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

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Message-ID: <20250724144738.77024c1d@jic23-huawei>
Date: Thu, 24 Jul 2025 14:47:38 +0100
From: Jonathan Cameron <jic23@...nel.org>
To: Lothar Rubusch <l.rubusch@...il.com>
Cc: eraretuya@...il.com, lars@...afoo.de, Michael.Hennerich@...log.com,
 dlechner@...libre.com, nuno.sa@...log.com, andy@...nel.org, corbet@....net,
 linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-doc@...r.kernel.org
Subject: Re: [PATCH v11 8/8] docs: iio: describe inactivity and free-fall
 detection on the ADXL345

On Sun, 20 Jul 2025 20:49:48 +0200
Lothar Rubusch <l.rubusch@...il.com> wrote:

> Hi
> 
> On Sun, Jul 6, 2025 at 6:16 PM Jonathan Cameron <jic23@...nel.org> wrote:
> >
> > On Wed,  2 Jul 2025 23:03:15 +0000
> > Lothar Rubusch <l.rubusch@...il.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@...il.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? 
I was lazy. The ODR... was meant to be ODR between 12.5 Hz and 400 Hz (so original text
for that bit).

> 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.

Sounds like you got my rubbish explanation.

> 
> > 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