drm/simpledrm: Synchronize access to GEM BOs

Synchronize CPU access to GEM BOs with other drivers when updating the
screen buffer. Imported buffers might otherwise contain stale data.
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220922130944.27138-6-tzimmermann@suse.de

drivers/gpu/drm/tiny/simpledrm.c

@@ -480,11 +480,15 @@ static void simpledrm_primary_plane_helper_atomic_update(struct drm_plane *plane
 	struct simpledrm_device *sdev = simpledrm_device_of_dev(dev);
 	struct drm_atomic_helper_damage_iter iter;
 	struct drm_rect damage;
-	int idx;
+	int ret, idx;
 
-	if (!drm_dev_enter(dev, &idx))
+	ret = drm_gem_fb_begin_cpu_access(fb, DMA_FROM_DEVICE);
+	if (ret)
 		return;
 
+	if (!drm_dev_enter(dev, &idx))
+		goto out_drm_gem_fb_end_cpu_access;
+
 	drm_atomic_helper_damage_iter_init(&iter, old_plane_state, plane_state);
 	drm_atomic_for_each_plane_damage(&iter, &damage) {
 		struct iosys_map dst = IOSYS_MAP_INIT_VADDR(sdev->screen_base);
@@ -499,6 +503,8 @@ static void simpledrm_primary_plane_helper_atomic_update(struct drm_plane *plane
 	}
 
 	drm_dev_exit(idx);
+out_drm_gem_fb_end_cpu_access:
+	drm_gem_fb_end_cpu_access(fb, DMA_FROM_DEVICE);
 }
 
 static void simpledrm_primary_plane_helper_atomic_disable(struct drm_plane *plane,