drm/sun4i: engine: Create an atomic_begin callback

We have to implement some display engine specific behaviours in
atomic_begin. Let's add a function for that.
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/44110951ae0cc13767fefc7fc1d9e2ec782d0a40.1516613040.git-series.maxime.ripard@free-electrons.com

drivers/gpu/drm/sun4i/sun4i_crtc.c

@@ -64,6 +64,7 @@ static void sun4i_crtc_atomic_begin(struct drm_crtc *crtc,
 {
 	struct sun4i_crtc *scrtc = drm_crtc_to_sun4i_crtc(crtc);
 	struct drm_device *dev = crtc->dev;
+	struct sunxi_engine *engine = scrtc->engine;
 	unsigned long flags;
 
 	if (crtc->state->event) {
@@ -73,7 +74,10 @@ static void sun4i_crtc_atomic_begin(struct drm_crtc *crtc,
 		scrtc->event = crtc->state->event;
 		spin_unlock_irqrestore(&dev->event_lock, flags);
 		crtc->state->event = NULL;
-	 }
+	}
+
+	if (engine->ops->atomic_begin)
+		engine->ops->atomic_begin(engine, old_state);
 }
 
 static void sun4i_crtc_atomic_flush(struct drm_crtc *crtc,

drivers/gpu/drm/sun4i/sunxi_engine.h

@@ -22,6 +22,19 @@ struct sunxi_engine;
  * implement the proper behaviour.
  */
 struct sunxi_engine_ops {
+	/**
+	 * @atomic_begin:
+	 *
+	 * This callback allows to prepare our engine for an atomic
+	 * update. This is mirroring the
+	 * &drm_crtc_helper_funcs.atomic_begin callback, so any
+	 * documentation there applies.
+	 *
+	 * This function is optional.
+	 */
+	void (*atomic_begin)(struct sunxi_engine *engine,
+			     struct drm_crtc_state *old_state);
+
 	/**
 	 * @atomic_check:
 	 *