drm/virtgpu api: define a dummy fence signaled event

The current virtgpu implementation of poll(..) drops events
when VIRTGPU_CONTEXT_PARAM_POLL_RINGS_MASK is enabled (otherwise
it's like a normal DRM driver).

This is because paravirtualized userspaces receives responses in a
buffer of type BLOB_MEM_GUEST, not by read(..).

To be in line with other DRM drivers and avoid specialized behavior,
it is possible to define a dummy event for virtgpu. Paravirtualized
userspace will now have to call read(..) on the DRM fd to receive the
dummy event.

Fixes: b10790434cf2 ("drm/virtgpu api: create context init feature")
Reported-by: Daniel Vetter <[e-mail]>
Signed-off-by: Gurchetan Singh <[e-mail]>
Reviewed-by: Daniel Vetter <[e-mail]>
Link: http://patchwork.freedesktop.org/patch/msgid/[e-mail]
Signed-off-by: Gerd Hoffmann <[e-mail]>

drivers/gpu/drm/virtio/virtgpu_drv.h

···
 	spinlock_t       lock;
 };
 
-#define VIRTGPU_EVENT_FENCE_SIGNALED_INTERNAL 0x10000000
 struct virtio_gpu_fence_event {
 	struct drm_pending_event base;
 	struct drm_event event;

drivers/gpu/drm/virtio/virtgpu_ioctl.c

···
 	if (!e)
 		return -ENOMEM;
 
-	e->event.type = VIRTGPU_EVENT_FENCE_SIGNALED_INTERNAL;
+	e->event.type = VIRTGPU_EVENT_FENCE_SIGNALED;
 	e->event.length = sizeof(e->event);
 
 	ret = drm_event_reserve_init(dev, file, &e->base, &e->event);

include/uapi/drm/virtgpu_drm.h

···
 	__u64 ctx_set_params;
 };
 
+/*
+ * Event code that's given when VIRTGPU_CONTEXT_PARAM_POLL_RINGS_MASK is in
+ * effect.  The event size is sizeof(drm_event), since there is no additional
+ * payload.
+ */
+#define VIRTGPU_EVENT_FENCE_SIGNALED 0x90000000
+
 #define DRM_IOCTL_VIRTGPU_MAP \
 	DRM_IOWR(DRM_COMMAND_BASE + DRM_VIRTGPU_MAP, struct drm_virtgpu_map)