Re: [RFC PATCH v1 0/4] iio: add Sensirion SLF3x liquid flow sensor support

[Wadim Mueller <wafgo01@gmail.com>]

On Tue, 26 May 2026 17:12:14 +0100
Jonathan Cameron <jic23@kernel.org> wrote:

> > 1. Subsystem.  Liquid-flow sensors don't seem to have a home in
> >    mainline yet.  iio/ feels like the natural place for me, but
> >    please correct me if hwmon (or somewhere else) is prefered.  
> 
> I'm fine with this type of sensor in IIO, but open to hear from others!

Thanks - keeping it in iio/ for v2.

> > 2. Channel type.  IIO has no flow channel type so far.  The series
> >    adds IIO_VOLUMEFLOW with _scale in litres per second per LSB,
> >    so drivers reporting smaller units only need a fractional scale.
> >    IIO_MASSFLOW (for gas-flow sensors) was left out on purpose --
> >    happy to add it in the same series if that's more usefull.  
> 
> Would be good to have a little more detail on difference etc to have
> that discussion.  Superficially I'm assuming this is a compressible vs
> non compressible thing?

Right - that was the distinction I had in mind:

  - IIO_VOLUMEFLOW: volumetric rate (m^3/s in SI, exposed as l/s),
    well-defined for an incompressible fluid where the volume at the
    sensor equals the volume delivered.  This matches all "liquid
    flow" sensors I'm aware of (and the SLF3S family in particular).

  - IIO_MASSFLOW: mass rate (kg/s), the natural unit for gases because
    the volumetric rate depends on temperature and pressure.  Most
    "gas flow" parts on the market (Sensirion SFM3xxx, SFC, MFC
    devices) actually report a "standard volume flow" (e.g. slm at
    0 degC and 1013 mbar) which is just a fixed-density restatement
    of mass flow.

I left IIO_MASSFLOW out of this series because I don't have a driver
needing it right now, but if you'd prefer I add it as a stub here
(name + ABI entry) so the two siblings land together, I can fold that
in for v2 -- happy either way.

> > 3. Subdirectory.  I put the driver in a new drivers/iio/flow/
> >    since there is no flow subsytem in iio yet.  If colocating with
> >    drivers/iio/pressure/ (next to sdp500) is preferred I'll respin.  
> 
> This is easy to change later if we make a wrong decision. For now I think
> iio/flow is fine.

Sticking with drivers/iio/flow/ for v2.

Thanks!

v2 is up, addressing the inline review feedback from you and Guenter.
Dropping the RFC tag since the three high-level questions are
answered.

Wadim