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

[Jonathan Cameron <jic23@kernel.org>]

On Sun, 6 Apr 2025 22:31:43 +0900
gyeyoung <gye976@gmail.com> wrote:

> Hello Jonathan,
> 
> On Sun, Apr 6, 2025 at 8:26 PM Jonathan Cameron <jic23@kernel.org> wrote:
> >
> > 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!)
> >  
> sorry, I'm not affiliated with the vendor,
> so I do not have any information other than the datasheet.
> And the datasheet doesn't specify any trade-offs related to adjusting the range.
> (only "± (50ppm+3% reading value)" is here)
> 
> I think this range setting would be better implemented in
> some other way (like module parameter) rather than IIO subsystem.

I'm not in general keen on module parameters effecting policy (and noise
vs range is definitely a policy thing) so I think we would want to figure out
a suitable sysfs ABI.

> For now I'll just drop this range setting code..
Makes sense  - can revisit when needed.
> 
> Regards,
> Gyeyoung