On Tue, Feb 23, 2016 at 08:04:32AM +1000, Peter Hutterer wrote:

On Mon, Feb 22, 2016 at 08:35:31AM -0800, Charles Mooney wrote:

quoted

On Sun, Feb 21, 2016 at 2:04 PM Peter Hutterer [off-list ref] wrote:

quoted

On Thu, Feb 18, 2016 at 10:19:30AM -0800, Dmitry Torokhov wrote:

quoted

On Tue, Feb 09, 2016 at 01:56:05PM -0800, Charles Mooney wrote:

quoted

Hello all,

I'm currently working with a touchpad vendor with a new device that
supports a limited form of hover detection.  Their sensor is able to
detect the presence or absence of a finger/hand/palm hovering over the
sensor without touching it, but is unable to report any more details
about it.  This is a more limited form of hover detection than some
devices which attach a hover state to each finger they see, and can
even report x/y coordinates to hovering finger.

Instead of using ABS_MT_DISTANCE, it appears that the correct event to
use would be ABS_DISTANCE, since the value is not tied to a specific
finger.  I would like to check with you all about how this value is
intended to be used, because it's not quite as obvious to me as I
first thought.

We need to handle three basic states:
  1. At least one finger is touching the pad.
  2. Something is hovering, but nothing is actually touching.
  3. Nothing is touching the pad and nothing is detected hovering over it either

It's seems clear to me that an ABS_DISTANCE of zero should indicate
state #1 and that any other legal positive value should indicate state
#2, but I'm less clear on what the best way to handle state #3 is.
Currently, I think the best strategy would be to use a value of
ABS_DISTANCE = -1 to indicate that there are no fingers seen (hovering
or otherwise), does that make sense?

If not this, how else would you suggest that this ought to be done?

As we discussed in person, I believe that reporting an "out of bounds"
value for ABS_DISTANCE when we have to use single-touch mode and thus do
not have a good way to invalidate a contact, is the easiest option.
Alternative would be to invent another SYN event, which I'd rather not.

So for devices that support hovering but can not report individual
hovering contacts we should declare 0..N as ABS_DISTANCE range and report
following values:

 - 0 when a finger is actually touching
 - 1..N for hovering fingers
 - return X < 0 or X > N when no fingers are detected at all; in
   practice I think we should simply report -1 in this case.

Benjamin, Peter, Henrik, any concerns?

on the touchpads that support hovering we're already using BTN_TOOL_FINGER
together with ABS_DISTANCE, without needing out-of-range reports.
BTN_TOUCH is the signal when a finger is physically touching (or ABS_PRESSURE
if it exists and clients care about it).

So the sequence Charles should send is:

3) <nothing> :)
2)
    EV_ABS ABS_DISTANCE <d> # for d > 0
    EV_KEY BTN_TOOL_FINGER 1
    EV_SYN SYN_REPORT 0
1)
    EV_ABS ABS_DISTANCE 0
    EV_ABS ABS_X <x>
    EV_ABS ABS_Y <y>
    ...
    EV_KEY BTN_TOUCH 1
    EV_SYN SYN_REPORT 0
2)
    EV_ABS ABS_DISTANCE <d> # for d > 0
    EV_KEY BTN_TOUCH 0
    EV_SYN SYN_REPORT 0
3)
    EV_KEY BTN_TOOL_FINGER 0
    EV_SYN SYN_REPORT 0

This should work with at least libinput, though I have to check what happens
when you don't send x/y on the first event. I think this would need a patch
in libinput, but that's doable. And it's the same sequence we also use for
e.g. pen tools that support hovering as well.

Cheers,
   Peter

Hi Peter,

It looks like you're suggesting to use "BTN_TOOL_FINGER" as a signal
as to weather or not the value in "ABS_DISTANCE" is valid or not.

1. No finger detected anywhere:
    BTN_TOOL_FINGER = 0
    ABS_DISTANCE = n/a
2. Finger seen hovering but not touching:
    BTN_TOOL_FINGER = 1
    ABS_DISTANCE > 0
2. Finger touching:
    BTN_TOOL_FINGER = 1
    ABS_DISTANCE = 0

you should set BTN_TOUCH here. This can be done based on some magic pressure
threshold if you have pressure, otherwise just unconditionally set it
whenever distance is 0.

quoted

Am I understanding that correctly?

yes, but IMO better to think it this way: BTN_TOOL_FINGER signals whether a
finger is detected by the device. everything else is just axis information
if and when it becomes available.
likewise, BTN_TOUCH signals whether the current tool (could be something
other than finger) logically touches the surface.

I wonder if this will not confuse clients that do not pay attention to
ABS_DISTANCE though... I take it that libinput and x synaptics drivers
won't be confused, mousedev in kernel relies on BTN_TOUCH, what about
others?

Thanks.

-- 
Dmitry