[PATCH] arm64: dts: sun50i-a64-pinephone: Add mount matrix for accelerometer

[Andrey Skvortsov]

Hi Dragan,

On 24-09-18 11:27, Dragan Simic wrote:
> Hello Andrey,
> 
> 
>   arm64: dts: allwinner: pinephone: Add mount matrix to accelerometer
> 
> The patch description should be reworded like this, reflown into
> proper line lengths, of course:
> 
>   The way InvenSense MPU-6050 accelerometer is mounted on the
>   user-facing side of the Pine64 PinePhone mainboard requires
>   the accelerometer's x- and y-axis to be swapped, and the
>   direction of the accelerometer's y-axis to be inverted.
> 
>   Rectify this by adding a mount-matrix to the accelerometer
>   definition in the PinePhone dtsi file.
> 
>   [andrey: Picked the patch description provided by dsimic]
>   Fixes: 91f480d40942 ("arm64: dts: allwinner: Add initial support for
> Pine64 PinePhone")
>   Cc: stable at vger.kernel.org

Thanks for the commit description, it's much better, than original
one.

> Please note the Fixes tag, which will submit this bugfix patch
> for inclusion into the long-term/stable kernels.
> 
> Also note that the patch description corrects the way inversion
> of the axis direction is described, which should also be corrected
> in the patch itself, as described further below.
> 
> After going through the InvenSense MPU-6050 datasheet, [1] the
> MPU-6050 evaluation board user guide, the PinePhone schematic,
> the PinePhone mainboard component placement, [2] and the kernel
> bindings documentation for mount-matrix, [3] I can conslude that
> only the direction of the accelerometer's y-axis is inverted,
> while the direction of the z-axis remain unchanged, according
> to the right-hand rule.

yes, it looks so on the first glance, but in MPU-6050 datasheet there
is also following information:

 7.8 Three-Axis MEMS Accelerometer with 16-bit ADCs and Signal
 Conditioning

 When the device is placed on a flat surface, it will measure
 0g on the X- and Y-axes and +1g on the Z-axis.

So sensors reports positive acceleration values for Z-axis, when
the gravity points to Z-minus. I see the same on device. positive
values are returned, when screen and IC point upwards (not the center
for gravity). 

In device tree mount-matrix documentation [3] there is

 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.

According to that, it looks like Z-axis here has to be inverted.

It applies to other axes as well. And because of that I came from (only Y-axis is inverted)

x' = -y
y' =  x
z' =  z

to inverted solution (Y-axis is kept, but X and Z are inverted).

x' =  y
y' = -x
z' = -z

probably should put this information into commit description.

> > > > Signed-off-by: Ondrej Jirman <megi at xff.cz>
> > > > Signed-off-by: Andrey Skvortsov <andrej.skvortzov at gmail.com>
> > > > ---
> > > >  arch/arm64/boot/dts/allwinner/sun50i-a64-pinephone.dtsi | 3 +++
> > > >  1 file changed, 3 insertions(+)
> > > >
> > > > diff --git a/arch/arm64/boot/dts/allwinner/sun50i-a64-pinephone.dtsi
> > > > b/arch/arm64/boot/dts/allwinner/sun50i-a64-pinephone.dtsi
> > > > index bc6af17e9267a..1da7506c38cd0 100644
> > > > --- a/arch/arm64/boot/dts/allwinner/sun50i-a64-pinephone.dtsi
> > > > +++ b/arch/arm64/boot/dts/allwinner/sun50i-a64-pinephone.dtsi
> > > > @@ -229,6 +229,9 @@ accelerometer at 68 {
> > > >  		interrupts = <7 5 IRQ_TYPE_EDGE_RISING>; /* PH5 */
> > > >  		vdd-supply = <&reg_dldo1>;
> > > >  		vddio-supply = <&reg_dldo1>;
> > > > +		mount-matrix = "0", "1", "0",
> > > > +				"-1", "0", "0",
> > > > +				"0", "0", "-1";
> > > >  	};
> > > >  };
> 
> With the above-described analysis in mind, the mount-matrix
> should be defined like this instead:
> 
> 		mount-matrix = "0", "1", "0",
> 			       "-1", "0", "0",
> 			       "0", "0", "1";


x' =  0 * x + 1 * y + 0 * z =  y
y' = -1 * x + 1 * y + 0 * z = -x
z' =  0 * z + 0 * y + 1 * z =  z

your description says, that only Y-axis is inverted, but this matrix,
imho, inverts original X axis as it was in original description.

> Please also note the line indentation that was changed a bit.
> 
> [1] https://rimgo.reallyaweso.me/vrBXQPq.png
> [2] https://rimgo.reallyaweso.me/uTmT1pr.png
> [3] https://www.kernel.org/doc/Documentation/devicetree/bindings/iio/mount-matrix.txt

-- 
Best regards,
Andrey Skvortsov