[PATCH 4/5] drm/atomic: document how to handle driver private objects

[Daniel Vetter <daniel.vetter@ffwll.ch>]

DK put some nice docs into the commit introducing driver private
state, but in the git history alone it'll be lost.

Also, since Ville remove the void* usage it's a good opportunity to
give the driver private stuff some tlc on the doc front.

Finally try to explain why the "let's just subclass drm_atomic_state"
approach wasn't the greatest, and annotate all those functions as
deprecated in favour of more standardized driver private states. Also
note where we could/should extend driver private states going forward
(atm neither locking nor synchronization is handled in core/helpers,
which isn't really all that great).

Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Rob Clark <robdclark@gmail.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
---
 Documentation/gpu/drm-kms.rst |  6 ++++++
 drivers/gpu/drm/drm_atomic.c  | 45 ++++++++++++++++++++++++++++++++++++++++---
 include/drm/drm_atomic.h      | 28 +++++++++++++++++++++++++++
 include/drm/drm_mode_config.h |  9 +++++++++
 4 files changed, 85 insertions(+), 3 deletions(-)

diff --git a/Documentation/gpu/drm-kms.rst b/Documentation/gpu/drm-kms.rst
index 307284125d7a..420025bd6a9b 100644
--- a/Documentation/gpu/drm-kms.rst
+++ b/Documentation/gpu/drm-kms.rst
@@ -271,6 +271,12 @@ Taken all together there's two consequences for the atomic design:
 Read on in this chapter, and also in :ref:`drm_atomic_helper` for more detailed
 coverage of specific topics.
 
+Handling Driver Private State
+-----------------------------
+
+.. kernel-doc:: drivers/gpu/drm/drm_atomic.c
+   :doc: handling driver private state
+
 Atomic Mode Setting Function Reference
 --------------------------------------
 
diff --git a/drivers/gpu/drm/drm_atomic.c b/drivers/gpu/drm/drm_atomic.c
index 37445d50816a..15e1a35c74a8 100644
--- a/drivers/gpu/drm/drm_atomic.c
+++ b/drivers/gpu/drm/drm_atomic.c
@@ -50,7 +50,8 @@ EXPORT_SYMBOL(__drm_crtc_commit_free);
  * @state: atomic state
  *
  * Free all the memory allocated by drm_atomic_state_init.
- * This is useful for drivers that subclass the atomic state.
+ * This should only be used by drivers which are still subclassing
+ * &drm_atomic_state and haven't switched to &drm_private_state yet.
  */
 void drm_atomic_state_default_release(struct drm_atomic_state *state)
 {
@@ -67,7 +68,8 @@ EXPORT_SYMBOL(drm_atomic_state_default_release);
  * @state: atomic state
  *
  * Default implementation for filling in a new atomic state.
- * This is useful for drivers that subclass the atomic state.
+ * This should only be used by drivers which are still subclassing
+ * &drm_atomic_state and haven't switched to &drm_private_state yet.
  */
 int
 drm_atomic_state_init(struct drm_device *dev, struct drm_atomic_state *state)
@@ -132,7 +134,8 @@ EXPORT_SYMBOL(drm_atomic_state_alloc);
  * @state: atomic state
  *
  * Default implementation for clearing atomic state.
- * This is useful for drivers that subclass the atomic state.
+ * This should only be used by drivers which are still subclassing
+ * &drm_atomic_state and haven't switched to &drm_private_state yet.
  */
 void drm_atomic_state_default_clear(struct drm_atomic_state *state)
 {
@@ -946,6 +949,42 @@ static void drm_atomic_plane_print_state(struct drm_printer *p,
 		plane->funcs->atomic_print_state(p, state);
 }
 
