libinput touchscreen maxTouchPoints

[Peter Hutterer]

On Wed, Jan 24, 2018 at 10:47:10AM +0100, Johannes Pointner wrote:
> On Wed, Jan 24, 2018 at 10:18 AM, Peter Hutterer
> <peter.hutterer at who-t.net> wrote:
> > On Wed, Jan 24, 2018 at 10:04:52AM +0100, Johannes Pointner wrote:
> >> Hello,
> >>
> >> I have noticed that since we are using xf86-input-libinput all our
> >> touchscreens (resistive and pcap) reporting a value of 15 for
> >> maxTouchPoints.
> >> After further looking into this I saw that xf86-input-libinput sets
> >> this value fix to 15. I also checked if it is possible to get this
> >> info from libinput but I wasn't able to find a possibility.
> >>
> >> Is this intentionally?
> >
> > mostly, it's largely a matter of not exporting information that doesn't need
> > to be exported. at least with touchpads libinput does a few tricks and
> > changes the number of touches available (not yet at runtime, but who knows)
> > - once we start exporting this we're a lot more limited in what we can do
> > internally to work around broken devices.
> >
> > Main question is though: what do you need this value for?
> We use this value which is available through the browser
> (https://developer.mozilla.org/en-US/docs/Web/API/Navigator/maxTouchPoints)
> to determine if it is a multitouch device or not.
> The same goes for Qt
> (http://doc.qt.io/qt-5/qtouchdevice.html#maximumTouchPoints) which has
> a libinput backend.

Actually, this is something we could fix in the driver by checking for
ABS_MT_SLOT and forcing it to 1 when missing. that wouldn't fix Qt though,
so it may be better to have this exported by libinput after all...
Need to think of the various special cases here.

Cheers,
   Peter