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

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Message-ID: <aIKlC-HlP3nX-ERA@smile.fi.intel.com>
Date: Fri, 25 Jul 2025 00:26:35 +0300
From: Andy Shevchenko <andriy.shevchenko@...el.com>
To: Jonathan Cameron <jic23@...nel.org>
Cc: Gustavo Silva <gustavograzs@...il.com>,
	Alex Lanzano <lanzano.alex@...il.com>,
	David Lechner <dlechner@...libre.com>,
	Nuno Sá <nuno.sa@...log.com>,
	Andy Shevchenko <andy@...nel.org>, linux-iio@...r.kernel.org,
	linux-kernel@...r.kernel.org, Lothar Rubusch <l.rubusch@...il.com>
Subject: Re: [PATCH v4 3/3] iio: imu: bmi270: add support for motion events

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@...el.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?

> 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?


-- 
With Best Regards,
Andy Shevchenko