drm/doc: document front-buffer rendering

Explain how to perform front-buffer rendering.

v2: apply Pekka's rewrite

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Simon Ser <contact@emersion.fr>
Link: https://lore.kernel.org/r/20250414085652.43904-1-contact@emersion.fr

diff --git a/drivers/gpu/drm/drm_blend.c b/drivers/gpu/drm/drm_blend.c
index 6e74de8334663caabac3edbeed5692e3eb388cdf..6852d73c931ce32e62062e2b8f8c5e38612d5210 100644 (file)
--- a/drivers/gpu/drm/drm_blend.c
+++ b/drivers/gpu/drm/drm_blend.c
@@ -75,6 +75,12 @@
  *     the currently visible vertical area of the &drm_crtc.
  * FB_ID:
  *     Mode object ID of the &drm_framebuffer this plane should scan out.
+ *
+ *     When a KMS client is performing front-buffer rendering, it should set
+ *     FB_ID to the same front-buffer FB on each atomic commit. This implies
+ *     to the driver that it needs to re-read the same FB again. Otherwise
+ *     drivers which do not employ continuously repeated scanout cycles might
+ *     not update the screen.
  * CRTC_ID:
  *     Mode object ID of the &drm_crtc this plane should be connected to.
  *