drm/plane: Extend damage tracking kernel-doc

The "Damage Tracking Properties" section in the documentation doesn't have
info about the two type of damage handling: frame damage vs buffer damage.

Add it to the section and mention that helpers only support frame damage,
and how drivers handling buffer damage can indicate that the damage clips
should be ignored.

Also add references to further documentation about the two damage types.

Suggested-by: Simon Ser <contact@emersion.fr>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Zack Rusin <zackr@vmware.com>
Acked-by: Sima Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20231123221315.3579454-5-javierm@redhat.com

1 files changed, 30 insertions, 0 deletions

diff --git a/drivers/gpu/drm/drm_plane.c b/drivers/gpu/drm/drm_plane.c
index f3f2eae83cca..9e8e4c60983d 100644
--- a/drivers/gpu/drm/drm_plane.c
+++ b/drivers/gpu/drm/drm_plane.c
@@ -1558,6 +1558,36 @@ out:
  * Drivers implementing damage can use drm_atomic_helper_damage_iter_init() and
  * drm_atomic_helper_damage_iter_next() helper iterator function to get damage
  * rectangles clipped to &drm_plane_state.src.
+ *
+ * Note that there are two types of damage handling: frame damage and buffer
+ * damage, the type of damage handling implemented depends on a driver's upload
+ * target. Drivers implementing a per-plane or per-CRTC upload target need to
+ * handle frame damage, while drivers implementing a per-buffer upload target
+ * need to handle buffer damage.
+ *
+ * The existing damage helpers only support the frame damage type, there is no
+ * buffer age support or similar damage accumulation algorithm implemented yet.
+ *
+ * Only drivers handling frame damage can use the mentioned damage helpers to
+ * iterate over the damaged regions. Drivers that handle buffer damage, must set
+ * &drm_plane_state.ignore_damage_clips for drm_atomic_helper_damage_iter_init()
+ * to know that damage clips should be ignored and return &drm_plane_state.src
+ * as the damage rectangle, to force a full plane update.
+ *
+ * Drivers with a per-buffer upload target could compare the &drm_plane_state.fb
+ * of the old and new plane states to determine if the framebuffer attached to a
+ * plane has changed or not since the last plane update. If &drm_plane_state.fb
+ * has changed, then &drm_plane_state.ignore_damage_clips must be set to true.
+ *
+ * That is because drivers with a per-plane upload target, expect the backing
+ * storage buffer to not change for a given plane. If the upload buffer changes
+ * between page flips, the new upload buffer has to be updated as a whole. This
+ * can be improved in the future if support for frame damage is added to the DRM
+ * damage helpers, similarly to how user-space already handle this case as it is
+ * explained in the following documents:
+ *
+ *     https://registry.khronos.org/EGL/extensions/KHR/EGL_KHR_swap_buffers_with_damage.txt
+ *     https://emersion.fr/blog/2019/intro-to-damage-tracking/
  */
 
 /**