sna/present: Skip the unflip if a no-op

If the screen is already scanning out from the desired framebuffer
(because we failed when flipping and already restored the mode), skip
the unflip.

References: https://bugs.freedesktop.org/show_bug.cgi?id=94829
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

src/sna/sna.h

@@ -467,6 +467,7 @@ extern bool sna_cursors_init(ScreenPtr screen, struct sna *sna);
 typedef void (*sna_flip_handler_t)(struct drm_event_vblank *e,
 				   void *data);
 
+extern bool sna_needs_page_flip(struct sna *sna, struct kgem_bo *bo);
 extern int sna_page_flip(struct sna *sna,
 			 struct kgem_bo *bo,
 			 sna_flip_handler_t handler,

src/sna/sna_display.c

@@ -6396,6 +6396,26 @@ static void sna_mode_restore(struct sna *sna)
 			   "Failed to restore display configuration\n");
 }
 
+bool sna_needs_page_flip(struct sna *sna, struct kgem_bo *bo)
+{
+	xf86CrtcConfigPtr config = XF86_CRTC_CONFIG_PTR(sna->scrn);
+	int i;
+
+	for (i = 0; i < sna->mode.num_real_crtc; i++) {
+		struct sna_crtc *crtc = config->crtc[i]->driver_private;
+
+		if (crtc->bo == NULL)
+			continue;
+
+		if (crtc->bo == bo)
+			continue;
+
+		return true;
+	}
+
+	return false;
+}
+
 int
 sna_page_flip(struct sna *sna,
 	      struct kgem_bo *bo,

src/sna/sna_present.c

@@ -830,6 +830,10 @@ reset_mode:
 
 	assert(sna_pixmap(screen->GetScreenPixmap(screen))->pinned & PIN_SCANOUT);
 
+	/* Are we unflipping after a failure that left our ScreenP in place? */
+	if (!sna_needs_page_flip(sna, bo))
+		goto notify;
+
 	if (sna->flags & SNA_HAS_ASYNC_FLIP) {
 		DBG(("%s: trying async flip restore\n", __FUNCTION__));
 		if (flip__async(sna, NULL, event_id, 0, bo))