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

[Jonathan Cameron <jic23@kernel.org>]

On Wed, 27 May 2026 16:34:58 +0200
Wadim Mueller <wafgo01@gmail.com> wrote:

> 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.
Makes sense.
> 
> 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.
> 

Nope. Good to discuss what it would look like in this over letter, but
the actual introduction needs to wait for a driver that uses it.

> > > 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