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

On Thu, Sep 29, 2016 at 12:25 AM, Benjamin Tissoires
[off-list ref] wrote:

On Sep 28 2016 or thereabouts, Roderick Colenbrander wrote:

quoted

On Wed, Sep 28, 2016 at 10:39 AM, Dmitry Torokhov
[off-list ref] wrote:

quoted

On Tue, Sep 27, 2016 at 4:38 PM, Roderick Colenbrander
[off-list ref] wrote:

quoted

From: Roderick Colenbrander <roderick.colenbrander@sony.com>

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 [off-list ref]
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.

If we decide to move forward in the direction proposed by this patch,
the spec could be updated
to limit the scope a bit or to make it wider.

quoted

quoted

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.

In our case we are interested in the motion functionality for some devices
with drivers already in the kernel, which we want to extend over time with
improved capabilities.

I understand your concerns about the scalability of ABS axes in general.
If someone were to come up with some crazy flight simulator joystick with many
weird axes, do you then add an ABS_X2, ABS_X3 etcetera? Similar what if
a controller for whatever reasons shipped with multiple gyroscopes,
accelerometers,
magnetic sensor, heartrate sensors etcetera?

A composite device would on the other hand be more of a pain for the different
userland APIs ranging from libinput, SDL2, Android and other embedded
platforms. It

That's already what we are doing for Wacom tablets (and some other
devices) both in the kernel and in libinput. Wacom digitizers are
exposed through 3 different device on average, one for the pen, one for
the touch and one for the buttons on the pad. Libinput then relies on
the notion of device group (a udev property) which can be tweaked when
the heuristic fails (through libwacom mainly).

Basically, libinput is not much of an issue, especially because we
ignore accel, gyro, and other weird axis, and because we already know
how to group composite devices.

For the others, yes, it'll be a pain. But only if there is an actual need of
grouping. If the sensors are the ones of the phone itself, having one or
several input nodes doesn't hurt that much. If the sensors are coming
from gamepads, then yes, there is a need for grouping, but hopefully the
device path should provide some good heuristic.

How are you currently ignoring accel, gyro and others axes? The main
concern I have is how to express the axes and how does userspace
detect these?

One of the examples I saw is the Wii driver, which supports accel,
gyro and others. It exposes both accel and gyro data through
ABS_RX/_RY/_RZ and adds "Accelerometer" or "Motion Plus" to the name.
In my opinion, applications shouldn't rely on string parsing for this
stuff. Not sure if I agree with using axes which were originally
intended for rotational exes for acceleration instead of just
ABS_X/_Y/_Z, though for gyro they would make sense.

On the other hand Wacom sets 'INPUT_PROP_ACCELEROMETER' and reports
acceleration data through ABS_X/_Y/_Z. This feels more reasonable to
me as originally ABS_X/_Y/_Z where meant for horizontal displacements.

There are probably some other examples as well. I'm trying to figure
what direction would make sense moving forward. I'm starting to accept
the need for composite devices. Questions:
- What axes to use for accel? Should this be X/Y/Z or RX/RY/RZ, as I
showed there is no real standard.
- What axes to use for gyro? I think we would need an additional
INPUT_PROP_GYROSCOPE.

For any of these axes should there be some sort of standard on
increments per unit. Originally ABS_X apparently was in units/mm and
ABS_RX in units/radian. Though most of the time actual units are
unknown... but it is also a pain for applications to know about every
device.

quoted

would be quite an extensive change. How would they even do the
stitching? You could
handle this through sysfs (not my favorite way) or maybe have a notion
of a 'master'
device being the current event node and some way to enumerate 'sensor'
nodes or something.

A simple udev property solves most of the grouping issues (based on the
sysfs path mostly).

The thing is currently, we are aware that the situation is not
satisfying, and we are seeing the limit of the ABS axis declarations. We
can find solutions (or workarounds) that works well enough, and adding
ABS_MAX2 might not be the best solution long term: especially because of
the slotted protocol inside ABS that messes things quite a bit.

If we were to expand to ABS_MAX2, in order to avoid conflicts with the
slotted protocol, we would need to reserve quite a few axis after
ABS_MAX for this purpose. But we can't say how many will be required.

Cheers,
Benjamin