Re: [PATCH v2 4/5] iio:magnetometer:ak8975: mounting matrix support

On 03/09/2016 09:46 PM, Jonathan Cameron wrote:

[snip...]

Definitely needs ABI docs in Documentation/ABI/testing/sysfs-bus-iio
as well. We need the reference directions to be well specified as
well - how do we define the main hardware orientation?  on yours presumably
the usual approach of using the screen is irrelevant so that definition
could be a little 'interesting' to write down?

quoted

The standard orientation for us is defined with respect to the drone's head
and the propellers plane, which does not suit the nominal use cases indeed...
Although it really deserves some more detailed documentation, I wonder if the
question is relevant.
Given that the DTS is board specific (correct me if I'm wrong), why not
consider the reference orientation as board specific as well (and make the
whole thing implicit) ?

I think that is fine for your sort of usecase - though might be nice to keep
a bit of documentation somewhere (perhaps as comments in the DTS), or perhaps
more centrally - would be nice for userspace libraries if at least all aircraft
had a similar 'default orientation'.

As you said earlier, things are never that simple. The standard orientation
may vary with types of UAVs (quad-rotor, fixed wings, bizarre home made 
UFOs...)

For the more 'standard' device with a screen I think we would want to standardise
if at all possible.  Most hand held devices have a 'natural' orientation
(front facing camera at the top for example).  In those cases we are basically looking
at whether the sensor is mounted on the back or the front of the main circuit board and
then of course it can be at any rotation in plane but typically is at 90 degrees
(hence the inv mpu limitations)

I'm guessing there are existing userspace mappings for this.  Perhaps in the Gnome
sensors stuff?

[snip...]
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