libinput Acer c720 Observations

Wade Berrier wberrier at
Wed Jun 10 20:44:54 PDT 2015

On Jun 10 14:43, Peter Hutterer wrote:
> On Fri, Jun 05, 2015 at 04:22:51PM -0600, Wade Berrier wrote:
> > Hello,
> > 
> > I'm running libinput 0.17 on Fedora 22 (compiled and installed the
> > rawhide src.rpm) on an Acer c720 chromebook.  The observations below
> > also apply to 0.15 that shipped with the distro, but I upgraded to see
> > if it changed the behavior.
> just fwiw, I'm pushing out updated packages on an almost daily basis (f22
> and rawhide) and ptraccel is the main things being tweaked at the moment.
> > The trackpad doesn't quite "feel right".  Here's some of the main
> > observations compared to when using the trackpad with ChromeOS and/or
> > the cmt driver:
> > 
> > 1. the pointer is very "fidgety".  Relatively small movements move the
> >    pointer around quite a bit.  If I adjust the pointer speed to the minimum
> >    (using "Mouse & Touchpad" gui) these small controlled movements seem
> >    much more accurate.  But, then it takes several "swipes" to move the
> >    pointer to the other side of the screen.
> > 
> > 2. at first it seemed that no matter what "pointer speed" setting I
> >    tried, finger distance always covered the same amount of screen
> >    distance, no matter the finger velocity.  ie: no acceleration
> >    handling.  I came across [1] and realized that wasn't the case.
> >    After additional tinkering, I was able to affect screen distance
> >    with finger speed, but it seemed very difficult to get into that
> >    state/window.
> Note: the below is gut feeling at this point:
> I think our biggest problem with touchpad acceleration is that we get too
> quickly into full acceleleration, so the gap between no acceleration and
> full acceleration is too narrow.
> if you always hit full acceleration, then you wouldn't notice the difference
> and it would feel like constant acceleration.

I logged the following bugs for these:

I hope they are useful.  It still seems a little subjective/arbitrary,
but they are there, fwiw.  Let me know if I can provide better
information, recordings, etc...

> > In short, the lowest pointer speed feels natural in localized
> > scenarios requiring high precision (ie: clicking on tiny stuff).  But,
> > when swiping faster to travel to the other side of the screen, a 40%
> > setting seems to "feel" about right.  It's interesting that ChromeOS
> > gets away with not even having a pointer speed adjustment.
> note that chromeos afaik only needs to support two touchpads, which makes
> the job a bit easier.

I'm not sure how many different touchpads they have to support, but
they sure have a lot of finely tuned configurations:

> > 3. the pointer moves around when doing "hard clicks".  Both on the way
> >    down and the way up.  It seems like ChromeOS "locks" the pointer in
> >    place during this case.  Without this, my 5 year old son is unable
> >    to hard click a button, whereas he's able to on ChromeOS.  Also
> >    interesting, is that in ChromeOS I can roll my fingertip in circles
> >    and the pointer barely, if at all, moves.  So, I guess this is an
> >    issue whether I'm clicking or not.
> file a bug for this please, with an evemu recording for one of these moving
> clicks. we have that feature in libinput, but you're clearly getting outside
> of it's boundaries.

Filed as

> > 4. I get spurious pointer movement and clicks from my right palm while my
> >    hands are in typing position.  The feature of disabling the mouse
> >    while typing partially helps. Also, the trackpad isn't centered
> >    between my hands on the c720, possibly making it only happen with my
> >    right hand.  On ChromeOS, I can tap my palm all over the pad and
> >    will get an occasional click but no pointer movement.
> same here, please file a bug with a couple of emu recordings. there's a plan
> for thumb detection (which would also detect palms) but it's not
> there yet.

Filed as

> > As an aside, it sounds like the mouse team at google pressure
> > calibrates [2] their hardware.  Is libinput normalizing pressure in
> > addition to dpi?  If so, does the hardware database also include
> > information for device specific pressure calibration?
> libinput doesn't use pressure on touchpads yet. it's somewhere on the 
> list, but so far we've gotten away without having to worry about it
> :)

I imagine this will become much more relevant at some point:

(I must admit, I tried this touchpad at the apple store and
surprisingly it feels incredibly intuitive, and I'm not a mac user.)

> > 
> > I realize this is all highly subjective to muscle memory and personal
> > preference, but I think it can be useful to compare to the excellent
> > experience that the ChromeOS driver offers.
> > 
> > So, where do I go from here?  Are my observations specific to the
> > c720?  Is there something I can try out or tweak?  If it were easy to
> > use the xf86-cmt driver on fedora [3] I probably would, but given
> > wayland will be using libinput it would be nice to get libinput
> > working nicely on this trackpad.
> as above, 3) is definitely a bug, 4) is a todo, 2 is being worked on, 1) as
> well but that too would likely benefit from a bug+evemu recording of such a
> small movement. these touchpads have a comparatively low resolution, so I
> wonder if we have a simple bug there.

The response and feedback is much appreciated.  Thanks,


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