (offset_val2 > 1000)
>
> What's the logic behind this one? > 1000000
> would be an interesting corner case, though I'm not sure we've ever
> explicitly disallowed it before.
>
> Why are we at 1000th of that for the check?
>

For these the idea was to go with one milli of precision.
I don't know if that's a good criteria but I wanted to start with
something. Do you have any suggestions?

> > +				return -EINVAL;
> > +			break;
> > +		case IIO_VAL_INT_PLUS_NANO:
> > +			if (offset_val2 > 1000000)
>
> Similar this is a bit odd.
>
> > +				return -EINVAL;
> > +		case IIO_VAL_FRACTIONAL:
> > +			if (offset_val2 != 1)
> > +				return -EINVAL;
>
> We could be more flexible on this, but I don't recall any
> channels using this so far.
>
> > +			break;
> > +		case IIO_VAL_FRACTIONAL_LOG2:
> > +			if (offset_val2)
> > +				return -EINVAL;
>
> Same in this case.
>

For these two cases, I went with what Peter suggested in the previous
version, to not break on valid implicit truncations.

What would be a good precision criteria for all offset types?

> > +			break;
> > +		default:
> > +			return -EINVAL;
> > +		}
> > +
> > +		raw64 += offset_val;
> > +	}
> >  
> >  	scale_type = iio_channel_read(chan, &scale_val, &scale_val2,
> >  					IIO_CHAN_INFO_SCALE);

Thanks for looking at this,
Liam