[PATCH v2] drm/virtio: Fix handling CONFIG_DRM_VIRTIO_GPU_KMS option

[Dmitry Osipenko <dmitry.osipenko@collabora.com>]

VirtIO-GPU got a new config option for disabling KMS. There were two
problems left unnoticed during review when the new option was added:

1. The IS_ENABLED(CONFIG_DRM_VIRTIO_GPU_KMS) check in the code was
inverted, hence KMS was disabled when it should be enabled and vice versa.

2. The disabled KMS crashed kernel with a NULL dereference in
drm_kms_helper_hotplug_event(), which shall not be invoked with a
disabled KMS.

Fix the inverted config option check in the code and skip handling the
VIRTIO_GPU_EVENT_DISPLAY sent by host when KMS is disabled in guest to fix
the crash.

Fixes: 72122c69d717 ("drm/virtio: Add option to disable KMS support")
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
---

Changelog:

v2: - Moved the "has_edid" under the "num_scanouts" condition, like was
      suggested by Gerd Hoffmann.

 drivers/gpu/drm/virtio/virtgpu_kms.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/virtio/virtgpu_kms.c b/drivers/gpu/drm/virtio/virtgpu_kms.c
index 874ad6c2621a..15f2519988e7 100644
--- a/drivers/gpu/drm/virtio/virtgpu_kms.c
+++ b/drivers/gpu/drm/virtio/virtgpu_kms.c
@@ -43,11 +43,13 @@ static void virtio_gpu_config_changed_work_func(struct work_struct *work)
 	virtio_cread_le(vgdev->vdev, struct virtio_gpu_config,
 			events_read, &events_read);
 	if (events_read & VIRTIO_GPU_EVENT_DISPLAY) {
-		if (vgdev->has_edid)
-			virtio_gpu_cmd_get_edids(vgdev);
-		virtio_gpu_cmd_get_display_info(vgdev);
-		virtio_gpu_notify(vgdev);
-		drm_helper_hpd_irq_event(vgdev->ddev);
+		if (vgdev->num_scanouts) {
+			if (vgdev->has_edid)
+				virtio_gpu_cmd_get_edids(vgdev);
+			virtio_gpu_cmd_get_display_info(vgdev);
+			virtio_gpu_notify(vgdev);
+			drm_helper_hpd_irq_event(vgdev->ddev);
+		}
 		events_clear |= VIRTIO_GPU_EVENT_DISPLAY;
 	}
 	virtio_cwrite_le(vgdev->vdev, struct virtio_gpu_config,
@@ -224,7 +226,7 @@ int virtio_gpu_init(struct virtio_device *vdev, struct drm_device *dev)
 	vgdev->num_scanouts = min_t(uint32_t, num_scanouts,
 				    VIRTIO_GPU_MAX_SCANOUTS);
 
-	if (IS_ENABLED(CONFIG_DRM_VIRTIO_GPU_KMS) || !vgdev->num_scanouts) {
+	if (!IS_ENABLED(CONFIG_DRM_VIRTIO_GPU_KMS) || !vgdev->num_scanouts) {
 		DRM_INFO("KMS disabled\n");
 		vgdev->num_scanouts = 0;
 		vgdev->has_edid = false;
-- 
2.39.2