Re: [PATCH v3 11/17] iio: core: Introduce _zeropoint for differential channels

[Jonathan Cameron <jic23@kernel.org>]

On Tue, 28 Jun 2022 14:17:33 +0200
Andy Shevchenko <andy.shevchenko@gmail.com> wrote:

> On Sun, Jun 26, 2022 at 2:20 PM Jonathan Cameron <jic23@kernel.org> wrote:
> >
> > From: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> >
> > Address an ABI gap for device where the offset of both lines in a
> > differential pair may be controlled so as to allow a wider range of
> > inputs, but without having any direct effect of the differential
> > measurement.
> >
> > _offset cannot be used as to remain in line with existing usage,
> > userspace would be expected to apply it as (_raw + _offset) * _scale
> > whereas _zeropoint is not. i.e. If we were computing the differential
> > in software it would be.
> > ((postive_raw + _zeropoint) - (negative_raw + zeropoint) + _offset) * _scale
> > = ((postive_raw - negative_raw) + _offset) * _scale
> > = (differential_raw + _offset) * _scale
> >
> > Similarly calibbias is expected to tweak the measurement seen, not
> > the adjust the two lines of the differential pair.
> >
> > Needed for in_capacitanceX-capacitanceY_zeropoint for the
> > AD7746 CDC driver.  
> 
> ...
> 
> > +What:          /sys/.../iio:deviceX/in_capacitanceY-capacitanceZ_zeropoint
> > +KernelVersion: 5.19  
> 
> 5.20?

:) Probably 5.21 as I'm not going to rush this in now, but good point none the less.

> 
> > +Contact:       linux-iio@vger.kernel.org
> > +Description:
> > +               For differential channels, this an offset that is applied
> > +               equally to both inputs. As the reading is of the difference
> > +               between the two inputs, this should not be applied to the _raw
> > +               reading by userspace (unlike _offset) and unlike calibbias
> > +               it does not affect the differential value measured because
> > +               the effect of _zeropoint cancels out across the two inputs
> > +               that make up the differential pair. It's purpose is to bring  
> 
> makes

No.  make is correct.
That's indeed an odd corner of English and honestly I'm not sure I could successfully
argue why it should be make :)

> 
> > +               the individual signals, before the differential is measured,
> > +               within the measurement range of the device. The naming is
> > +               chosen because if the separate inputs that make the
> > +               differential pair are drawn on a graph in their
> > +               _raw  units, this is the value that the zero point on the
> > +               measurement axis represents. It is expressed with the
> > +               same scaling as _raw.  
>