clients/dmabuf-intel: use three buffers

Use three buffers like simple-dmabuf-v4l instead of just two.

This is required, because when a frame callback arrives, the just
committed buffer is only on its way to the screen, while the previous
buffer is still being scanned out. It will take for the page flip to
complete, before the previous buffer is release. However, we want to be
able to repaint already at the frame callback, so three buffers can be
necessary.

This patch fixes weston-simple-dmabuf-intel to not abort with "Both
buffers busy at redraw()." when hardware overlays are used and the
surface gets directly scanned out.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Yong Bakos <ybakos@humanoriented.com>

clients/simple-dmabuf-intel.c

@@ -74,12 +74,14 @@ struct buffer {
 	unsigned long stride;
 };
 
+#define NUM_BUFFERS 3
+
 struct window {
 	struct display *display;
 	int width, height;
 	struct wl_surface *surface;
 	struct xdg_surface *xdg_surface;
-	struct buffer buffers[2];
+	struct buffer buffers[NUM_BUFFERS];
 	struct buffer *prev_buffer;
 	struct wl_callback *callback;
 };
@@ -341,7 +343,7 @@ create_window(struct display *display, int width, int height)
 		assert(0);
 	}
 
-	for (i = 0; i < 2; ++i) {
+	for (i = 0; i < NUM_BUFFERS; ++i) {
 		ret = create_dmabuf_buffer(display, &window->buffers[i],
 		                               width, height);
 
@@ -360,7 +362,7 @@ destroy_window(struct window *window)
 	if (window->callback)
 		wl_callback_destroy(window->callback);
 
-	for (i = 0; i < 2; i++) {
+	for (i = 0; i < NUM_BUFFERS; i++) {
 		if (!window->buffers[i].buffer)
 			continue;
 
@@ -379,16 +381,13 @@ destroy_window(struct window *window)
 static struct buffer *
 window_next_buffer(struct window *window)
 {
-	struct buffer *buffer;
+	int i;
 
-	if (!window->buffers[0].busy)
-		buffer = &window->buffers[0];
-	else if (!window->buffers[1].busy)
-		buffer = &window->buffers[1];
-	else
-		return NULL;
+	for (i = 0; i < NUM_BUFFERS; i++)
+		if (!window->buffers[i].busy)
+			return &window->buffers[i];
 
-	return buffer;
+	return NULL;
 }
 
 static const struct wl_callback_listener frame_listener;
@@ -403,7 +402,7 @@ redraw(void *data, struct wl_callback *callback, uint32_t time)
 	if (!buffer) {
 		fprintf(stderr,
 			!callback ? "Failed to create the first buffer.\n" :
-			"Both buffers busy at redraw(). Server bug?\n");
+			"All buffers busy at redraw(). Server bug?\n");
 		abort();
 	}