On Tue, Mar 12, 2013 at 7:18 PM, Yufeng Shen [off-list ref] wrote:

On Tue, Mar 12, 2013 at 4:59 AM, Benjamin Tissoires
[off-list ref] wrote:

quoted

On Tue, Mar 12, 2013 at 2:36 AM, Peter Hutterer
[off-list ref] wrote:

quoted

On Mon, Mar 11, 2013 at 04:34:27PM -0400, Yufeng Shen wrote:

quoted

I have ran into cases where I want to make a touch end event to have a
touch cancel indication.

This comes from trying to solve the problem of :

If the touch sequence happens before the system suspends, and the touch
release event is
never received after the system resumes, userspace MT state tracking could
be in a bad state.

( see #5 from
https://bugs.launchpad.net/ubuntu/+source/xserver-xorg-input-synaptics/+bug/968845
for an example of how this could happen from lid close/open on MBA)

ftr, this a bug in the driver and should be fixed now.

quoted

One possible workaround is to let the touch device driver to release all
existing touches on
resume, which has the effect of clearing all the MT states in userspace
touch stacks.
But the touch release/end event often will result in some gesture being
recognized and performed,
like a tap-to-click being generated.

So I am wondering what's the best way to solve the problem of clearing the
touch states with
minimal side effect. One way I can think of is to have MTB protocol add
support of
a touch cancel indication on touch release, e.g. making TRACKING_ID = -2
meaning that
the touch release is synthesized from the system and really has the meaning
of releasing and canceling the current touch, while TRACKING_ID = -1
meaning that the touch release is reported back from the device.

And from Xf86-input driver level, we can add a corresponding TouchCancel
for this.

I can handle touch-cancel events in the synaptics driver to avoid
tap-to-click but further details get a bit nasty.

To actually add TouchCancel to the client-protocol means a new XI protocol
revision, plus the stuff in the server _and_ the stuff in the client. that
is quite some lag time here, and if a client cannot handle TouchCancel all
we can do is do a TouchEnd - which will still trigger the gesture.

even if you update the touch clients you're still lacking any solution for
pointer-emulated clients. again, here we can only do a ButtonRelease event
which again will trigger whatever it did.

All the above can be implemented though. In fact, I suspect the protocol
part is the easy bit (just a flag on TouchEnd) but the server part is
reasonably nasty.

the real counter-argument is that I think it is a partial solution only.
From an X perspective touches also end when you vt-switch away from the
server (device is disabled). but the kernel won't cancel the touch event for
that. Or when the device is disabled by the client ("disable touchpad while
typing" feature), So we'd have to maintain both implicit cancel and explicit
cancel in the driver anyway.

so yeah, I don't think adding this to the kernel would provide any
significant benefit since we still need to handle all the other cases
anyway.

If the same effect is seen when VT-switching, it's definitively a user
space synchronization problem, not a kernel problem.

I think we have all the pieces in term of protocol in the kernel for
this use case:
When coming back from resume, the kernel should guarantee that the
current input state is correct. If fingers are still present, then
their slots are still assigned, if they are missing, their slots
should be silently released (as if the released occurs while
sleeping). This is something the kernel can work on.
As for the user-space, when coming back from a situation where
inconsistency may have occurred (VT-switching, sleep/resume, events
dropped due to a SYN_DROPPED event, or device disabled by the client),
the user-space driver has to retrieve the current state of the kernel
driver through the correct ioctls. If it doesn't do it, then that
means that he is not following the evdev protocol. It's up to it to
notify or not the toolkit/gesture recognizer that events have been
dropped.

what's your suggestion on implementing "notify the toolkit/gesture
recognizer that events have been dropped" ?

For xf86-input-synaptics, then this is just an internal call to stop
the scrolling, tapping and others.

For external gesture recognition engines and toolkits, then it's the
exact same problem you were raising at the beginning of the thread: I
doubt Xinput 2.2 can handle it smoothly.
But in any case, this is something that will need further thoughts.
Maybe there could be a generic XI event: XIAbortedEvents.
Peter would answer better than me on this subject.

Benjamin