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

On Mon, Mar 21, 2016 at 5:21 PM, Gregor Boirie [off-list ref] wrote:

Hi Rob,

On Mon, Mar 21, 2016 at 09:58:46AM -0500, Rob Herring wrote:

quoted

On Thu, Mar 17, 2016 at 11:43 AM, Gregor Boirie
[off-list ref] wrote:

quoted

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.

Given it is an ABI, yes I think so. We don't want to end up with a
bunch of similar yet different interfaces.

quoted

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 ?

You have to maintain the ABI. If both interfaces can co-exist, then
you can have both and mark the old one as deprecated. In time we can
remove the old one.

quoted

quoted

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" ?

Sure.

Rob