drm/sun4i: backend: Document the engine operations

Our operations were missing some documentation to explain what was expected
from them.

Let's make that clearer.
Reviewed-by: NChen-Yu Tsai <wens@csie.org>
Signed-off-by: NMaxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/fdcd8ec3ae9ecd73ef089ede5218d3a41b49be05.1516613040.git-series.maxime.ripard@free-electrons.com

drivers/gpu/drm/sun4i/sunxi_engine.h

@@ -15,12 +15,58 @@ struct drm_device;
 
 struct sunxi_engine;
 
+/**
+ * struct sunxi_engine_ops - helper operations for sunXi engines
+ *
+ * These hooks are used by the common part of the DRM driver to
+ * implement the proper behaviour.
+ */
 struct sunxi_engine_ops {
+	/**
+	 * @commit:
+	 *
+	 * This callback will trigger the hardware switch to commit
+	 * the new configuration that has been setup during the next
+	 * vblank period.
+	 *
+	 * This function is optional.
+	 */
 	void (*commit)(struct sunxi_engine *engine);
+
+	/**
+	 * @layers_init:
+	 *
+	 * This callback is used to allocate, initialize and register
+	 * the layers supported by that engine.
+	 *
+	 * This function is mandatory.
+	 *
+	 * RETURNS:
+	 *
+	 * The array of struct drm_plane backing the layers, or an
+	 * error pointer on failure.
+	 */
 	struct drm_plane **(*layers_init)(struct drm_device *drm,
 					  struct sunxi_engine *engine);
 
+	/**
+	 * @apply_color_correction:
+	 *
+	 * This callback will enable the color correction in the
+	 * engine. This is useful only for the composite output.
+	 *
+	 * This function is optional.
+	 */
 	void (*apply_color_correction)(struct sunxi_engine *engine);
+
+	/**
+	 * @disable_color_correction:
+	 *
+	 * This callback will stop the color correction in the
+	 * engine. This is useful only for the composite output.
+	 *
+	 * This function is optional.
+	 */
 	void (*disable_color_correction)(struct sunxi_engine *engine);
 };