[Wayland-bugs] [Bug 97526] Handle wacom tablet delayed serial events

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=97526

            Bug ID: 97526
           Summary: Handle wacom tablet delayed serial events
           Product: Wayland
           Version: unspecified
          Hardware: Other
                OS: All
            Status: ASSIGNED
          Severity: normal
          Priority: medium
         Component: libinput
          Assignee: wayland-bugs at lists.freedesktop.org
          Reporter: peter.hutterer at who-t.net
                CC: killertofu at gmail.com

Created attachment 126092
  --> https://bugs.freedesktop.org/attachment.cgi?id=126092&action=edit
evemu log for a delayed serial

Some sensors take a few events until they can read the serial number from the
pen. This means that the serial is initially 0, then updates to the real serial
number later.

See also:
https://lists.freedesktop.org/archives/wayland-devel/2016-August/030751.html

There are a couple of options to deal with this, based on the assumption that
once we announce a tool in libinput, we cannot afterwards change its serial
number.

1) delay initialization of a tool until the serial comes. this would result in
the device not reacting until potentially well into the interaction
2) disable serial reporting on affected tablets. this requires a hwdb
3) ignore late serials. we get a serial of 0 and that's a good hint that we
just don't know yet. we can then ignore whatever serial comes next
4) a combination of 1 with memory - delay the initial sequence and remember the
last-seen tool. Re-use that for the next unknown sequence and where incorrect
fake a prox out + prox in for the new tool

Option 1) is complicated for making sure button events are correct. 2) requires
updating of the hwdb. 3) is simple, 4) is probably the best user experience and
should be implemented once 3) proves insufficient (e.g. use of multiple tools
on such tablet)

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