drm/sun4i: backend: Document the engine operations

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);

};