feat(hal): add render_start_cb to disp_drv (#3274)

* feat(hal): add render_start_cb to disp_drv

* update doc for render_start_cb

* Update docs/porting/display.md

* remove disp_refr parameter

Co-authored-by: pengyiqiang <pengyiqiang@xiaomi.com>
Co-authored-by: Gabor Kiss-Vamosi <kisvegabor@gmail.com>

docs/porting/display.md

@@ -109,6 +109,7 @@ It can be used if the display controller can refresh only areas with specific he
 This way the buffers used in `lv_disp_draw_buf_t` can be smaller to hold only the required number of bits for the given area size. Note that rendering with `set_px_cb` is slower than normal rendering.
 - `monitor_cb` A callback function that tells how many pixels were refreshed and in how much time. Called when the last chunk is rendered and sent to the display.
 - `clean_dcache_cb` A callback for cleaning any caches related to the display.
+- `render_start_cb` A callback function that notifies the display driver that rendering has started. It also could be used to wait for VSYNC to start rendering. It's useful if rendering is faster than a VSYNC period.
 
 LVGL has built-in support to several GPUs (see `lv_conf.h`) but if something else is required these functions can be used to make LVGL use a GPU:
 - `gpu_fill_cb` fill an area in the memory with a color.

src/core/lv_refr.c

@@ -519,6 +519,11 @@ static void lv_refr_areas(void)
         }
     }
 
+    /*Notify the display driven rendering has started*/
+    if(disp_refr->driver->render_start_cb) {
+        disp_refr->driver->render_start_cb(disp_refr->driver);
+    }
+
     disp_refr->driver->draw_buf->last_area = 0;
     disp_refr->driver->draw_buf->last_part = 0;
     disp_refr->rendering_in_progress = true;

src/hal/lv_hal_disp.h

@@ -133,6 +133,9 @@ typedef struct _lv_disp_drv_t {
     /** OPTIONAL: called when driver parameters are updated */
     void (*drv_update_cb)(struct _lv_disp_drv_t * disp_drv);
 
+    /** OPTIONAL: called when start rendering */
+    void (*render_start_cb)(struct _lv_disp_drv_t * disp_drv);
+
     /** On CHROMA_KEYED images this color will be transparent.
      * `LV_COLOR_CHROMA_KEY` by default. (lv_conf.h)*/
     lv_color_t color_chroma_key;