[PATCH v3 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>
Reviewed-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
---

Changes in v3:
- Fix typo in the kernel-doc (Simon Ser).
- Add a paragraph explaining what the problem in the kernel is and
  make it clear that the refeference documents are related to how
  user-space handles this case (Thomas Zimmermann).

 drivers/gpu/drm/drm_plane.c | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/drivers/gpu/drm/drm_plane.c b/drivers/gpu/drm/drm_plane.c
index 24e7998d1731..87edd6c3c5a4 100644
--- a/drivers/gpu/drm/drm_plane.c
+++ b/drivers/gpu/drm/drm_plane.c
@@ -1442,6 +1442,32 @@ 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 mentioned 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.
+ *
+ * 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/
  */
 
 /**
-- 
2.41.0