Re: [PATCH 2/2] Input: add motion-tracking ABS_* bits and docs

From: jic23
Date: Thu Sep 29 2016 - 04:56:12 EST

On 28.09.2016 18:39, Dmitry Torokhov wrote:
On Tue, Sep 27, 2016 at 4:38 PM, Roderick Colenbrander
<roderick@xxxxxxxxxx> wrote:
From: Roderick Colenbrander <roderick.colenbrander@xxxxxxxx>

This patch introduces new axes for acceleration and angular velocity.
David Herrmann's work served as a base, but we extended the specification
with various changes inspired by real devices and challenges we see
when doing motion tracking.

- Changed unit of acceleration to G instead of m/s^2. We felt that m/s^2
is not the appropriate unit to return, because accelerometers are most
often calibrated based on gravity.
I'd so like to believe they are referenced to a standard g rather than
whatever is true at the plant. Almost no consumer parts are able to internally
adjust their calibrations (or at least it's undocumented if they are).
They return values in multiples of
G and since we don't know the device location on earth, we should not
blindly multiply by '9.8' for accuracy reasons. Such conversion is left
to userspace.
Hmm. If userspace is involved in trimming the readings, then the units don't matter.
Ah well, not important really. However, there is a question of generalised
interfaces. There are plenty of other devices (distance sensors etc) which measure
in m. For IIO at least we have to support those as well so SI units are
preferred. I remember having the same discussion long long ago!
- Resolution field is used for acceleration and gyro to report precision.
The previous spec, specified to map 1 unit to e.g. 0.001 deg/s or 0.001 m/s^2.
This is of course simpler for applications, but unit definition is a bit
arbitrary. Previous axes definitions used the resolution field, which
felt more consistent.
- Added section on timestamps, which are important for accurate motion
tracking purposes. The use of MSC_TIMESTAMP was recommended in this
situation to get access to the hardware timestamp if available.
The moment you are into doing motion tracking I'm really thinking shoving it
through input makes little sense... This is one of the main things
IIO is set up to do...

- Changed motion axes to be defined as a right-handed coordinate system.
Due to this change the gyro vectors are now defined as counter-clockwise.
The overall changes makes the definitions consistent with computer graphics.

[PATCH 4/4] Input: add motion-tracking ABS_* bits and docs
David Herrmann <dh.herrmann@xxxxxxxxx>
Tue Dec 17 07:48:54 PST 2013

Motion sensors are getting quite common in mobile devices. To avoid
returning accelerometer data via ABS_X/Y/Z and irritating the Xorg
mouse-driver, this adds separate ABS_* bits for that.

We have IIO for motions sensors that are not strictly human input
devices; I believe there is also IIO->input bridge where generic IIO
sensors could be mapped to input device if they are supposed to be
used as such in given product.
Yeah, *looks guilty* I've been failing to actually submit the input
bridge for a quite some time due to the open question of how the heck
we describe the connectivity (device tree etc) after all the fuss
over the hwmon bridge.

Need to get this moving again - in principle it is very straight forward.
The infrastructure is used for a few other purposes so is reasonably
well tested.

Basically there has been very push on this previously so good
if someone wants to pick it up and push it forward.

(basically I did it as a proof of concept years ago but didn't care
that much as I don't have a screen on any of the boards I care about).

This is needed if gaming devices want to report their normal data plus
accelerometer/gyro data. Usually, ABS_X/Y are already used by analog
sticks, so need separate definitions, anyway.

I am not sure if this direction is sustainable. We can't keep adding
more and more ABS axes every time we add another control to something
that is basically a composite device. What if you add another stick?
Magnetometer? Some other sensor?

I think the only reasonable way it to come up with a notion of
"composite" input device consisting of several event nodes and have
userspace "assemble" it all together.
That sounds sensible if we are going to pipe this stuff out to userspace.

If the actual need is to do fusion on the various sensors then I'd be
inclined to pull it from an IIO device directly then use uinput to
push the fused result back in so that it emerges from input as one
would expect a magic 'this is pose' data type.

There is obviously the open question of how to describe the various
axis if they don't make sense to be passed directly to input.
So there are valid points raised by this email.

Signed-off-by: David Herrmann <dh.herrmann@xxxxxxxxx>
Signed-off-by: Roderick Colenbrander <roderick.colenbrander@xxxxxxxx>
Documentation/input/gamepad.txt | 9 +-
Documentation/input/motion-tracking.txt | 176 ++++++++++++++++++++++++++++++++
include/uapi/linux/input-event-codes.h | 7 ++
3 files changed, 190 insertions(+), 2 deletions(-)
create mode 100644 Documentation/input/motion-tracking.txt

diff --git a/Documentation/input/gamepad.txt b/Documentation/input/gamepad.txt
index 3f6d8a5..ed13782 100644
--- a/Documentation/input/gamepad.txt
+++ b/Documentation/input/gamepad.txt
@@ -57,6 +57,9 @@ Most gamepads have the following features:
- Rumble
Many devices provide force-feedback features. But are mostly just
simple rumble motors.
+ - Motion-tracking
+ Gamepads may include motion-tracking sensors like accelerometers and
+ gyroscopes.

