Re: Questions: IIO type for absorbance and float values in channels?

[Stefan Lengfeld <stefan.lengfeld@inovex.de>]

Hi Jonathan, 

On Tue, Jun 21, 2022 at 01:29:09PM +0100, Jonathan Cameron wrote:
> > Is there another kernel subsystem/UAPI that fits this type of sensor
> > better?  (My guess would be: No, IIO is the correct subsystem.)
> 
> Based on your description, IIO is the best option.

Great. This means that I'm on the right track.


> > Question 1:
> > 
> > The sensor measures the absorbance[1] of a material. It's the negative
> > logarithmic quotient of the received and emitted light. It's a unit-less value.
> > 
> > As far as I can see there is currently no predefined IIO type in the kernel for
> > this.  I have search the list [2], but nothing matches.
> > 
> > Do I overlook something or what channel type should I use for this sensor
> > measurement value?
> 
> So the closest we've seen to this is some of the health sensors (pulse oximeter
> targeting devices) but in that case the processing is done with a custom userspace
> library so we just pass values on in fixed point.

I have look at the 'oximeter' drivers in the IIO framework, namely
afe440x and max3010x. All use the IIO type IIO_INTENSITY and the
max30100x drivers also use the modifiers IIO_MOD_LIGHT_*.

You also referenced the modifiers in your response to Andy.


> So probably needs a new channel type.

Ok. This makes it a bit harder for us, because we want to provide a
kernel driver that also works for older kernels.

Is there a private IIO type range that can be use in the meantime,
until a new IIO type is accepted and merged upstream?

Another side note/question: Is there a documented mapping between the
IIO types and physical SI units?  I have found nothing about that in the
IIO documentation. It would have helped me to better understand the
differences between the various types.

Just for reference. I mentioned the absorbance, but there is also
another equivalent physical value: the refelctance[1]. It's the same
value just without the negative logarithm.


> > So effectively the result of a
> > measurement can be a float-point value after it's computed in userspace.
> > 
> > Are floats really not supported as channel values? What is the recommend
> > approach to deal with these measurement values?
> 
> So, in kernel we basically can't do floats (not quite true, but near enough that
> it doesn't matter).  Given IIO can have in kernel consumers, that more or
> less rules out true floating point handling.

Yes, float point math is frowned in the kernel. I have read the kernel
hacking documentation [2]. :-) But I also know that the functions
kernel_fpu_begin() and kernel_fpu_end() exist (including there
performance problems ;-) ).


> We have in the past discussed simply passing them on to userspace (and
> hence ruling out any interpretation of the value) but so far no one has added
> the necessary support.  I'm not against adding that support, but I would
> want to see the code / docs to fully understand why we are doing it.
> It will be a special case in various code paths hence the extra consideration
> needed.

Ack on that. My take way is that it's currently not implemented but
theoretical acceptable upstream to pass float point values from the chip
to userspace. But it would require a good/valid show case and extra work
in the IIO subsystem.

I take this information to the sensor engineers. Maybe we can rework the
communication/data protocol to pass the measurement as integer values
with a separate scale factor that does not require raw float point
values.


> > I know that it's possible to provide a 'scale' attribute per channel to
> > userspace[4], which can be a float point number.
> 
> More fixed point - though we have a couple of precision options. The aim there
> is to ensure we only need a few bits of custom code to consume these values
> in kernel. Raw value can also be fixed point.

Yes. I already saw and tested the IIO_VAL_* defines (like
IIO_VAL_INT_PLUS_NANO or IIO_VAL_FRACTIONAL) that can be used to provide
fixed point scale values to userspace. They work great and maybe we can
use them.

Thanks again!

Kind Regards, 
Stefan

[1]: https://en.wikipedia.org/wiki/Reflectance
[2]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/kernel-hacking/hacking.rst?id=9d2231c5d74e13b2a0546fee6737ee4446017903#n134