drm/vc4: Don't wait for vblank when updating the cursor

Commonly used desktop environments such as xfce4 and gnome
on debian sid can flood the graphics drivers with cursor
updates.  Because the current implementation is waiting
for a vblank between cursor updates, this will cause the
display to hang for a long time since a typical refresh
rate is only 60Hz.

This is unnecessary and unexpected by user mode software,
so simply swap out the cursor frame buffer without waiting.
Signed-off-by: Michael Zoran <mzoran@crowfest.net>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: http://patchwork.freedesktop.org/patch/msgid/20170224015431.24583-1-mzoran@crowfest.net

drivers/gpu/drm/vc4/vc4_plane.c

@@ -20,6 +20,7 @@
 
 #include "vc4_drv.h"
 #include "vc4_regs.h"
+#include "drm_atomic.h"
 #include "drm_atomic_helper.h"
 #include "drm_fb_cma_helper.h"
 #include "drm_plane_helper.h"
@@ -769,12 +770,6 @@ vc4_update_plane(struct drm_plane *plane,
 	if (!plane_state)
 		goto out;
 
-	/* If we're changing the cursor contents, do that in the
-	 * normal vblank-synced atomic path.
-	 */
-	if (fb != plane_state->fb)
-		goto out;
-
 	/* No configuring new scaling in the fast path. */
 	if (crtc_w != plane_state->crtc_w ||
 	    crtc_h != plane_state->crtc_h ||
@@ -783,6 +778,11 @@ vc4_update_plane(struct drm_plane *plane,
 		goto out;
 	}
 
+	if (fb != plane_state->fb) {
+		drm_atomic_set_fb_for_plane(plane->state, fb);
+		vc4_plane_async_set_fb(plane, fb);
+	}
+
 	/* Set the cursor's position on the screen.  This is the
 	 * expected change from the drm_mode_cursor_universal()
 	 * helper.