Re: [PATCH v11 8/8] docs: iio: describe inactivity and free-fall detection on the ADXL345

[Lothar Rubusch <l.rubusch@gmail.com>]

Hi

On Sun, Jul 6, 2025 at 6:16 PM Jonathan Cameron <jic23@kernel.org> wrote:
>
> On Wed,  2 Jul 2025 23:03:15 +0000
> Lothar Rubusch <l.rubusch@gmail.com> wrote:
>
> > Describe the inactivity detection additionally using the free-fall
> > register. Due to the controversial discussions on the mailing list, this
> > section of the documentation will be committed separately to allow for a
> > more focused and detailed elaboration of the topic.
> >
> > Signed-off-by: Lothar Rubusch <l.rubusch@gmail.com>
> > ---
> >  Documentation/iio/adxl345.rst | 25 +++++++++++++++++++++++++
> >  1 file changed, 25 insertions(+)
> >
> > diff --git a/Documentation/iio/adxl345.rst b/Documentation/iio/adxl345.rst
> > index 8ee01b8b87f4..c5525267ea12 100644
> > --- a/Documentation/iio/adxl345.rst
> > +++ b/Documentation/iio/adxl345.rst
> > @@ -150,6 +150,31 @@ functions, so that one follows the other. The auto-sleep function puts the
> >  sensor into sleep mode when inactivity is detected, reducing power consumption
> >  to the sub-12.5 Hz rate.
> >
> > +The inactivity time is configurable between 1 and 255 seconds. In addition to
> > +inactivity detection, the sensor also supports free-fall detection, which, from
> > +the IIO perspective, is treated as a fall in magnitude across all axes. In
> > +sensor terms, free-fall is defined using an inactivity period ranging from 0.000
> > +to 1.000 seconds.
> > +
> > +The driver behaves as follows:
> > +* If the configured inactivity period is 1 second or more, the driver uses the
> > +  sensor's inactivity register. This allows the event to be linked with
> > +  activity detection, use auto-sleep, and be either AC- or DC-coupled.
> > +
> > +* If the inactivity period is less than 1 second, the event is treated as plain
> > +  inactivity or free-fall detection. In this case, auto-sleep and coupling
> > +  (AC/DC) are not applied.
> > +
> > +* If an inactivity time of 0 seconds is configured, the driver selects a
> > +  heuristically determined default period (greater than 1 second) to optimize
> > +  power consumption. This also uses the inactivity register.
> > +
> > +Note: It is recommended to use the activity, inactivity, or free-fall registers
> > +when operating with an ODR between 12.5 Hz and 400 Hz.
>
> This seems a tiny bit backwards.   It is recommend that the activity, inactivity or
> free-fall registers are only used when operating with an ODR...
>

Ehm, Doesn't the sensor always have an ODR? The real question is which
ODR should be configured. There are recommendations for specific
features. I may have either misunderstood or misdocumented this part.

> As currently written it seems to be recommending that if you want those sampling
> frequencies you should also enable one of these detectors.
>

Ah, no. The other way around, when someone wants one of events
detected, the recommended frequencies should be used. I'll have a look
at it.

> Reminds me of the classic London underground sign that said "Dogs must be
> carried." which raised the question of how people with out dogs were meant to travel.
>
> Otherwise this new section looks good to me.  Thanks,
>
> Jonathan
>...

Best,
Lothar