drm/atomic: initial support for asynchronous plane update

In some cases, like cursor updates, it is interesting to update the
plane in an asynchronous fashion to avoid big delays. The current queued
update could be still waiting for a fence to signal and thus block any
subsequent update until its scan out. In cases like this if we update the
cursor synchronously through the atomic API it will cause significant
delays that would even be noticed by the final user.

This patch creates a fast path to jump ahead the current queued state and
do single planes updates without going through all atomic steps in
drm_atomic_helper_commit(). We take this path for legacy cursor updates.

For now only single plane updates are supported, but we plan to support
multiple planes updates and async PageFlips through this interface as well
in the near future.

v6:	- move check code to drm_atomic_helper.c (Daniel Vetter)

v5:
	- improve comments (Eric Anholt)

v4:
	- fix state->crtc NULL check (Archit Taneja)

v3:
	- fix iteration on the wrong crtc state
	- put back code to forbid updates if there is a queued update for
	the same plane (Ville Syrjälä)
	- move size checks back to drivers (Ville Syrjälä)
	- move ASYNC_UPDATE flag addition to its own patch (Ville Syrjälä)

v2:
	- allow updates even if there is a queued update for the same
	plane.
        - fixes on the documentation (Emil Velikov)
        - unconditionally call ->atomic_async_update (Emil Velikov)
        - check for ->atomic_async_update earlier (Daniel Vetter)
        - make ->atomic_async_check() the last step (Daniel Vetter)
        - add ASYNC_UPDATE flag (Eric Anholt)
        - update state in core after ->atomic_async_update (Eric Anholt)
	- update docs (Eric Anholt)

Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Rob Clark <robdclark@gmail.com>
Cc: Eric Anholt <eric@anholt.net>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Reviewed-by: Archit Taneja <architt@codeaurora.org> (v5)
Acked-by: Eric Anholt <eric@anholt.net> (v5)
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170630180322.29007-2-gustavo@padovan.org

1 files changed, 50 insertions, 0 deletions

diff --git a/include/drm/drm_modeset_helper_vtables.h b/include/drm/drm_modeset_helper_vtables.h
index c85124f687ba..06569845708c 100644
--- a/include/drm/drm_modeset_helper_vtables.h
+++ b/include/drm/drm_modeset_helper_vtables.h
@@ -1135,6 +1135,56 @@ struct drm_plane_helper_funcs {
 	 */
 	void (*atomic_disable)(struct drm_plane *plane,
 			       struct drm_plane_state *old_state);
+
+	/**
+	 * @atomic_async_check:
+	 *
+	 * Drivers should set this function pointer to check if the plane state
+	 * can be updated in a async fashion. Here async means "not vblank
+	 * synchronized".
+	 *
+	 * This hook is called by drm_atomic_async_check() to establish if a
+	 * given update can be committed asynchronously, that is, if it can
+	 * jump ahead of the state currently queued for update.
+	 *
+	 * RETURNS:
+	 *
+	 * Return 0 on success and any error returned indicates that the update
+	 * can not be applied in asynchronous manner.
+	 */
+	int (*atomic_async_check)(struct drm_plane *plane,
+				  struct drm_plane_state *state);
+
+	/**
+	 * @atomic_async_update:
+	 *
+	 * Drivers should set this function pointer to perform asynchronous
+	 * updates of planes, that is, jump ahead of the currently queued
+	 * state and update the plane. Here async means "not vblank
+	 * synchronized".
+	 *
+	 * This hook is called by drm_atomic_helper_async_commit().
+	 *
+	 * An async update will happen on legacy cursor updates. An async
+	 * update won't happen if there is an outstanding commit modifying
+	 * the same plane.
+	 *
+	 * Note that unlike &drm_plane_helper_funcs.atomic_update this hook
+	 * takes the new &drm_plane_state as parameter. When doing async_update
+	 * drivers shouldn't replace the &drm_plane_state but update the
+	 * current one with the new plane configurations in the new
+	 * plane_state.
+	 *
+	 * FIXME:
+	 *  - It only works for single plane updates
+	 *  - Async Pageflips are not supported yet
+	 *  - Some hw might still scan out the old buffer until the next
+	 *  vblank, however we let go of the fb references as soon as
+	 *  we run this hook. For now drivers must implement their own workers
+	 *  for deferring if needed, until a common solution is created.
+	 */
+	void (*atomic_async_update)(struct drm_plane *plane,
+				    struct drm_plane_state *new_state);
 };
 
 /**