[PATCH] protocol: Add gaming input protocol

[Roderick Colenbrander]

Hi Jingkui,

I heard about your team's efforts on the wayland gaming input protocol
proposal. I wasn't on the list prior and had difficulties replying to
the thread directly, so sorry for likely starting a new thread.

I applaud work being done in this field and we had a close look over
the spec. Ourselves we are quite interested in gamepad / joystick
support in Wayland for various use cases. We carefully reviewed the
spec and understand some of the decisions made, but overall we feel
the spec is a bit limiting for general gamepad support. First, I will
go over general feedback to the 'core gamepad functionality' and then
into more extended features, which should be considered as well.

The spec uses a 'standard gamepad' as described in the W3C spec. Use
of a standard gamepad simplifies the whole axis and button mapping
situation, which is a pain. However, mapping gamepads to this standard
gamepad is in my opinion not a good idea. The majority of controllers
for Xbox / PS3 / PS4 and some of the common PC models kind of map to
this. Speaking of my company's Dualshock 4 it for example has an extra
button which the spec doesn't handle. The Steam Controller for example
has extra triggers also not handled. Aside from this there are many
existing controllers which should work like the Wii Mote or the
recently announced Joypads for the Nintendo NX, but they don't provide
enough buttons or axes.

I see the client side ultimately getting implemented across SDL2, Wine
and other applications and if really meant as a general gamepad
interface, I don't think it is a good idea. I don't have a good
alternative yet, except for giving up on the standardized gamepad and
maybe bubbling up hardware ids (so applications can do the mapping,
though probably undesired) or maybe proposing a general axes / button
mapping, but even that is tricky as well.

The second half of my feedback is more centered on extended
functionality. These features are not always available on each device,
but this kind of data should come from a gamepad API as applications
shouldn't use another channel to get this data. Some of it which has
been common for quite some time:
- Rumble / Force feedback
- Led support
- Motion sensors
- Touchpad

Rumble / force feedback has been supported since at least PS2 days or
before and is common on gamepads.

Led support is in general about the LEDs found on many controllers for
Xbox, PS3, PS4, Wii and others. Depending on the model this can at
minimum be used for showing the player number (often 1-4) and on some
devices even for color animations (e.g. on DS4). On Linux this is
currently often done through sysfs and I think libinput may be able to
read this now as well.

Motion sensors have been supported on DS3, DS4, Steam Controller and
others. Typically this are accelerometers and gyroscopes presented as
raw or calibrated data.

Touchpads are more recent features as found on DS4 and Steam
Controller. Speaking for the DS4 we currently report that on Linux
using hid-sony as a device separate from the controller (has to do
with evdev limitations). Not sure how a gamepad API needs to handle
that whether it needs to merge the devices or not. If reporting as
separate devices, we would need a way to 'tie' them together from an
application perspective if there are multiple gamepads. Some sort of
hardware id or something could be used.

To summarize for core gamepad functionality, I think the gamepad API
as is is a bit limiting. There many other devices which we could
support fine on e.g. Desktop Linux and there are popular existing
devices which don't map exactly. Then there is the situation about
extended features and how to handle them. Myself I'm new to Wayland
and need to see what other protocols for like tablets and other
devices are doing, but maybe some sort of capability bitmap could make
sense. My team and I ourselves need most of this functionality as
well, so we won't mind helping out in some way.

Thanks,
Roderick

-- 
Roderick Colenbrander
Senior Manager of Software Engineering
Gaikai, a Sony Interactive Entertainment Company
roderick at gaikai.com