On Tue, 28 Jun 2022 14:17:33 +0200 Andy Shevchenko <andy.shevchenko@xxxxxxxxx> wrote: > On Sun, Jun 26, 2022 at 2:20 PM Jonathan Cameron <jic23@xxxxxxxxxx> wrote: > > > > From: Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx> > > > > 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@xxxxxxxxxxxxxxx > > +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. >