3. Detection
@@ -138,8 +141,6 @@ Triggers:
Upper trigger buttons are reported as BTN_TR or ABS_HAT1X (right) and BTN_TL
or ABS_HAT1Y (left). Lower trigger buttons are reported as BTN_TR2 or
ABS_HAT2X (right/ZR) and BTN_TL2 or ABS_HAT2Y (left/ZL).
- If only one trigger-button combination is present (upper+lower), they are
- reported as "right" triggers (BTN_TR/ABS_HAT1X).
(ABS trigger values start at 0, pressure is reported as positive values)

@@ -155,5 +156,9 @@ Menu-Pad:
Rumble is advertised as FF_RUMBLE.

+ Motion-tracking is defined in ./Documentation/input/motion-tracking.txt and
+ gamepads shall comply to the rules defined there.
Written 2013 by David Herrmann <dh.herrmann@xxxxxxxxx>
diff --git a/Documentation/input/motion-tracking.txt b/Documentation/input/motion-tracking.txt
new file mode 100644
index 0000000..d34a290
--- /dev/null
+++ b/Documentation/input/motion-tracking.txt
@@ -0,0 +1,176 @@
+ Motion Tracking API
+1. Intro
+Motion tracking devices produce device motion events generated from an
+accelerometer, gyroscope or compass. These data can be returned to user-space
+via input events. This document defines how these data are reported.
+2. Devices
+In this document, a "device" is one of:
+ - accelerometer
+ - gyroscope
+ - compass
+These devices returned their information via different APIs in the past. To
+unify them and define a common API, a set of input evdev codes was created. Old
+drivers might continue using their API, but developers are encouraged to use
+the input evdev API for new drivers.
Agreed, it was a mess which is one of the reasons IIO came about. Another
being that there are lots of usecases for these devices which have nothing to
do with user input, where the filtering etc that occurs there is superfluous.

+2.1 Axes
+Movement data is usually returned as absolute data for the 3 axes of a device.
+In this context, the three axes are defined in a right-handed coordinate
+system as:
+ - X: Axis goes from the left to the right side of the device
+ - Y: Axis goes from the bottom to the top of the device
+ - Z: Axis goes from the back to the front of the device
+The front of a device is the side faced to the user. For a mobile-phone it
+would be the screen. For devices without a screen, the top is usually the
+side with the most buttons on it.
You need the means to describe a transformation from the chip to device space.
It can be spectacularly non obvious. See the mounting matrix stuff we have
in IIO. Doing the nasty floating point maths to fix this stuff is definitely
a usespace job!
+ Example: Mobile-Phone
+ +-------------------------------------------------------------------------+
+ | TOP |
+ | |
+ | |
+ | +---------------------------+ |
+ | |\ ________________________ \ .__ |
+ | \ \ \ \ \ |\ |
+ | \ \ \ __ \ \ \ RIGHT|
+ | \ \ \ /| \ \ \__ |
+ | \ \ \ __/ \ \ |\ |
+ | \ \ \ /| \ \ \ (Y Axis) |
+ | \ \ \ __/ (Z axis) \ \ \__ |
+ | \ \ \ /| \ \ |\ |
+ | LEFT \ \ \ / \ \ \ |
+ | \ \ \ FRONT \ \ \ |
+ | \ \ \ \ \ |
+ | \ \ \_______________________\ \ |
+ | \ \ ___ \ |
+ | /\ \ \__\ \ |
+ | __/ \ +---------------------------+ |
+ | /| \|___________________________| |
+ | / BACK |
+ | (X axis) |
+ | ------->------->------->-------> |
+ | |
+ | |
+ | BOTTOM |
+ +-------------------------------------------------------------------------+
+Rotation-data is reported as counter-clockwise rotation on an axis when viewed
+from the top of the axis, as given by the right hand rule. For a given axis,
+the reported rotation would be:
+ ____
+ //|
+ // | (axis)
+ //
+ //
+ . // __
+ / // /\
+ | // |
+ \ // / (counter-clockwise rotation)
+ *.___.*
+ //
+ //
+2.2 Calibration
+Motion sensors are often highly sensitive and need precise calibration. Users
+are advised to perform neutral-point calibration themselves or to implement a
+state-machine to normalize input data automatically.
+Kernel devices may perform their own calibration and/or normalization. However,
+this is usually sparse and, if implemented, transparent to the user.
+There is currently no way to feed calibration data into the kernel in a generic
+way. Proposals welcome!
There is in IIO and always has been.
+2.3 Units
+(NOTE: This section describes an experimental API. Currently, no device complies
+to these rules so this might change in the future.)
+Reported data shall be returned as:
+ - Acceleration: 1/(input_absinfo.resolution) G
+ - Rotation: 1/(input_absinfo.resolution) degree per second
+Acceleration is reported in units of G as opposed to m/s^2, because acceleration
+sensors internally work based on gravitation.

