Re: [PATCH 1/9] iio: document bindings for mounting matrices

[H. Nikolaus Schaller]

> Am 20.02.2019 um 17:10 schrieb Jonathan Cameron <jic23@xxxxxxxxxx>:
> 
> On Wed, 20 Feb 2019 15:00:48 +0100
> "H. Nikolaus Schaller" <hns@xxxxxxxxxxxxx> wrote:
> 
>> From: Linus Walleij <linus.walleij@xxxxxxxxxx>
>> 
>> The mounting matrix for sensors was introduced in
>> commit dfc57732ad38 ("iio:core: mounting matrix support")
>> 
>> However the device tree bindings are very terse and since this is
>> a widely applicable property, we need a proper binding for it
>> that the other bindings can reference. This will also be useful
>> for other operating systems and sensor engineering at large.
>> 
>> I think all 3D sensors should support it, the current situation
>> is probably that the mounting information is confined in magic
>> userspace components rather than using the mounting matrix, which
>> is not good for portability and reuse.
>> 
>> Cc: Gregor Boirie <gregor.boirie@xxxxxxxxxx>
>> Cc: Sebastian Reichel <sre@xxxxxxxxxx>
>> Cc: Samu Onkalo <samu.onkalo@xxxxxxxxx>
>> Cc: devicetree@xxxxxxxxxxxxxxx
>> Signed-off-by: Linus Walleij <linus.walleij@xxxxxxxxxx>
> Hmm. I looked back and seems there were still some outstanding questions
> on this last time around.
> 
> https://lore.kernel.org/linux-iio/a6d866f2-ee20-282b-def0-f65de2177aee@xxxxxxxxxx/
> 
> Particularly hard as ever to define the magnetic planes when near the
> magnetic poles when in 3D.
> 
> That needs cleaning up ideally before we apply this.

Agreed. I already placed my proposal for improvement on top of that,
but I did not try to solve the magnetic planes issue.

Maybe Linus could squeeze the patches and edit a new one that solves
all issues.

What I am most interested at the moment is to get an unambigous
description for accelerometers first. The otehr topic was that I
did not clearly see how the matrix is applied to the raw values
(one could do row index first or column index first).

