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

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Date:   Thu, 10 Jun 2021 10:06:55 +0100
From:   Jonathan Cameron <Jonathan.Cameron@...wei.com>
To:     Liam Beguin <liambeguin@...il.com>
CC:     Jonathan Cameron <jic23@...nel.org>, <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

On Wed, 09 Jun 2021 17:40:47 -0400
"Liam Beguin" <liambeguin@...il.com> wrote:

> 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?

@Peter, what were your thoughts on this.

I was thinking we'd just not worry about how much truncation was happening
and just silently eat it.

J
> 
> > > +			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