+/**
+ * DOC: handling driver private state
+ *
+ * Very often the DRM objects exposed to userspace in the atomic modeset api
+ * (&drm_connector, &drm_crtc and &drm_plane) do not map neatly to the
+ * underlying hardware. Especially for any kind of shared resources (e.g. shared
+ * clocks, scaler units, bandwidth and fifo limits shared among a group of
+ * planes or CRTCs, and so on) it makes sense to model these as independent
+ * objects. Drivers then need to similar state tracking and commit ordering for
+ * such private (since not exposed to userpace) objects as the atomic core and
+ * helpers already provide for connectors, planes and CRTCs.
+ *
+ * To make this easier on drivers the atomic core provides some support to track
+ * driver private state objects using struct &drm_private_obj, with the
+ * associated state struct &drm_private_state.
+ *
+ * Similar to userspace-exposed objects, state structures can be acquired by
+ * calling drm_atomic_get_private_obj_state(). Since this function does not take
+ * care of locking, drivers should wrap it for each type of private state object
+ * they have with the required call to drm_modeset_lock() for the corresponding
+ * &drm_modeset_lock.
+ *
+ * All private state structures contained in a &drm_atomic_state update can be
+ * iterated using for_each_oldnew_private_obj_in_state(),
+ * for_each_old_private_obj_in_state() and for_each_old_private_obj_in_state().
+ * Drivers are recommend to wrap these for each type of driver private state
+ * object they have, filtering on &drm_private_obj.funcs using for_each_if(), at
+ * least if they want to iterate over all objects of a given type.
+ *
+ * An earlier way to handle driver private state was by subclassing struct
+ * &drm_atomic_state. But since that encourages non-standard ways to implement
+ * the check/commit split atomic requires (by using e.g. "check and rollback or
+ * commit instead" of "duplicate state, check, then either commit or release
+ * duplicated state) it is deprecated in favour of using &drm_private_state.
+ */
+
 /**
  * drm_atomic_private_obj_init - initialize private object
  * @obj: private object
diff --git a/include/drm/drm_atomic.h b/include/drm/drm_atomic.h
index 5afd6e364fb6..8e4829a25c1b 100644
--- a/include/drm/drm_atomic.h
+++ b/include/drm/drm_atomic.h
@@ -189,12 +189,40 @@ struct drm_private_state_funcs {
 				     struct drm_private_state *state);
 };
 
+/**
+ * struct drm_private_obj - base struct for driver private atomic object
+ *
+ * A driver private object is initialized by calling
+ * drm_atomic_private_obj_init() and cleaned up by calling
+ * drm_atomic_private_obj_fini().
+ *
+ * Currently only tracks the state update functions and the opaque driver
+ * private state itself, but in the future might also track which
+ * &drm_modeset_lock is required to duplicate and update this object's state.
+ */
 struct drm_private_obj {
+	/**
+	 * @state: Current atomic state for this driver private object.
+	 */
 	struct drm_private_state *state;
 
+	/**
+	 * @funcs:
+	 *
+	 * Functions to manipulate the state of this driver private object, see
+	 * &drm_private_state_funcs.
+	 */
 	const struct drm_private_state_funcs *funcs;
 };
 
+/**
+ * struct drm_private_state - base struct for driver private object state
+ * @state: backpointer to global drm_atomic_state
+ *
+ * Currently only contains a backpointer to the overall atomic upated, but in
+ * the future also might hold synchronization information similar to e.g.
+ * &drm_crtc.commit.
+ */
 struct drm_private_state {
 	struct drm_atomic_state *state;
 };
diff --git a/include/drm/drm_mode_config.h b/include/drm/drm_mode_config.h
index c6639e8e5007..2cb6f02df64a 100644
--- a/include/drm/drm_mode_config.h
+++ b/include/drm/drm_mode_config.h
@@ -269,6 +269,9 @@ struct drm_mode_config_funcs {
 	 * state easily. If this hook is implemented, drivers must also
 	 * implement @atomic_state_clear and @atomic_state_free.
 	 *
+	 * Subclassing of &drm_atomic_state is deprecated in favour of using
+	 * &drm_private_state and &drm_private_obj.
+	 *
 	 * RETURNS:
 	 *
 	 * A new &drm_atomic_state on success or NULL on failure.
@@ -290,6 +293,9 @@ struct drm_mode_config_funcs {
 	 *
 	 * Drivers that implement this must call drm_atomic_state_default_clear()
 	 * to clear common state.
+	 *
+	 * Subclassing of &drm_atomic_state is deprecated in favour of using
+	 * &drm_private_state and &drm_private_obj.
 	 */
 	void (*atomic_state_clear)(struct drm_atomic_state *state);
 
@@ -302,6 +308,9 @@ struct drm_mode_config_funcs {
 	 *
 	 * Drivers that implement this must call
 	 * drm_atomic_state_default_release() to release common resources.
+	 *
+	 * Subclassing of &drm_atomic_state is deprecated in favour of using
+	 * &drm_private_state and &drm_private_obj.
 	 */
 	void (*atomic_state_free)(struct drm_atomic_state *state);
 };
-- 
2.15.1

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