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

From: Roderick Colenbrander
Date: Tue Sep 27 2016 - 19:57:34 EST

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. 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.
- 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.
- 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.

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.

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)

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

+Motion-tracking:
+ 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.
+
+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.
+
+ 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!
+
+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. 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.
+
+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
+deltas.
+
+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 gmail.com>
+ (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
--
2.7.4