Re: [PATCH v3 2/3] iio:magnetometer:ak8975: mounting matrix support

[Gregor Boirie <gregor.boirie-ITF29qwbsa/QT0dZR+AlfA@public.gmane.org>]

Hi Rob,

On Mon, Mar 21, 2016 at 09:58:46AM -0500, Rob Herring wrote:
> On Thu, Mar 17, 2016 at 11:43 AM, Gregor Boirie
> <gregor.boirie-ITF29qwbsa/QT0dZR+AlfA@public.gmane.org> wrote:
> > Expose a rotation matrix to indicate userspace the chip placement with
> > respect to the overall hardware system. This is needed to adjust
> > coordinates sampled from a magnetometer chip when its position deviates
> > from the main hardware system.
> >
> > Final coordinates computation is delegated to userspace since:
> > * computation may involve floating point arithmetics ;
> > * it allows an application to combine adjustments with arbitrary
> >   transformations.
> >
> > This 3 dimentional space rotation matrix is expressed as 3x3 array of
> > strings to support floating point numbers. It may be retrieved from a
> > "in_magn_matrix" sysfs attribute file. It is declared into ak8975 DTS
> > entry as a "matrix" property.
>
> Why is the sysfs interface specific to ak8975?
AFAIK, this is just the first IIO driver implementation relying on floating
point numbers. Should a single driver be enough to justify a "generic" API,
I suppose code could be factored out in a similar way to over sampling rate
support. People could call this on a per-driver basis.

>
> Furthermore, why is it specific to magnetometer? Couldn't
> accelerometers need the same thing? There's a thread discussing a
> similar matrix on android-x86[1].
Same may apply to gyro / accelero / imu and magnetometers at least.
inv_mpu_core.c already implements such a rotation matrix exposed as 3x3 integers
array. Should we be smart enough to keep this compatible with existing userspace
apps ?

>
> > diff --git a/Documentation/devicetree/bindings/iio/magnetometer/ak8975.txt b/Documentation/devicetree/bindings/iio/magnetometer/ak8975.txt
> > index 34a3206..f936f86 100644
> > --- a/Documentation/devicetree/bindings/iio/magnetometer/ak8975.txt
> > +++ b/Documentation/devicetree/bindings/iio/magnetometer/ak8975.txt
> > @@ -9,6 +9,7 @@ Optional properties:
> >
> >    - gpios : should be device tree identifier of the magnetometer DRDY pin
> >    - vdd-supply: an optional regulator that needs to be on to provide VDD
> > +  - matrix: an optional 3x3 mounting rotation matrix
>
> Perhaps "rotation-matrx" would be a better name in case there's ever
> any other matrix needed.
What about "mounting-matrix" ?
> 
> Rob
> 
> [1] https://groups.google.com/forum/#!msg/android-x86/LaKhV8cT69o/G1rrVU0_JAAJ