[PATCH v2 4/5] drm/plane: Extend damage tracking kernel-doc

[Javier Martinez Canillas <javierm@redhat.com>]

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>
---
- Dropped Simon Ser's Reviwed-by tag because the text changed to adapt
  to the approach Thomas Zimmermann suggested for v2.

(no changes since v1)

 drivers/gpu/drm/drm_plane.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/drivers/gpu/drm/drm_plane.c b/drivers/gpu/drm/drm_plane.c
index 24e7998d1731..3b1b8bca3065 100644
--- a/drivers/gpu/drm/drm_plane.c
+++ b/drivers/gpu/drm/drm_plane.c
@@ -1442,6 +1442,26 @@ int drm_mode_page_flip_ioctl(struct drm_device *dev,
  * 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 mentiored damage helpers to
+ * iterate over the damaged regions. Drivers that handle buffer damage, need to
+ * set &struct drm_plane_state.ignore_damage_clips as an indication to
+ * drm_atomic_helper_damage_iter_init() that the damage clips should be ignored.
+ * In that case, the returned damage rectangle is the &drm_plane_state.src since
+ * a full plane update should happen.
+ *
+ * For more information about the two type of damage, see:
+ *     https://registry.khronos.org/EGL/extensions/KHR/EGL_KHR_swap_buffers_with_damage.txt
+ *     https://emersion.fr/blog/2019/intro-to-damage-tracking/
  */
 
 /**
-- 
2.41.0