[PATCH v2] drm/virtio: conditionally allocate virtio_gpu_fence

[Gurchetan Singh]

We don't want to create a fence for every command submission.  It's
only necessary when userspace provides a waitable token for submission.
This could be:

1) bo_handles, to be used with VIRTGPU_WAIT
2) out_fence_fd, to be used with dma_fence apis
3) a ring_idx provided with VIRTGPU_CONTEXT_PARAM_POLL_RINGS_MASK
   + DRM event API
4) syncobjs in the future

The use case for just submitting a command to the host, and expected
no response.  For example, gfxstream has GFXSTREAM_CONTEXT_PING that
just wakes up the host side worker threads.  There's also
CROSS_DOMAIN_CMD_SEND which just sends data to the Wayland server.

This prevents the need to signal the automatically created
virtio_gpu_fence.

Signed-off-by: Gurchetan Singh <gurchetansingh at chromium.org>
Reviewed-by: <dmitry.osipenko at collabora.com>
---
 v2: Fix indent (Dmitry)

 drivers/gpu/drm/virtio/virtgpu_submit.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/virtio/virtgpu_submit.c b/drivers/gpu/drm/virtio/virtgpu_submit.c
index cf3c04b16a7a..8c7e15c31164 100644
--- a/drivers/gpu/drm/virtio/virtgpu_submit.c
+++ b/drivers/gpu/drm/virtio/virtgpu_submit.c
@@ -168,9 +168,13 @@ static int virtio_gpu_init_submit(struct virtio_gpu_submit *submit,
 
 	memset(submit, 0, sizeof(*submit));
 
-	out_fence = virtio_gpu_fence_alloc(vgdev, fence_ctx, ring_idx);
-	if (!out_fence)
-		return -ENOMEM;
+	if ((exbuf->flags & VIRTGPU_EXECBUF_FENCE_FD_OUT) ||
+	    ((exbuf->flags & VIRTGPU_EXECBUF_RING_IDX) &&
+	    (vfpriv->ring_idx_mask & BIT_ULL(ring_idx))) ||
+	    exbuf->num_bo_handles)
+		out_fence = virtio_gpu_fence_alloc(vgdev, fence_ctx, ring_idx);
+	else
+		out_fence = NULL;
 
 	err = virtio_gpu_fence_event_create(dev, file, out_fence, ring_idx);
 	if (err) {
-- 
2.41.0.162.gfafddb0af9-goog