drm/atomic: Document history of drm_atomic_state

After some discussions on the mailing-list for an earlier revision of
the series, it was suggested to document the evolution of
drm_atomic_state and its use by drivers to explain some of the confusion
one might still encounter when reading the framework code.

Suggested-by: Simona Vetter <simona.vetter@ffwll.ch>
Link: https://lore.kernel.org/dri-devel/Z4jtKHY4qN3RNZNG@phenom.ffwll.local/
Reviewed-by: Simona Vetter <simona.vetter@ffwll.ch>
Link: https://lore.kernel.org/r/20250213-bridge-connector-v3-1-e71598f49c8f@kernel.org
Signed-off-by: Maxime Ripard <mripard@kernel.org>

1 files changed, 31 insertions, 0 deletions

diff --git a/include/drm/drm_atomic.h b/include/drm/drm_atomic.h
index 1ded9a8d4e84..4c673f0698fe 100644
--- a/include/drm/drm_atomic.h
+++ b/include/drm/drm_atomic.h
@@ -357,6 +357,37 @@ struct __drm_private_objs_state {
  * States are added to an atomic update by calling drm_atomic_get_crtc_state(),
  * drm_atomic_get_plane_state(), drm_atomic_get_connector_state(), or for
  * private state structures, drm_atomic_get_private_obj_state().
+ *
+ * NOTE: struct drm_atomic_state first started as a single collection of
+ * entities state pointers (drm_plane_state, drm_crtc_state, etc.).
+ *
+ * At atomic_check time, you could get the state about to be committed
+ * from drm_atomic_state, and the one currently running from the
+ * entities state pointer (drm_crtc.state, for example). After the call
+ * to drm_atomic_helper_swap_state(), the entities state pointer would
+ * contain the state previously checked, and the drm_atomic_state
+ * structure the old state.
+ *
+ * Over time, and in order to avoid confusion, drm_atomic_state has
+ * grown to have both the old state (ie, the state we replace) and the
+ * new state (ie, the state we want to apply). Those names are stable
+ * during the commit process, which makes it easier to reason about.
+ *
+ * You can still find some traces of that evolution through some hooks
+ * or callbacks taking a drm_atomic_state parameter called names like
+ * "old_state". This doesn't necessarily mean that the previous
+ * drm_atomic_state is passed, but rather that this used to be the state
+ * collection we were replacing after drm_atomic_helper_swap_state(),
+ * but the variable name was never updated.
+ *
+ * Some atomic operations implementations followed a similar process. We
+ * first started to pass the entity state only. However, it was pretty
+ * cumbersome for drivers, and especially CRTCs, to retrieve the states
+ * of other components. Thus, we switched to passing the whole
+ * drm_atomic_state as a parameter to those operations. Similarly, the
+ * transition isn't complete yet, and one might still find atomic
+ * operations taking a drm_atomic_state pointer, or a component state
+ * pointer. The former is the preferred form.
  */
 struct drm_atomic_state {
 	/**