drm/amd/display: Add Freesync video documentation

Recently, we added support for an experimental feature named Freesync
video; for more details on that, refer to:

commit 6f59f229f8 ("drm/amd/display: Skip modeset for front porch change")
commit d10cd527f5 ("drm/amd/display: Add freesync video modes based on preferred modes")
commit 0eb1af2e82 ("drm/amd/display: Add module parameter for freesync video mode")

Nevertheless, we did not document it in detail in our driver. This
commit introduces a kernel-doc and expands the module parameter
description.

Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Reviewed by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

Documentation/gpu/amdgpu-dc.rst

@@ -66,3 +66,9 @@ Display Core
 ============
 
 **WIP**
+
+FreeSync Video
+--------------
+
+.. kernel-doc:: drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
+   :doc: FreeSync Video

drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c

@@ -837,8 +837,23 @@ module_param_named(tmz, amdgpu_tmz, int, 0444);
 
 /**
  * DOC: freesync_video (uint)
- * Enabled the optimization to adjust front porch timing to achieve seamless mode change experience
- * when setting a freesync supported mode for which full modeset is not needed.
+ * Enable the optimization to adjust front porch timing to achieve seamless
+ * mode change experience when setting a freesync supported mode for which full
+ * modeset is not needed.
+ *
+ * The Display Core will add a set of modes derived from the base FreeSync
+ * video mode into the corresponding connector's mode list based on commonly
+ * used refresh rates and VRR range of the connected display, when users enable
+ * this feature. From the userspace perspective, they can see a seamless mode
+ * change experience when the change between different refresh rates under the
+ * same resolution. Additionally, userspace applications such as Video playback
+ * can read this modeset list and change the refresh rate based on the video
+ * frame rate. Finally, the userspace can also derive an appropriate mode for a
+ * particular refresh rate based on the FreeSync Mode and add it to the
+ * connector's mode list.
+ *
+ * Note: This is an experimental feature.
+ *
  * The default value: 0 (off).
  */
 MODULE_PARM_DESC(

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -5646,6 +5646,36 @@ static void apply_dsc_policy_for_stream(struct amdgpu_dm_connector *aconnector,
 }
 #endif
 
+/**
+ * DOC: FreeSync Video
+ *
+ * When a userspace application wants to play a video, the content follows a
+ * standard format definition that usually specifies the FPS for that format.
+ * The below list illustrates some video format and the expected FPS,
+ * respectively:
+ *
+ * - TV/NTSC (23.976 FPS)
+ * - Cinema (24 FPS)
+ * - TV/PAL (25 FPS)
+ * - TV/NTSC (29.97 FPS)
+ * - TV/NTSC (30 FPS)
+ * - Cinema HFR (48 FPS)
+ * - TV/PAL (50 FPS)
+ * - Commonly used (60 FPS)
+ * - Multiples of 24 (48,72,96 FPS)
+ *
+ * The list of standards video format is not huge and can be added to the
+ * connector modeset list beforehand. With that, userspace can leverage
+ * FreeSync to extends the front porch in order to attain the target refresh
+ * rate. Such a switch will happen seamlessly, without screen blanking or
+ * reprogramming of the output in any other way. If the userspace requests a
+ * modesetting change compatible with FreeSync modes that only differ in the
+ * refresh rate, DC will skip the full update and avoid blink during the
+ * transition. For example, the video player can change the modesetting from
+ * 60Hz to 30Hz for playing TV/NTSC content when it goes full screen without
+ * causing any display blink. This same concept can be applied to a mode
+ * setting change.
+ */
 static struct drm_display_mode *
 get_highest_refresh_rate_mode(struct amdgpu_dm_connector *aconnector,
 			  bool use_probed_modes)