Documentation/gpu: Document amdgpu_dm_visual_confirm debugfs entry

Display core provides a feature that makes it easy for users to debug
Multiple planes by enabling a visual notification at the bottom of each
plane. This commit introduces how to use such a feature.
Reviewed-by: Yann Dirson <ydirson@free.fr>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

Documentation/gpu/amdgpu/display/dc-debug.rst

+========================
 Display Core Debug tools
 ========================
 
-TODO
+DC Debugfs
+==========
+
+Multiple Planes Debug
+---------------------
+
+If you want to enable or debug multiple planes in a specific user-space
+application, you can leverage a debug feature named visual confirm. For
+enabling it, you will need::
+
+  echo 1 > /sys/kernel/debug/dri/0/amdgpu_dm_visual_confirm
+
+You need to reload your GUI to see the visual confirmation. When the plane
+configuration changes or a full update occurs there will be a colored bar at
+the bottom of each hardware plane being drawn on the screen.
+
+* The color indicates the format - For example, red is AR24 and green is NV12
+* The height of the bar indicates the index of the plane
+* Pipe split can be observed if there are two bars with a difference in height
+  covering the same plane
+
+Consider the video playback case in which a video is played in a specific
+plane, and the desktop is drawn in another plane. The video plane should
+feature one or two green bars at the bottom of the video depending on pipe
+split configuration.
+
+* There should **not** be any visual corruption
+* There should **not** be any underflow or screen flashes
+* There should **not** be any black screens
+* There should **not** be any cursor corruption
+* Multiple plane **may** be briefly disabled during window transitions or
+  resizing but should come back after the action has finished