drm/crtc: Add a generic infrastructure to fake VBLANK events

In some cases CRTCs are active but are not able to generating events, at
least not at every frame at it's expected to.
This is typically the case when the CRTC is feeding a writeback connector
that has no job queued. In this situation the CRTC is usually stopped
until a new job is queued, and this can lead to timeouts when part of
the pipeline is updated but no new jobs are queued to the active
writeback connector.

In order to solve that, we add a ->no_vblank flag to drm_crtc_state
and ask the CRTC drivers to set it to true when they know they're not
able to generate VBLANK events. The core drm_atomic_helper_fake_vblank()
helper can then be used to fake VBLANKs at commit time.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180703075022.15138-6-boris.brezillon@bootlin.com

1 files changed, 23 insertions, 0 deletions

diff --git a/include/drm/drm_crtc.h b/include/drm/drm_crtc.h
index 23eddbccab10..17f4f93340b8 100644
--- a/include/drm/drm_crtc.h
+++ b/include/drm/drm_crtc.h
@@ -119,6 +119,29 @@ struct drm_crtc_state {
         bool zpos_changed : 1;
         bool color_mgmt_changed : 1;
 
+        /**
+         * @no_vblank:
+         *
+         * Reflects the ability of a CRTC to send VBLANK events. This state
+         * usually depends on the pipeline configuration, and the main usuage
+         * is CRTCs feeding a writeback connector operating in oneshot mode.
+         * In this case the VBLANK event is only generated when a job is queued
+         * to the writeback connector, and we want the core to fake VBLANK
+         * events when this part of the pipeline hasn't changed but others had
+         * or when the CRTC and connectors are being disabled.
+         *
+         * __drm_atomic_helper_crtc_duplicate_state() will not reset the value
+         * from the current state, the CRTC driver is then responsible for
+         * updating this field when needed.
+         *
+         * Note that the combination of &drm_crtc_state.event == NULL and
+         * &drm_crtc_state.no_blank == true is valid and usually used when the
+         * writeback connector attached to the CRTC has a new job queued. In
+         * this case the driver will send the VBLANK event on its own when the
+         * writeback job is complete.
+         */
+        bool no_vblank : 1;
+
         /* attached planes bitmask:
          * WARNING: transitional helpers do not maintain plane_mask so
          * drivers not converted over to atomic helpers should not rely