[PATCH 1/2] drm/virtio: preserve damage clips for rendered resources

[Lyude Paul]

From: Christian Hergert <christian@xxxxxxxxxxxxxxxxxx>

virtio_gpu_plane_atomic_check() unconditionally ignores FB_DAMAGE_CLIPS
when the framebuffer changes. This forces a full resource flush on every
page flip when a compositor rotates through multiple framebuffers, even
when userspace supplies accumulated per-buffer damage.

The full update is required for dumb buffers because their contents are
uploaded per buffer. Rendered resources are already coherent on the host,
so replacing their damage with the full plane only discards useful
information.

Restrict ignore_damage_clips to dumb buffers. This allows damage-aware
compositors to propagate partial updates through RESOURCE_FLUSH while
preserving the existing full-upload behavior for dumb buffers.

Signed-off-by: Christian Hergert <christian@xxxxxxxxxxxxxxxxxx>
Reviewed-by: Lyude Paul <lyude@xxxxxxxxxx>
Signed-off-by: Lyude Paul <lyude@xxxxxxxxxx>
---
 drivers/gpu/drm/virtio/virtgpu_plane.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/virtio/virtgpu_plane.c b/drivers/gpu/drm/virtio/virtgpu_plane.c
index 1d1b27ece62a7..d9579a4b4714c 100644
--- a/drivers/gpu/drm/virtio/virtgpu_plane.c
+++ b/drivers/gpu/drm/virtio/virtgpu_plane.c
@@ -103,6 +103,7 @@ static int virtio_gpu_plane_atomic_check(struct drm_plane *plane,
 										 plane);
 	struct drm_plane_state *old_plane_state = drm_atomic_get_old_plane_state(state,
 										 plane);
+	struct virtio_gpu_object *bo;
 	bool is_cursor = plane->type == DRM_PLANE_TYPE_CURSOR;
 	struct drm_crtc_state *crtc_state;
 	int ret;
@@ -113,9 +114,12 @@ static int virtio_gpu_plane_atomic_check(struct drm_plane *plane,
 	/*
 	 * Ignore damage clips if the framebuffer attached to the plane's state
 	 * has changed since the last plane update (page-flip). In this case, a
-	 * full plane update should happen because uploads are done per-buffer.
+	 * full plane update should happen for dumb buffers because uploads are
+	 * done per-buffer. Rendered resources are already coherent on the host,
+	 * so preserve userspace's accumulated per-buffer damage for those.
 	 */
-	if (old_plane_state->fb != new_plane_state->fb)
+	bo = gem_to_virtio_gpu_obj(new_plane_state->fb->obj[0]);
+	if (old_plane_state->fb != new_plane_state->fb && bo->dumb)
 		new_plane_state->ignore_damage_clips = true;
 
 	crtc_state = drm_atomic_get_crtc_state(state,
-- 
2.55.0