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

[Jonathan Cameron <jic23@kernel.org>]

On 22/03/16 12:38, Rob Herring wrote:
> On Mon, Mar 21, 2016 at 5:21 PM, Gregor Boirie <gregor.boirie@parrot.com> wrote:
>> 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@parrot.com> 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.
> 
> Given it is an ABI, yes I think so. We don't want to end up with a
> bunch of similar yet different interfaces.
> 
Absolutely.  Most interfaces get made up based on one implementation ;)
A second is always nice, but here the interface is obvious enough we don't
need to wait.
>>> 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.
If you want to (or someone else does) it would be good to add the new abi to
inv_mpu_core as well and indeed mark the old one as deprecated. 
The cost of keeping it is negligible, so we may never actually bother to
remove it. 
> 
>>>> 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.
Works for me as well.
> 
> Rob
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