BR and thanks,
Nikolaus

> 
> Jonathan
> 
>> ---
>> .../devicetree/bindings/iio/mount-matrix.txt  | 108 ++++++++++++++++++
>> 1 file changed, 108 insertions(+)
>> create mode 100644 Documentation/devicetree/bindings/iio/mount-matrix.txt
>> 
>> diff --git a/Documentation/devicetree/bindings/iio/mount-matrix.txt b/Documentation/devicetree/bindings/iio/mount-matrix.txt
>> new file mode 100644
>> index 000000000000..a3714727f739
>> --- /dev/null
>> +++ b/Documentation/devicetree/bindings/iio/mount-matrix.txt
>> @@ -0,0 +1,108 @@
>> +Mounting matrix
>> +
>> +The mounting matrix is a device tree property used to orient any IIO device
>> +that produce three-dimensional data in relation to the world where it is
>> +deployed.
>> +
>> +The purpose of the mounting matrix is to translate the sensor frame of
>> +reference into the device frame of reference using a translation matrix as
>> +defined in linear algebra.
>> +
>> +The typical usecase is that where a component has an internal representation
>> +of the (x,y,z) triplets, such as different registers to read these coordinates,
>> +and thus implying that the component should be mounted in a certain orientation
>> +relative to some specific device frame of reference.
>> +
>> +For example a device with some kind of screen, where the user is supposed to
>> +interact with the environment using an accelerometer, gyroscope or magnetometer
>> +mounted on the same chassis as this screen, will likely take the screen as
>> +reference to (x,y,z) orientation, with (x,y) corresponding to these axes on the
>> +screen and (z) being depth, the axis perpendicular to the screen.
>> +
>> +For a screen you probably want (x) coordinates to go from negative on the left
>> +to positive on the right and (z) depth to be negative under the screen and
>> +positive in front of it, toward the face of the user.
>> +
>> +A sensor can be mounted in any angle along the axes relative to the frame of
>> +reference. This means that the sensor may be flipped upside-down, left-right,
>> +or tilted at any angle relative to the frame of reference.
>> +
>> +Another frame of reference is how the device with its sensor relates to the
>> +external world, the environment where the device is deployed. Usually the data
>> +from the sensor is used to figure out how the device is oriented with respect
>> +to this world. When using the mounting matrix, the sensor and device orientation
>> +becomes identical and we can focus on the data as it relates to the surrounding
>> +world.
>> +
>> +Device-to-world examples for some three-dimensional sensor types:
>> +
>> +- Accelerometers have their world frame of reference toward the center of
>> +  gravity, usually to the core of the planet. A reading of the (x,y,z) values
>> +  from the sensor will give a projection of the gravity vector through the
>> +  device relative to the center of the planet, i.e. relative to its surface at
>> +  this point. Up and down in the world relative to the device frame of
>> +  reference can thus be determined. and users would likely expect a value of
>> +  9.81 m/s^2 upwards along the (z) axis, i.e. out of the screen when the device
>> +  is held with its screen flat on the planets surface and 0 on the other axes,
>> +  as the gravity vector is projected 1:1 onto the sensors (z)-axis.
>> +
>> +- Magnetometers (compasses) have their world frame of reference relative to the
>> +  geomagnetic field. The system orientation vis-a-vis the world is defined with
>> +  respect to the local earth geomagnetic reference frame where (y) is in the
>> +  ground plane and positive towards magnetic North, (x) is in the ground plane,
>> +  perpendicular to the North axis and positive towards the East and (z) is
>> +  perpendicular to the ground plane and positive upwards.
>> +
>> +- Gyroscopes detects the movement relative the device itself. The angular
>> +  velocity is defined as orthogonal to the plane of rotation, so if you put the
>> +  device on a flat surface and spin it around the z axis (such as rotating a
>> +  device with a screen lying flat on a table), you should get a negative value
>> +  along the (z) axis if rotated clockwise, and a positive value if rotated
>> +  counter-clockwise according to the right-hand rule.
>> +
>> +So unless the sensor is ideally mounted, we need a means to indicate the
>> +relative orientation of any given sensor of this type with respect to the
>> +frame of reference.
>> +
>> +To achieve this, use the device tree property "mount-matrix" for the sensor.
>> +This supplies a 3x3 rotation matrix in the strict linear algebraic sense,
>> +to orient the senor axes relative to a desired point of reference. This means
>> +the resulting values from the sensor, after scaling to proper units, should be
>> +multiplied by this matrix to give the proper vectors values in three-dimensional
>> +space, relative to the device or world point of reference.
>> +
>> +For more information, consult:
>> +https://en.wikipedia.org/wiki/Rotation_matrix
>> +
>> +The mounting matrix has the layout:
>> +
>> + (x0, y0, z0)
>> + (x1, y1, z1)
>> + (x2, y2, z3)
>> +
>> +And it is represented as an array of strings containing the real values for
>> +producing the transformation matrix. The real values use a decimal point and
>> +a minus (-) to indicate a negative value.
>> +
>> +Examples:
>> +
>> +Identity matrix (nothing happens to the coordinates, which means the device was
>> +mechanically mounted in an ideal way and we need no transformation):
>> +
>> +mount-matrix = "1", "0", "0",
>> +               "0", "1", "0",
>> +               "0", "0", "1";
>> +
>> +The sensor is mounted 30 degrees (Pi/6 radians) tilted along the X axis, so we
>> +compensate by performing a -30 degrees rotation around the X axis:
>> +
>> +mount-matrix = "1", "0", "0",
>> +               "0", "0.866", "0.5",
>> +               "0", "-0.5", "0.866";
>> +
>> +The sensor is flipped 180 degrees (Pi radians) around the Z axis, i.e. mounted
>> +upside-down:
>> +
>> +mount-matrix = "0.998", "0.054", "0",
>> +               "-0.054", "0.998", "0",
>> +               "0", "0", "1";
>