epicardiumy: fix epicardium_disp_framebuffer API

epicardium/epicardium.h

@@ -283,12 +283,13 @@ struct disp_framebuffer {
    *
    * .. code-block:: cpp
    *
-   * 	struct disp_framebuffer *fb = epic_disp_framebuffer();
+   * 	struct disp_framebuffer **fb;
+   * 	epic_disp_framebuffer(fb);
    * 	uint16_t red = 0b1111100000000000;
    * 	for (int y = 0; y < DISP_HEIGHT; y++) {
    * 		for (int x = 0; x < DISP_WIDTH; x++) {
-   * 			fb->fb[y][x][0] = red >> 8;
-   * 			fb->fb[y][x][1] = red & 0xFF;
+   * 			(*fb)->fb[y][x][0] = red >> 8;
+   * 			(*fb)->fb[y][x][1] = red & 0xFF;
    * 		}
    * 	}
    * 	epic_disp_update();
@@ -318,6 +319,10 @@ API(API_DISP_CLOSE, int epic_disp_close());
 /**
  * Causes the changes that have been written to the framebuffer
  * to be shown on the display
+ * :return: ``0`` on success or a negative value in case of an error:
+ *
+ *    - ``-EBUSY``: Display was already locked from another task.
+ *    - ``-EAGAIN``: Display is currently being updated.
  */
 API(API_DISP_UPDATE, int epic_disp_update());
 /**
@@ -445,9 +450,12 @@ API(API_DISP_CIRC,
  * Returns the back framebuffer (the display buffer that's currently inactive
  * that will be shown after the next call to ``epic_disp_update``.
  *
- * :returns: pointer to a framebuffer
+ * :param fb: output: current framebuffer
+ * :return: ``0`` on success or negative value in case of an error:
+ *
+ *    - ``-EBUSY``: Display was already locked from another task.
  */
-API(API_DISP_FRAMEBUFFER, struct disp_framebuffer *epic_disp_framebuffer());
+API(API_DISP_FRAMEBUFFER, int epic_disp_framebuffer(struct disp_framebuffer **fb));
 
 
 /**

epicardium/modules/display.c

@@ -119,15 +119,23 @@ int epic_disp_update()
 	int cl = check_lock();
 	if (cl < 0) {
 		return cl;
-	} else {
-		LCD_Update();
-		return 0;
 	}
+
+	if (LCD_Update()) {
+		return -EAGAIN;
+	}
+	return 0;
 }
 
-struct disp_framebuffer *epic_disp_framebuffer()
+int epic_disp_framebuffer(struct disp_framebuffer **fb)
 {
-	return (struct disp_framebuffer *)LCD_Framebuffer();
+	int cl = check_lock();
+	if (cl < 0) {
+		return cl;
+	}
+
+	*fb = (struct disp_framebuffer *)LCD_Framebuffer();
+	return 0;
 }
 
 int epic_disp_open()