doc: Add frame timing documentation

doc/frame-timing.md

@@ -0,0 +1,52 @@
+# Frame Timing {#frame-timing}
+
+<!--
+Copyright 2021, Collabora, Ltd. and the Monado contributors
+SPDX-License-Identifier: BSL-1.0
+-->
+
+
+A "brief" overview of the various time-points that a frame goes through, from
+when the application gets go ahead to render the frame to when pixels are turned
+into photons. This only a single frame, where all of the timings are hit and the
+application is single threaded. The HMD also only turns on the display during
+the vblank period, meaning the pixel to photon transformation is delayed from
+scanout starting to the vblank period (like for the Index).
+
+* `xrWaitFrame` returns to the application, referred to as **wake_up**.
+* The app does a logic step to move the simulation to the next predicted
+  display time.
+* `xrBeginFrame` is called by the application, referred to as **begin**.
+* The app renders the current views using the GPU.
+* `xrEndFrame` is called by the application submitting the views.
+* The compositor wakes up to do it's distorting rendering and any warping,
+  checking if the applications rendering has finished. When the compositor
+  submits the work to the GPU, referred to as **submit**.
+* The compositor queues it's swapbuffer to the display engine.
+* Scanout starts, the kernel checked if the compositors rendering was completed
+  in time. We refer to this time as **present**, this seems to be common.
+* During the vblank period the display lights up turning the pixels into
+  photons. We refer to this time as **display**, same as in OpenXR.
+
+
+The names for timepoints are chosen to align with the naming in
+[`VK_GOOGLE_display_timing`][], reading that extension can provide further
+information.
+
+
+## Main compositor perspective
+
+ * @ref xrt_comp_wait_frame - It is within this function that the frame timing is
+   predicted, see @ref u_frame_timing_predict. The compositor will then wait to
+   **waku_up** time and then return from this function.
+ * @ref xrt_comp_begin_frame - The app or IPC server calls this function when it
+   is done with CPU work and ready to do GPU work.
+ * @ref xrt_comp_discard_frame - The frame is discarded.
+ * @ref xrt_comp_layer_begin - Called during transfers of layers.
+ * @ref xrt_comp_layer_stereo_projection - This and other layer functions are
+   called to list the layers the compositor should render.
+ * @ref xrt_comp_layer_commit - The compositor starts to render the frame,
+   trying to hit the **present** time.
+
+
+[`VK_GOOGLE_display_timing`]: https://www.khronos.org/registry/vulkan/specs/1.2-extensions/man/html/VK_GOOGLE_display_timing.html

doc/mainpage.md

@@ -17,6 +17,7 @@ getting started information and general documentation.
 * @ref vulkan-extensions
 * @ref writing-driver (**not complete**)
 * @ref ipc-design
+* @ref frame-timing
 
 ## Source layout