[PATCH v4] 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 expecting
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.

In addition, VIRTGPU_EXECBUF_RING_IDX is checked when creating a
DRM event object.  VIRTGPU_CONTEXT_PARAM_POLL_RINGS_MASK is
already defined in terms of per-context rings.  It was theoretically
possible to create a DRM event on the global timeline (ring_idx == 0),
if the context enabled DRM event polling.  However, that wouldn't
work and userspace (Sommelier).  Explicitly disallow it for
clarity.

Signed-off-by: Gurchetan Singh <gurchetansingh at chromium.org>
---
 v2: Fix indent (Dmitry)
 v3: Refactor drm fence event checks to avoid possible NULL deref (Dmitry)
 v4: More detailed commit message about addition drm fence event checks (Dmitry)

 drivers/gpu/drm/virtio/virtgpu_submit.c | 28 +++++++++++++------------
 1 file changed, 15 insertions(+), 13 deletions(-)

diff --git a/drivers/gpu/drm/virtio/virtgpu_submit.c b/drivers/gpu/drm/virtio/virtgpu_submit.c
index cf3c04b16a7a..004364cf86d7 100644
--- a/drivers/gpu/drm/virtio/virtgpu_submit.c
+++ b/drivers/gpu/drm/virtio/virtgpu_submit.c
@@ -64,13 +64,9 @@ static int virtio_gpu_fence_event_create(struct drm_device *dev,
 					 struct virtio_gpu_fence *fence,
 					 u32 ring_idx)
 {
-	struct virtio_gpu_fpriv *vfpriv = file->driver_priv;
 	struct virtio_gpu_fence_event *e = NULL;
 	int ret;
 
-	if (!(vfpriv->ring_idx_mask & BIT_ULL(ring_idx)))
-		return 0;
-
 	e = kzalloc(sizeof(*e), GFP_KERNEL);
 	if (!e)
 		return -ENOMEM;
@@ -161,21 +157,27 @@ static int virtio_gpu_init_submit(struct virtio_gpu_submit *submit,
 				  struct drm_file *file,
 				  u64 fence_ctx, u32 ring_idx)
 {
+	int err;
+	struct virtio_gpu_fence *out_fence;
 	struct virtio_gpu_fpriv *vfpriv = file->driver_priv;
 	struct virtio_gpu_device *vgdev = dev->dev_private;
-	struct virtio_gpu_fence *out_fence;
-	int err;
+	bool drm_fence_event = (exbuf->flags & VIRTGPU_EXECBUF_RING_IDX) &&
+			       (vfpriv->ring_idx_mask & BIT_ULL(ring_idx));
 
 	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) || drm_fence_event ||
+	     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) {
-		dma_fence_put(&out_fence->f);
-		return err;
+	if (drm_fence_event) {
+		err = virtio_gpu_fence_event_create(dev, file, out_fence, ring_idx);
+		if (err) {
+			dma_fence_put(&out_fence->f);
+			return err;
+		}
 	}
 
 	submit->out_fence = out_fence;
-- 
2.41.0.255.g8b1d071c50-goog