[RFC] Multi-Touch (MT) support - arbitration or not

[Chris Bagwell]

On Fri, Nov 5, 2010 at 1:47 PM, Ping Cheng <pinglinux at gmail.com> wrote:
> Recent changes and discussion about MT support at LKML, UDS, and
> xorg-devel encouraged me to migrate Wacom MT devices to the slot-based
> MT protocol (introduced in kernel 2.6.36). Since Wacom supports both
> digitizer and touch devices, I need to decide how to report touch data
> when the pen is in proximity.
>
> My goal is to understand how X server would like the MT data to be
> reported from the kernel. I hope to keep kernel and X server driver MT
> support in sync so we can avoid unnecessary confusion or extra work in
> the userland.
>
> The existing solution for single touch events is to arbitrate touch
> when pen is in prox. This is based on the assumption that we do not
> want to have two cursors competing on the screen.
>
> With the introduction of MT, the touch data are most likely translated
> into something other than pointer events. So, reporting both pen and
> touch data makes sense now. However, I want to assure a smooth
> tansition from single touch to MT for end users so they still get the
> single touch behavior as they used to be. I gathered the following
> approaches:
>
> 1.     Arbitrate all touch data in the kernel.
>
> This is the simplest solution for device driver developers. But I do
> not feel it is end user and userland client friendly.
>
> 2.     Report first finger touch as ABS_X/Y events when pen is not in
> prox.  Arbitrating single touch data when pen is in prox. Pen data is
> reported as ABS_X/Y events. Both ABS_X/Y for pen or the first finger
> and ABS_MT_* for MT data are reported.
>
> This approach reduces the overhead in dealing with two cursors in userland.
>
> 3.    Report first finger touch as ABS_X/Y events when pen is not in prox;
>       Report pen data as ABS_X/Y events when there is no finger touch;
>       Report touch data as MT_TOOL_TOUCH and pen data as MT_TOOL_PEN
> events when both pen and touch data are received. No ABS_X/Y are
> reported when pen and tocuh or multi-touch data are received.
>
> I feel this one makes sense to userland since pen can be considered as
> another touch.
>
> 4.    Report first finger touch as ABS_X/Y events when pen is not in prox;
>       Report pen data as ABS_X/Y events when there is no finger touch;
>       Report touch data as MT_TOOL_TOUCH and pen data as MT_TOOL_PEN
> events when both pen and touch data are received. ABS_X/Y are also
> reported for pen when both pen and tocuh data are received.
>
> This one makes sense to userland too. It eases the backward
> compatibility support for those clients that don't support MT at all.
>
> Which approach do you like? Or do you have other suggestions share?
>
> Ping

Here is my input on topic.  I'll summarize that I'm leaning towards
your option #1 right now.

First, I think we need to decide if non-MT aware apps/X drivers are to
be supported or if we require them to be "fixed" to work reliably with
devices such as Wacom Bamboo that split device into two inputs yet
need coordination across inputs.

I prefer that non-MT aware are supported and so we should continue the
current arbitration in kernel over the ST events.  Sending
un-arbitrated MT events to userland I think is mostly OK.

Next, you bring up an interesting point on tablet side.  Currently, it
doesn't send MT events because it only tracks 1 stylus tool at a time.
 Should it send MT events anyways for single tool to help userland
because its events are meant to be associated with another inputs MT
events?  Thats a good question for MT people to consider.

Thinking about that issue though makes we think of option #5 to
consider.  We could combine these 2 input devices into single input
device and send all events as MT events.  I assume userland has easier
time arbitrating within single input device.  I'm not sure what it
would take to combine these in kernel though... and there is some
value in 2 inputs since  just the touchpad can currently be re-routed
to xf86-input-synaptics.

Chris