[Bug 1882851] [PATCH 1/2] drm/virtio: fix unblank

[Gerd Hoffmann <1882851@bugs.launchpad.net>]

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: 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 | 11 +++++++++++
 drivers/gpu/drm/virtio/virtgpu_plane.c   |  4 +++-
 3 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/virtio/virtgpu_drv.h b/drivers/gpu/drm/virtio/virtgpu_drv.h
index 9ff9f4ac0522..4ab1b0ba2925 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 needs_modeset;
 };
 #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 2c2742b8d657..6c26b41f4e0d 100644
--- a/drivers/gpu/drm/virtio/virtgpu_display.c
+++ b/drivers/gpu/drm/virtio/virtgpu_display.c
@@ -123,6 +123,17 @@ static int virtio_gpu_crtc_atomic_check(struct drm_crtc *crtc,
 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
+	 * independant 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 = {
diff --git a/drivers/gpu/drm/virtio/virtgpu_plane.c b/drivers/gpu/drm/virtio/virtgpu_plane.c
index 52d24179bcec..65757409d9ed 100644
--- a/drivers/gpu/drm/virtio/virtgpu_plane.c
+++ b/drivers/gpu/drm/virtio/virtgpu_plane.c
@@ -163,7 +163,9 @@ 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->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,
-- 
2.18.4

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https://bugs.launchpad.net/bugs/1882851

Title:
  QEMU video freezes with "Guest disabled display" (virtio driver)

Status in QEMU:
  New

Bug description:
  I am using Arch Linux as my Guest and Host OS, after starting qemu
  with the following command:

    $ qemu-system-x86_64 -enable-kvm -hda arch-zoom.qcow2 -m 4G -vga
  virtio

  and waiting for a screen blank, I get this message:

    Guest disabled display

  And nothing happens after that, I can move the mouse or hit any key,
  and the message is still there.

  I can still reboot the VM but that's not optimal.

  I can reproduce this with the latest QEMU release (5.0.0) or git master, 
  I also tried this with older releases (4.0.0, 3.0.0) and the issue is still there.

  I can't reproduce this with other video drivers (std, qxl).

  With std/qxl the screen will blank a bit and then continue as normal.

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