diff --git a/protocol/presentation_timing.xml b/protocol/presentation_timing.xml
index 5752c134..4058d479 100644
--- a/protocol/presentation_timing.xml
+++ b/protocol/presentation_timing.xml
@@ -164,11 +164,47 @@
- Currently no flags are defined. Some flags will be added
- into presentation_feedback version 1 before it is released.
+ These flags provide information about how the presentation of
+ the related content update was done. The intent is to help
+ clients assess the reliability of the feedback and the visual
+ quality with respect to possible tearing and timings. The
+ flags are:
+
+ VSYNC:
+ The presentation was synchronized to the "vertical retrace" by
+ the display hardware such that tearing does not happen.
+ Relying on user space scheduling is not acceptable for this
+ flag. If presentation is done by a copy to the active
+ frontbuffer, then it must guarantee that tearing cannot
+ happen.
+
+ HW_CLOCK:
+ The display hardware provided measurements that the hardware
+ driver converted into a presentation timestamp. Sampling a
+ clock in user space is not acceptable for this flag.
+
+ HW_COMPLETION:
+ The display hardware signalled that it started using the new
+ image content. The opposite of this is e.g. a timer being used
+ to guess when the display hardware has switched to the new
+ image content.
+
+ ZERO_COPY:
+ The presentation of this update was done zero-copy. This means
+ the buffer from the client was given to display hardware as
+ is, without copying it. Compositing with OpenGL counts as
+ copying, even if textured directly from the client buffer.
+ Possible zero-copy cases include direct scanout of a
+ fullscreen surface and a surface on a hardware overlay.
-
+
+
+
+