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.

Cc: 1882851@bugs.launchpad.net
Fixes: 3954ff10e06e ("drm/virtio: skip set_scanout if framebuffer didn't change")
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
---
 drivers/gpu/drm/virtio/virtgpu_drv.h     | 1 +
 drivers/gpu/drm/virtio/virtgpu_display.c | 1 +
 drivers/gpu/drm/virtio/virtgpu_plane.c   | 4 +++-
 3 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/virtio/virtgpu_drv.h b/drivers/gpu/drm/virtio/virtgpu_drv.h
index 9ff9f4ac0522..7b0c319f23c9 100644
--- a/drivers/gpu/drm/virtio/virtgpu_drv.h
+++ b/drivers/gpu/drm/virtio/virtgpu_drv.h

@@ -138,6 +138,7 @@ struct virtio_gpu_output {
 	int cur_x;
 	int cur_y;
 	bool enabled;
+	bool need_update;
 };
 #define drm_crtc_to_virtio_gpu_output(x) \
 	container_of(x, struct virtio_gpu_output, crtc)

diff --git a/drivers/gpu/drm/virtio/virtgpu_display.c b/drivers/gpu/drm/virtio/virtgpu_display.c
index cc7fd957a307..378be5956b30 100644
--- a/drivers/gpu/drm/virtio/virtgpu_display.c
+++ b/drivers/gpu/drm/virtio/virtgpu_display.c

@@ -100,6 +100,7 @@ static void virtio_gpu_crtc_atomic_enable(struct drm_crtc *crtc,
 	struct virtio_gpu_output *output = drm_crtc_to_virtio_gpu_output(crtc);
 
 	output->enabled = true;
+	output->need_update = true;
 }
 
 static void virtio_gpu_crtc_atomic_disable(struct drm_crtc *crtc,

diff --git a/drivers/gpu/drm/virtio/virtgpu_plane.c b/drivers/gpu/drm/virtio/virtgpu_plane.c
index 52d24179bcec..5948031a9ce8 100644
--- a/drivers/gpu/drm/virtio/virtgpu_plane.c
+++ b/drivers/gpu/drm/virtio/virtgpu_plane.c

@@ -163,7 +163,8 @@ static void virtio_gpu_primary_plane_update(struct drm_plane *plane,
 	    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->need_update) {

 		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,

@@ -178,6 +179,7 @@ static void virtio_gpu_primary_plane_update(struct drm_plane *plane,
 					   plane->state->src_h >> 16,
 					   plane->state->src_x >> 16,
 					   plane->state->src_y >> 16);
+		output->need_update = false;
 	}
 
 	virtio_gpu_cmd_resource_flush(vgdev, bo->hw_res_handle,

-- 
2.18.4