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

[Gregor Boirie <gregor.boirie@parrot.com>]

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?
>> 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...]