[PATCH 01/14] drm/atomic: Document atomic state lifetime

[Maxime Ripard]

How drm_atomic_state structures and the various entity structures are
allocated and freed isn't really trivial, so let's document it.

Signed-off-by: Maxime Ripard <mripard@xxxxxxxxxx>
---
 Documentation/gpu/drm-kms.rst |  6 +++++
 drivers/gpu/drm/drm_atomic.c  | 52 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 58 insertions(+)

diff --git a/Documentation/gpu/drm-kms.rst b/Documentation/gpu/drm-kms.rst
index 2292e65f044c3bdebafbb8f83dfe7ac12e831273..017c7b196ed7ead4cf5fa8572e1f977d9e00dda8 100644
--- a/Documentation/gpu/drm-kms.rst
+++ b/Documentation/gpu/drm-kms.rst
@@ -280,10 +280,16 @@ structure, ordering of committing state changes to hardware is sequenced using
 :c:type:`struct drm_crtc_commit <drm_crtc_commit>`.
 
 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
 -----------------------------
 
 .. kernel-doc:: drivers/gpu/drm/drm_atomic.c
    :doc: handling driver private state
diff --git a/drivers/gpu/drm/drm_atomic.c b/drivers/gpu/drm/drm_atomic.c
index 4283ab4d06c581727cc98b1dc870bf69691ea654..92c6afc8f22c8307a59dc266aacdb8e03351409d 100644
--- a/drivers/gpu/drm/drm_atomic.c
+++ b/drivers/gpu/drm/drm_atomic.c
@@ -45,10 +45,62 @@
 #include <drm/drm_colorop.h>
 
 #include "drm_crtc_internal.h"
 #include "drm_internal.h"
 
+/**
+ * DOC: state lifetime
+ *
+ * &struct drm_atomic_state represents an update to video pipeline
+ * state. Despite its confusing name, it's actually a transient object
+ * that holds a state update as a collection of pointer to individual
+ * objects states. &struct drm_atomic_state has a much shorter lifetime
+ * than the objects states, since it's only allocated while preparing,
+ * checking or doing the update, while object states are allocated while
+ * the state will be, or is active in the hardware.
+ *
+ * Their respective lifetimes are:
+ *
+ * - at reset time, the object reset implementation will allocate a new,
+ *   default, state and will store it in the object state pointer.
+ *
+ * - whenever a new update is needed:
+ *
+ *   + we allocate a new &struct drm_atomic_state using drm_atomic_state_alloc().
+ *
+ *   + we copy the state of each affected entity into our &struct
+ *     drm_atomic_state using drm_atomic_get_plane_state(),
+ *     drm_atomic_get_crtc_state(), drm_atomic_get_connector_state(), or
+ *     drm_atomic_get_private_obj_state(). That state can then be
+ *     modified.
+ *
+ *     At that point, &struct drm_atomic_state stores three state
+ *     pointers for that particular entity: the old, new, and existing
+ *     (called "state") states. 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 existing state points to the state we'll
+ *     eventually have to free when the drm_atomic_state will be
+ *     destroyed, but points to the new state for now.
+ *
+ *   + After the state is populated, it is checked. If the check is
+ *     successful, the update is committed. Part of the commit is a call
+ *     to drm_atomic_helper_swap_state() which will turn the new states
+ *     into the active states. Doing so involves updating the objects
+ *     state pointer (&drm_crtc.state or similar) to point to the new
+ *     state, and the existing states will now point to the old states,
+ *     that used to be active but isn't anymore.
+ *
+ *   + When the commit is done, and when all references to our &struct
+ *     drm_atomic_state are put, drm_atomic_state_clear() runs and will
+ *     free all the old states.
+ *
+ *   + Now, we don't have any active &struct drm_atomic_state anymore,
+ *     and only the entity active states remain allocated.
+ */
+
 void __drm_crtc_commit_free(struct kref *kref)
 {
 	struct drm_crtc_commit *commit =
 		container_of(kref, struct drm_crtc_commit, ref);
 

-- 
2.53.0