[RFC wayland] protocol: Add high-resolution wl_touch timestamp event

[Alexandros Frantzis]

wl_touch events currently use a 32-bit timestamp with millisecond
resolution. In some cases, notably latency measurements, this resolution
is too coarse-grained to be useful.

This protocol update adds a wl_touch.timestamp event, which is emitted
just before an up, motion or down touch event. The timestamp event
contains a high-resolution, and ideally higher-accuracy, version of the
'time' argument of the up/motion/down event that follows.

>From a client implementation perspective clients can just ignore this event if
they don't care about the improved resolution. Clients that care just need to
keep track of the last timestamp event they receive and associate it with the
next up/motion/down event that arrives.

Some points for discussion:

1. Should there be a request to explicitly enable/disable the timestamp event
   (to reduce extra event overhead when improved timestamps are not needed)?

2. Should we introduce similar timestamp events for keyboard and pointer?

3. Should this be a separate protocol instead of a core update (being separate
   would also take care of point (1))?

A proof of concept implementation for weston can be found at:

https://git.collabora.com/cgit/user/alf/weston.git/log/?h=wl-touch-v7

Note that the weston implementation depends on a patchset that changes weston
to use higher resolution timestamps internally. The patchset been submitted
for review at:

https://lists.freedesktop.org/archives/wayland-devel/2017-November/035851.html

Signed-off-by: Alexandros Frantzis <alexandros.frantzis at collabora.com>
---
 protocol/wayland.xml | 27 +++++++++++++++++++++++----
 1 file changed, 23 insertions(+), 4 deletions(-)

diff --git a/protocol/wayland.xml b/protocol/wayland.xml
index aabc7ae..b01811e 100644
--- a/protocol/wayland.xml
+++ b/protocol/wayland.xml
@@ -1657,7 +1657,7 @@
     </request>
    </interface>
 
-  <interface name="wl_seat" version="6">
+  <interface name="wl_seat" version="7">
     <description summary="group of input devices">
       A seat is a group of keyboards, pointer and touch devices. This
       object is published as a global during start up, or when such a
@@ -1766,7 +1766,7 @@
 
   </interface>
 
-  <interface name="wl_pointer" version="6">
+  <interface name="wl_pointer" version="7">
     <description summary="pointer input device">
       The wl_pointer interface represents one or more input devices,
       such as mice, which control the pointer location and pointer_focus
@@ -2089,7 +2089,7 @@
     </event>
   </interface>
 
-  <interface name="wl_keyboard" version="6">
+  <interface name="wl_keyboard" version="7">
     <description summary="keyboard input device">
       The wl_keyboard interface represents one or more keyboards
       associated with a seat.
@@ -2200,7 +2200,7 @@
     </event>
   </interface>
 
-  <interface name="wl_touch" version="6">
+  <interface name="wl_touch" version="7">
     <description summary="touchscreen input device">
       The wl_touch interface represents a touchscreen
       associated with a seat.
@@ -2342,6 +2342,25 @@
       <arg name="id" type="int" summary="the unique ID of this touch point"/>
       <arg name="orientation" type="fixed" summary="angle between major axis and positive surface y-axis in degrees"/>
     </event>
+
+    <!-- Version 7 additions -->
+
+    <event name="timestamp" since="7">
+      <description summary="high-resolution timestamp event">
+        The timestamp event is sent just before a wl_touch.up, wl_touch.motion
+        or wl_touch.down event and provides a high-resolution version of the
+        time argument of the event that follows.
+
+        The timestamp provided by this event is at least as accurate
+        as the timestamp provided by the wl_touch event that follows.
+      </description>
+      <arg name="tv_sec_hi" type="uint"
+           summary="high 32 bits of the seconds part of the timestamp"/>
+      <arg name="tv_sec_lo" type="uint"
+           summary="low 32 bits of the seconds part of the timestamp"/>
+      <arg name="tv_nsec" type="uint"
+           summary="nanoseconds part of the timestamp"/>
+    </event>
   </interface>
 
   <interface name="wl_output" version="3">
-- 
2.14.1