linux-kernel - Re: [PATCH v1 2/9] iio: inkern: error out on unsupported offset type

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Message-Id: <CBRGVFX9GJMZ.22AK6P7RNXMES@shaak>
Date:   Mon, 31 May 2021 09:31:49 -0400
From:   "Liam Beguin" <liambeguin@...il.com>
To:     "Peter Rosin" <peda@...ntia.se>, <jic23@...nel.org>,
        <lars@...afoo.de>, <pmeerw@...erw.net>
Cc:     <linux-kernel@...r.kernel.org>, <linux-iio@...r.kernel.org>,
        <devicetree@...r.kernel.org>, <robh+dt@...nel.org>
Subject: Re: [PATCH v1 2/9] iio: inkern: error out on unsupported offset
 type

Hi Peter,

On Mon May 31, 2021 at 5:45 AM EDT, Peter Rosin wrote:
> Hi!
>
> On 2021-05-30 02:59, Liam Beguin 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 when an
> > unsupported offset type is detected.
> > 
> > Signed-off-by: Liam Beguin <lvb@...hos.com>
> > ---
> >  drivers/iio/inkern.c | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/iio/inkern.c b/drivers/iio/inkern.c
> > index 4b6a8e11116a..dede4536d499 100644
> > --- a/drivers/iio/inkern.c
> > +++ b/drivers/iio/inkern.c
> > @@ -595,8 +595,12 @@ static int iio_convert_raw_to_processed_unlocked(struct iio_channel *chan,
> >  	int ret;
> >  
> >  	ret = iio_channel_read(chan, &offset, NULL, IIO_CHAN_INFO_OFFSET);
> > -	if (ret >= 0)
> > +	if (ret == IIO_VAL_INT) {
> >  		raw64 += offset;
> > +	} else if (ret >= 0) {
> > +		dev_err(&chan->indio_dev->dev, "unsupported offset type");
> > +		return -EINVAL;
> > +	}
> >  
> >  	scale_type = iio_channel_read(chan, &scale_val, &scale_val2,
> >  					IIO_CHAN_INFO_SCALE);
> > 
>
> This breaks the implicit truncation that happens for drivers that have
> offsets of type IIO_VAL_INT_PLUS_{MICRO_DB,MICRO,NANO}
>
> Implicit truncation might be more appropriate than an error?
>
> However, to error out on fractional offsets etc still seem appropriate,
> but
> there are corner cases where the existing code did the right thing. E.g.
> a denominator of one or a fractional-log2 of zero, but a big denominator
> and
> a smaller numerator would also just result in a relatively harmless
> truncation.
>
> I don't know if it's really right to just break that?

Apologies for missing these. You're right that this change shouldn't
break what used to work implicitly. I'll rework this.

Liam

>
> Cheers,
> Peter