XInput 2 Multi-touch maxing out at 32 touch-points

Alex Averill alex.averill at
Wed May 27 17:50:43 UTC 2020

Hello Peter,
I apologize for the delay in responding to this, I hit a bug that I wanted
to track down and wasn't able to get access to the hardware again until
earlier today.

When I run libinput record I end up hitting a Max of 16 point and hit an
assertion that fails:

libinput-record: ../tools/libinput-record.c:385: handle_evdev_frame:
Assertion `slot < sizeof(d->touch.slot_state) * 8' failed.

I was able to track that to the file that the error is mentioning (

I was also able to recompile libinput and remove that assertion. By
removing that assertion I was able to record 80 touchpoints. Would you be
able to provide some guidance on what removing that assertion may have on
other portions of libinput? I would be happy to work through the necessary
work for testing and then submit a pull request, though this would be my
first work on Libinput.


On Thu, May 14, 2020 at 12:59 AM Peter Hutterer <peter.hutterer at>

> On Tue, May 12, 2020 at 04:06:17PM -0600, Alex Averill wrote:
> > Hello,
> > I am currently working on building an application using XInput 2 and
> > RawTouch events to capture touch positions and pass them to another
> > application. I am using a 3M Touchscreen that is reporting through X11
> that
> > it supports 80 points, however I am only ever able to capture at most 32
> > touchpoints.
> 32 sounds suspiciously like a mask somewhere but I couldn't find
> anything skimming over the relevant bits.. Are you getting 32 slots from
> the device though? libinput record will tell you, look at the
> ABS_MT_SLOT value and inside that the ABS_MT_TRACKING_ID. Any slot with
> a tracking ID != -1 has a current touch, check how many you can produce.
> libinput record output is YAML and easy enough to parse.
> Cheers,
>   Peter
> > Is there an inherent limit to the total number of
> > touchpoints/RawTouchEvents supported by XInput 2?
> >
> > Thanks,
> > Alex Averill
> > _______________________________________________
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