drm/atomic: Document atomic commit lifetime

How drm_atomic_commit and the various entity structures are allocated
and freed isn't really trivial. Document it.

Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patch.msgid.link/20260526-drm-mode-config-init-v6-1-852346394200@kernel.org
Signed-off-by: Maxime Ripard <mripard@kernel.org>

Documentation/gpu/drm-kms.rst

@@ -287,6 +287,12 @@ structure, ordering of committing state changes to hardware is sequenced using
 Read on in this chapter, and also in :ref:`drm_atomic_helper` for more detailed
 coverage of specific topics.
 
+Atomic State Lifetime
+---------------------
+
+.. kernel-doc:: drivers/gpu/drm/drm_atomic.c
+   :doc: state lifetime
+
 Handling Driver Private State
 -----------------------------
 

drivers/gpu/drm/drm_atomic.c

@@ -47,6 +47,78 @@
 #include "drm_crtc_internal.h"
 #include "drm_internal.h"
 
+/**
+ * DOC: state lifetime
+ *
+ * &drm_atomic_commit represents an update to modeset pipeline state.
+ * It's a transient object that holds a state update as a collection of
+ * pointers to individual objects' states. &drm_atomic_commit has a much
+ * shorter lifetime than the objects' states, since it's only allocated
+ * while preparing, checking or committing the update, while object
+ * states are allocated when preparing the update and kept alive as long
+ * as they are active in the device.
+ *
+ * Their respective lifetimes are:
+ *
+ * - at reset time, the object reset implementation allocates a new
+ *   default state and stores it in the object state pointer.
+ *
+ * - whenever a new update is needed:
+ *
+ *   + drm_atomic_commit_alloc() allocates a new &drm_atomic_commit
+ *     instance.
+ *
+ *   + The code triggering the commit (ioctl, client modeset,
+ *     drm_atomic_helper_reset_crtc(), etc.) copies the current active
+ *     state of all entities affected by the update into this new
+ *     &drm_atomic_commit using drm_atomic_get_plane_state(),
+ *     drm_atomic_get_crtc_state(), drm_atomic_get_connector_state(), or
+ *     drm_atomic_get_private_obj_state(). This new state can then be
+ *     modified.
+ *
+ *     At that point, &drm_atomic_commit stores three state pointers for
+ *     any affected entity: the "old" and "new" states, and
+ *     state_to_destroy. The old state is the state currently active in
+ *     the hardware, which is either the one initialized by reset() or a
+ *     newer one if a commit has been made. The new state is the state
+ *     we just allocated and we might eventually commit to the hardware.
+ *     The state_to_destroy points to the state we'll eventually have to
+ *     free when the drm_atomic_commit will be destroyed, and points to
+ *     the new state for now since the old state is still the active
+ *     state.
+ *
+ *   + After the calling code populated the commit with the entities
+ *     states, it updates the new states with the new values we need to
+ *     commit. The new commit instance is now ready.
+ *
+ *   + Then we have two branches depending on the calling code intent:
+ *
+ *     - If the calling code only wants to check that the commit would
+ *       work (for example because of the DRM_MODE_ATOMIC_TEST_ONLY
+ *       flag). It calls drm_atomic_check_only(), which in turn checks
+ *       all these states by invoking atomic_check on all affected
+ *       pipeline stages.
+ *
+ *     - If the calling code actually wants to trigger a commit, it
+ *       calls drm_atomic_commit(). The first stage is the check
+ *       mentioned above, and if the check is successful, it performs
+ *       the commit. Part of the commit is a call to
+ *       drm_atomic_helper_swap_state() which turns the new states into
+ *       the active states. After swapping states, each object's state
+ *       pointer now refers to the formerly new state. The
+ *       state_to_destroy now refers to the formerly old state.
+ *
+ *   + Once done, and when the last reference to our &drm_atomic_commit
+ *     is given up through drm_atomic_commit_put(), it calls
+ *     __drm_atomic_commit_free(). In turn, __drm_atomic_commit_free()
+ *     calls drm_atomic_commit_clear() that will free all
+ *     state_to_destroy (ie. old states), and it finally frees
+ *     &drm_atomic_commit instance.
+ *
+ *   + Now, we don't have any active &drm_atomic_commit anymore, and
+ *     only the entity active states remain allocated.
+ */
+
 void __drm_crtc_commit_free(struct kref *kref)
 {
 	struct drm_crtc_commit *commit =