No they don't. They work based on acceleration. Gravity just happens to cause
acceleration. Please don't confuse this issue. Take a sensor calibrated in
Cambridge UK (it's a bit of local joke round here and catches undergraduate
physicists every year apparently) and carry it to almost anywhere else in the
world and you'll find that the value goes up.

Since the conversion to m/s^2 is
+location dependent, applications should either approximate the conversion
+factor as 9.8 m/s^2 or if more precision is desired obtain a scaling factor
+by other means e.g. GPS.
+However, for most devices the reported units are unknown (more precisely: no
+one has the time to measure them and figure them out). Therefore, user-space
+shall use abs-minimum and abs-maximum to calculate relative data and use that
+instead. Devices which return wrong units may be fixed in the future to comply
+to these rules.
Most devices they are known for, it's a characteristic of the relevant chip and
typically factory calibrated (or based on design rules).
+2.4 Timestamps
+For motion tracking purposes the time delta between consecutive motion events
+is important for mathematical operations such as differentiation and integration.
+The time delta could be derived from the 'time' field in 'struct input_event' by
+subtracting the time between consecutive events. However, this timestamp may not
+provide enough accuracy depending on the use case, since it is based upon time of
+processing within the input layer versus time of arrival in the kernel or the
+time the hardware sent the data. There is often a small variable time difference
+between these.
+Optionally, hardware may provide a hardware timestamp produced at the time it
+sampled the motion sensors. This timestamp is is exposed through
+'MSC_TIMESTAMP' event, which provides timing information in microseconds.
+If available, MSC_TIMESTAMP is the recommended approach for calculation of time
This is actually pretty unusual, but as you might expect we have supported it
in IIO from the start.
+3.1 Accelerometer
+Accelerometers measure movement acceleration of devices. Any combination of the
+three available axes can be used. Usually, all three are supported.
+Data is provided as absolute acceleration. A positive integer defines the
+acceleration in the direction of an axis. A negative integer defines
+acceleration in the opposite direction.
+The evdev ABS codes used are:
+ - ABS_ACCEL_X: X axis
+ - ABS_ACCEL_Y: Y axis
+ - ABS_ACCEL_Z: Z axis
+3.2 Gyroscope
+A gyroscope measures rotational speed (*not* acceleration!). Any combination of
+the three available axes can be used. Usually, all three are supported.
+Data is provided as absolute speed. A positive integer defines the rotational
+speed in counter-clockwise order around a given axis when viewed from the top of
+the axis. A negative integer defines it in clockwise order.
+The evdev ABS codes used are:
+ - ABS_GYRO_X: X axis (also: Pitch)
+ - ABS_GYRO_Y: Y axis (also: Roll)
+ - ABS_GYRO_Z: Z axis (also: Azimuth/Yaw)
+3.3 Compass
+(NOTE: No compass device currently uses the evdev input subsystem. Thus, this
+API is only a proposal, it hasn't been implemented, yet.)
+A compass measures the ambient magnetic field of the three defined axes. This
+makes the data self-contained and independent of the current device position.
+Any combination of the three axes can be used. Usually all three are supported,
+otherwise, it's not really useful as a compass.
+Proposed evdev ABS codes are:
+ - ABS_COMPASS_X: X axis
+ - ABS_COMPASS_Y: Y axis
+ - ABS_COMPASS_Z: Z axis
+ (c) 2013 David Herrmann <dh.herrmann at>
+ (c) 2016 Roderick Colenbrander <roderick.colenbrander@xxxxxxxx>
diff --git a/include/uapi/linux/input-event-codes.h b/include/uapi/linux/input-event-codes.h
index 7bf2a2e..0cacfe7 100644
--- a/include/uapi/linux/input-event-codes.h
+++ b/include/uapi/linux/input-event-codes.h
@@ -763,6 +763,13 @@
#define ABS_MAX 0x3f
#define ABS_CNT (ABS_MAX+1)

+#define ABS_GYRO_X 0x40 /* Gyroscope X axis */
+#define ABS_GYRO_Y 0x41 /* Gyroscope Y axis */
+#define ABS_GYRO_Z 0x42 /* Gyroscope Z axis */
+#define ABS_ACCEL_X 0x43 /* Accelerometer X axis */
+#define ABS_ACCEL_Y 0x44 /* Accelerometer Y axis */
+#define ABS_ACCEL_Z 0x45 /* Accelerometer Z axis */
* Due to API restrictions the legacy evdev API only supports ABS values up to
* ABS_MAX/CNT. Use the extended *ABS2 ioctls to operate on the full range of