Re: [PATCH v4 3/3] iio: imu: bmi270: add support for motion events

[Jonathan Cameron <jic23@kernel.org>]

On Sun, 27 Jul 2025 16:49:09 +0100
Jonathan Cameron <jic23@kernel.org> wrote:

> On Fri, 25 Jul 2025 00:26:35 +0300
> Andy Shevchenko <andriy.shevchenko@intel.com> wrote:
> 
> > On Thu, Jul 24, 2025 at 04:22:27PM +0100, Jonathan Cameron wrote:  
> > > On Wed, 16 Jul 2025 12:53:55 +0300
> > > Andy Shevchenko <andriy.shevchenko@intel.com> wrote:    
> > > > On Tue, Jul 15, 2025 at 08:55:35PM -0300, Gustavo Silva wrote:    
> > > > > On Tue, Jul 15, 2025 at 10:49:25AM +0300, Andy Shevchenko wrote:      
> > > > > > On Tue, Jul 15, 2025 at 10:37:22AM +0300, Andy Shevchenko wrote:      
> > > > > > > On Fri, Jul 11, 2025 at 08:36:03PM -0300, Gustavo Silva wrote:      
> > 
> > ...
> >   
> > > > > > > > +/* 9.81 * 1000000 m/s^2 */
> > > > > > > > +#define BMI270_G_MEGA_M_S_2				9810000      
> > > > > > > 
> > > > > > > I thought this is MICRO...      
> > > > > > 
> > > > > > Btw, what if we use the device on poles and on equator (or even on orbital
> > > > > > station)? I'm wondering if this constant should be defined in units.h or
> > > > > > even in uAPI that user space may add a correction if needed.
> > > > > >       
> > > > > I certainly hadn't thought about these scenarios.
> > > > > FWIW, the accelerometer scale values also assume g = 9.81 m/s^2.
> > > > > For example, 0.000598 = 2 * 9.81 / 32768      
> > > > 
> > > > Right, but this should be supplied to user space somehow. OTOH the measure error
> > > > may be high enough (what is the precision of the measurements by the way?) that
> > > > it will neglect the differences in the 'g' constant.
> > > > 
> > > > All the details are given in [1].
> > > > 
> > > > [1]: https://en.wikipedia.org/wiki/Gravity_of_Earth#:~:text=The%20precise%20strength%20of%20Earth's,/s2)%20by%20definition.    
> > > 
> > > These sensors don't measure relative to g.    
> > 
> > What do they measure? Any links for me to study?  
> m/s^2 (well actually not, but they derive that from change in properties of a capacitor like structure as it bends under
> the force generated by the acceleration.  Which in the vertical direction is g if the device isn't moving.
> 
> https://www.analog.com/en/resources/technical-articles/accelerometer-and-gyroscopes-sensors-operation-sensing-and-applications.html
> looks good as an intro from a very quick glance.
> 
> >   
> > > That's annoying marketing which is why I held firm for m/s^2 for IIO :)
> > > So what they measure for a given acceleration does not change depending
> > > on where we are on earth. You should use a 'fixed' standard value for
> > > conversion from marketing values in g to m/s^2..    
> > 
> > Hmm... But shouldn't that marketing value be exposed to user space?  
> 
> yes, but its 9.81 where ever you are. Or whatever the datasheet says they use.
> These things are rarely that well calibrated, so the exact value gets lost in the
> errors!
> 
> Jonathan
> 
I got lost in this thread, so perhaps am assuming there is a v5 to come.

Maybe just send one anyway even if no changes as it'll jump up my list to look at.

Jonathan

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