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

[jic23]

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)
> >
> >  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.
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
> > +deltas.
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 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
>
> Thanks.