On Wed, 22 Jun 2022 17:42:25 +0200 Stefan Lengfeld <stefan.lengfeld@xxxxxxxxx> wrote: > 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? No unfortunately. Such ranges only work with a bunch of 'vendor id' type magic to allow avoidance of clashes so tend to be complex unless you expect to have long term vendor specific types. The result is that I know a bunch of vendors are carrying backports of core IIO changes to enable new sensor types (and features sometimes), obviously a bit harder to do if you are providing a single sensor driver. > > 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. Documentation/ABI/testing/sysfs-bus-iio has most stuff. We haven't done a great job at cross reference between the other docs and there. > > 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. Yup. > > 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. Would be great if that is a fair representation of what is going on in the device. If the device is doing something hideously non linear using an actual FPU then it might be very challenging. > > > > > 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! Good luck. Looking forward to seeing the code + more details on your sensor! Jonathan > > 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
