drm/exynos: added comments and code clean.

this patch adds the following comments and code clean.
- add comment of exynos_drm_crtc_apply() call at page flip time.
- add comment that when exynos_drm_fbdev_reinit() is called,
  why num_connector is 0 and also the framebuffers should be destroyed.
- remove buf_off member from struct exynos_drm_overlay because this member
  isn't used anymore.

Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/exynos/exynos_drm_crtc.c

@@ -279,6 +279,12 @@ static int exynos_drm_crtc_page_flip(struct drm_crtc *crtc,
 			goto out;
 		}
 
+		/*
+		 * the values related to a buffer of the drm framebuffer
+		 * to be applied should be set at here. because these values
+		 * first, is set to shadow registers and then to
+		 * real registers at vsync front porch period.
+		 */
 		exynos_drm_crtc_apply(crtc);
 
 		dev_priv->pageflip_event = true;

drivers/gpu/drm/exynos/exynos_drm_drv.h

@@ -82,7 +82,6 @@ struct exynos_drm_overlay_ops {
  * @paddr: bus(accessed by dma) physical memory address to this overlay
  *		and this is physically continuous.
  * @vaddr: virtual memory addresss to this overlay.
- * @buf_off: start offset of framebuffer to be displayed.
  * @default_win: a window to be enabled.
  * @color_key: color key on or off.
  * @index_color: if using color key feature then this value would be used
@@ -111,7 +110,6 @@ struct exynos_drm_overlay {
 	unsigned int pitch;
 	dma_addr_t paddr;
 	void __iomem *vaddr;
-	unsigned int buf_off;
 
 	bool default_win;
 	bool color_key;

drivers/gpu/drm/exynos/exynos_drm_fbdev.c

@@ -393,6 +393,10 @@ int exynos_drm_fbdev_reinit(struct drm_device *dev)
 	if (!private)
 		return -EINVAL;
 
+	/*
+	 * if all sub drivers were unloaded then num_connector is 0
+	 * so at this time, the framebuffers also should be destroyed.
+	 */
 	if (!dev->mode_config.num_connector) {
 		exynos_drm_fbdev_fini(dev);
 		return 0;
@@ -429,7 +433,7 @@ int exynos_drm_fbdev_reinit(struct drm_device *dev)
 		 * re-configure the fb helper. it means that this function
 		 * has been called by the specific drivers.
 		 */
-		return exynos_drm_fbdev_init(dev);
+		ret = exynos_drm_fbdev_init(dev);
 	}
 
 	return ret;