On Wed, Jul 06, 2011 at 08:47:59PM +0200, Henrik Rydberg wrote:

quoted

quoted

(C) A third, compromise implementation might be to do something like this:
  (1) When >=2 fingers are on the pad, report 2 MT-B slots, where:
    (a) slot[0] reports finger 1
    (b) slot[1] reports finger 2
    (c) Total number of fingers on pad is indicated by EV_KEY/BTN_TOOL_* is set.
  (2) If 1 finger is removed, the tracking_id for its slot is invalidated;
      but, the finger that remains stays in its same slot with the
same tracking_id.
  (3) Whenever the number of fingers changes from/to 3 or more
fingers, both slots are always invalidated.
      New tracking_ids are assigned to both slots which contain the
two fingers now being reported.
  (4) No special driver property is required for this, not even "semi-mt".
      It should be as pure MT-B as we can get (plus BTN_TOOL_* hack).

I believe there is a strong userspace assumption that BTN_TOOL_* has
no meaning for real MT devices. Rightfully so, IMO. Hence, I think
semi-mt needs to be used here as well.

I think we need to adjust userspace to pay attention to BTN_TOOL_* for
MT-B too so that if number of slots advertised does not match
BTN_TOOL_* capabilities that means that the device does not privide
tracking data for all contacts.

Luckily this should be backward-compatible (i.e. older userspace will
ignore "extended" fingercounts, newer will pay attention to it).

quoted

quoted

This is the best option in my opinion. We will present 2 finger position
data plus extended finger count.

We never did put all the details of the bounding box coordinates in
writing, so perhaps this is an opportunity to both fix that and extend
usability to the case so described. The only question is whether there
are applications out there which now assume min/max instead of contact
positions. If anyone knows, please speak up. :-) Otherwise, I am very
much for Daniel's case C, with Dmitry's modification.

In short: Use the semi-MT property, and send two suitable fingers
along with it.

Umm... but it is my understanding that 2 fingers will provide real
tracking data, not bounding box, so why would we set semi-MT?

Maybe we have different notions of what semi-MT property conveys? For me
semi-MT indicates that the device provides 2 coordinates for bounding
box.  However if semi-MT is not set does not mean that the device
provides true tracking for all contacts, but only for advertised slots.
There still may be additional data transmitted.

Thanks.

-- 
Dmitry