drm/virtio: fix unblank

When going through a disable/enable cycle without changing the
framebuffer the optimization added by commit 3954ff10e06e ("drm/virtio:
skip set_scanout if framebuffer didn't change") causes the screen stay
blank. Add a bool to force an update to fix that.

v2: use drm_atomic_crtc_needs_modeset() (Daniel).

Cc: [e-mail]
Fixes: 3954ff10e06e ("drm/virtio: skip set_scanout if framebuffer didn't change")
Signed-off-by: Gerd Hoffmann <[e-mail]>
Tested-by: Jiri Slaby <[e-mail]>
Tested-by: Diego Viola <[e-mail]>
Acked-by: Daniel Vetter <[e-mail]>
Link: http://patchwork.freedesktop.org/patch/msgid/[e-mail]
(cherry picked from commit 1bc371cd0ec907bab870cacb6e898105f9c41dc8)

drivers/gpu/drm/virtio/virtgpu_display.c

···
 static void virtio_gpu_crtc_atomic_flush(struct drm_crtc *crtc,
 					 struct drm_crtc_state *old_state)
 {
+	struct virtio_gpu_output *output = drm_crtc_to_virtio_gpu_output(crtc);
+
+	/*
+	 * virtio-gpu can't do modeset and plane update operations
+	 * independent from each other.  So the actual modeset happens
+	 * in the plane update callback, and here we just check
+	 * whenever we must force the modeset.
+	 */
+	if (drm_atomic_crtc_needs_modeset(crtc->state)) {
+		output->needs_modeset = true;
+	}
 }
 
 static const struct drm_crtc_helper_funcs virtio_gpu_crtc_helper_funcs = {

drivers/gpu/drm/virtio/virtgpu_drv.h

···
 	int cur_x;
 	int cur_y;
 	bool enabled;
+	bool needs_modeset;
 };
 #define drm_crtc_to_virtio_gpu_output(x) \
 	container_of(x, struct virtio_gpu_output, crtc)

drivers/gpu/drm/virtio/virtgpu_plane.c

···
 	    plane->state->src_w != old_state->src_w ||
 	    plane->state->src_h != old_state->src_h ||
 	    plane->state->src_x != old_state->src_x ||
-	    plane->state->src_y != old_state->src_y) {
+	    plane->state->src_y != old_state->src_y ||
+	    output->needs_modeset) {
+		output->needs_modeset = false;
 		DRM_DEBUG("handle 0x%x, crtc %dx%d+%d+%d, src %dx%d+%d+%d\n",
 			  bo->hw_res_handle,
 			  plane->state->crtc_w, plane->state->crtc_h,