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

drivers/gpu/drm/drm_atomic_helper.c

@@ -2053,6 +2053,45 @@ void drm_atomic_helper_wait_for_dependencies(struct drm_atomic_state *old_state)
 }
 EXPORT_SYMBOL(drm_atomic_helper_wait_for_dependencies);
 
+/**
+ * drm_atomic_helper_fake_vblank - fake VBLANK events if needed
+ * @old_state: atomic state object with old state structures
+ *
+ * This function walks all CRTCs and fake VBLANK events on those with
+ * &drm_crtc_state.no_vblank set to true and &drm_crtc_state.event != NULL.
+ * The primary use of this function is writeback connectors working in oneshot
+ * mode and faking VBLANK events. In this case they only fake the VBLANK event
+ * when a job is queued, and any change to the pipeline that does not touch the
+ * connector is leading to timeouts when calling
+ * drm_atomic_helper_wait_for_vblanks() or
+ * drm_atomic_helper_wait_for_flip_done().
+ *
+ * This is part of the atomic helper support for nonblocking commits, see
+ * drm_atomic_helper_setup_commit() for an overview.
+ */
+void drm_atomic_helper_fake_vblank(struct drm_atomic_state *old_state)
+{
+	struct drm_crtc_state *new_crtc_state;
+	struct drm_crtc *crtc;
+	int i;
+
+	for_each_new_crtc_in_state(old_state, crtc, new_crtc_state, i) {
+		unsigned long flags;
+
+		if (!new_crtc_state->no_vblank)
+			continue;
+
+		spin_lock_irqsave(&old_state->dev->event_lock, flags);
+		if (new_crtc_state->event) {
+			drm_crtc_send_vblank_event(crtc,
+						   new_crtc_state->event);
+			new_crtc_state->event = NULL;
+		}
+		spin_unlock_irqrestore(&old_state->dev->event_lock, flags);
+	}
+}
+EXPORT_SYMBOL(drm_atomic_helper_fake_vblank);
+
 /**
  * drm_atomic_helper_commit_hw_done - setup possible nonblocking commit
  * @old_state: atomic state object with old state structures

include/drm/drm_atomic_helper.h

@@ -100,6 +100,7 @@ int __must_check drm_atomic_helper_swap_state(struct drm_atomic_state *state,
 int drm_atomic_helper_setup_commit(struct drm_atomic_state *state,
 				   bool nonblock);
 void drm_atomic_helper_wait_for_dependencies(struct drm_atomic_state *state);
+void drm_atomic_helper_fake_vblank(struct drm_atomic_state *state);
 void drm_atomic_helper_commit_hw_done(struct drm_atomic_state *state);
 void drm_atomic_helper_commit_cleanup_done(struct drm_atomic_state *state);
 

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