Re: [PATCH v1 3/5] ABI: iio: add new ABI doc for mhz19b

[Jonathan Cameron <jic23@kernel.org>]

On Sun, 6 Apr 2025 12:20:48 +0100
Jonathan Cameron <jic23@kernel.org> wrote:

> On Sat, 5 Apr 2025 22:47:45 +0900
> gyeyoung <gye976@gmail.com> wrote:
> 
> > Hello Jonathan, thank you for the review.
> >   
> > > > diff --git a/Documentation/ABI/testing/sysfs-bus-iio-chemical-mhz19b b/Documentation/ABI/testing/sysfs-bus-iio-chemical-mhz19b
> > > > new file mode 100644
> > > > index 000000000000..6cdfd34be016
> > > > --- /dev/null
> > > > +++ b/Documentation/ABI/testing/sysfs-bus-iio-chemical-mhz19b
> > > > @@ -0,0 +1,7 @@
> > > > +What:                /sys/bus/iio/devices/co2_range
> > > > +Date:                April 2025
> > > > +KernelVersion:       6.14
> > > > +Contact:     Gyeyoung Baek <gye976@gmail.com>
> > > > +Description:
> > > > +             Writing a value adjust maximum measurable PPM.
> > > > +             should be 2000 or 5000.    
> > >
> > > I haven't checked but assume this also results in a scaling of the
> > > measure _raw values?  If so the control should be via the standard
> > > ABI scale.  If you need to be able to establish the range, provide
> > > the _available for the _raw via the read_avail() callback and setting
> > > appropriate bit in info_mask_separate_available
> > >    
> > 
> > In this device, changing the measurement range does not affect the
> > unit or scaling.
> > As far as I know, increasing the range just leads to a decrease in accuracy.  
> 
> That's unusual but fair enough.  hardwaregain is perhaps appropriate
> as this doesn't really map to calibscale which is the other thing close
> to this.
> 
Actually - any idea what the gain is doing?  Is it adjusting a analog
amplfier, or messing with the integration time (I have little idea
how these sensors work!) 

Jonathan


> >   
> > > General rule is don't introduce new ABI unless it is impossible to
> > > provide the same information via existing interfaces.  The decision
> > > to use scale rather than range info to control channel scaling was
> > > made a very long time ago and having a mixture of the two would
> > > make for very complex userspace code.    
> > 
> > I’ve reviewed the sysfs-bus-iio documentation, I think there is
> > no suitable interface for this case. So I'll drop this option.
> > 
> > Thanks,
> > Gyeyoung  
>