[RFC] Multitouch proposal v2 + implementation

[Daniel Stone]

Hi,

On Mon, Sep 20, 2010 at 10:29:49AM -0700, Ping Cheng wrote:
> On Sun, Sep 19, 2010 at 9:45 PM, Daniel Stone <daniel at fooishbar.org> wrote:
> > This is my second draft of the multitouch proposal, along with a fairly
> > complete implementation; patchsets for inputproto, xserver,
> > xf86-input-evdev, libXi and xinput follow.
> >
> > This is more or less half way between Peter's proposal and my original
> > one: it doesn't use valuators anymore, but it has a different, and
> > hopefully more workable, approach to grabbing.
> 
> I have a corner case that I'd like to know how you address it.
> 
> How do we deal with the single touch case for multi-touch devices?
> Shouldn't we still post valuator for single touch events so MT devices
> can perform single touch functions while the touch is not grabbed by
> the client? The single touch use case can occur before and after the
> client grabs the MT events.
> 
> Maybe you have already discussed and covered this case in your
> proposal. But I am not familiar with the X server (and evdev) code. So
> I am taking a shortcut to get my question answered - asking the expert
> :).

Actually, I haven't discussed that in the spec, only briefly with Peter.
Core emulation is actually somewhat tricky, as you need to guarantee
that you're the only one sending core events.  The following scenario is
the kind of thing I'm worried about:
    * T+0: TouchBegin started on SD 1, sent to client A via sync grab
    * T+5: Core event sent from SD 2, sent to client B via sync grab on
           MD 3, device frozen
    * T+6: More core events enqueued from SD 2 -> MD 3
    * T+10: Client A uses ReplayTouch on touch stream, no other
            listeners found; core events emulated, routed through MD 3

MD 3's core events queue for client B is now going to going to contain
events from T+5, T+6, T+7 ... and then T+0.

So I think for core emulation, the only safe way to do it is going to be
to decide at event generation time whether it's going to be a core event
or a touch event.  I've seen some kernel drivers which emulate ABS_[XY]
events for the first finger down, which seems like the most sensible
semantic I can think of.

Does that make sense?

Cheers,
Daniel
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