hwrng: virtio - always add a pending request

If we ensure we have already some data available by enqueuing
again the buffer once data are exhausted, we can return what we
have without waiting for the device answer.

Signed-off-by: Laurent Vivier <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Michael S. Tsirkin <[e-mail]>

drivers/char/hw_random/virtio-rng.c

···
 	struct virtqueue *vq;
 	char name[25];
 	int index;
-	bool busy;
 	bool hwrng_register_done;
 	bool hwrng_removed;
 	/* data transfer */
···
 		return;
 
 	vi->data_idx = 0;
-	vi->busy = false;
 
 	complete(&vi->have_data);
 }
 
-/* The host will fill any buffer we give it with sweet, sweet randomness. */
-static void register_buffer(struct virtrng_info *vi)
+static void request_entropy(struct virtrng_info *vi)
 {
 	struct scatterlist sg;
+
+	reinit_completion(&vi->have_data);
+	vi->data_avail = 0;
+	vi->data_idx = 0;
 
 	sg_init_one(&sg, vi->data, sizeof(vi->data));
 
···
 	memcpy(buf, vi->data + vi->data_idx, size);
 	vi->data_idx += size;
 	vi->data_avail -= size;
+	if (vi->data_avail == 0)
+		request_entropy(vi);
 	return size;
 }
 
···
 	 * so either size is 0 or data_avail is 0
 	 */
 	while (size != 0) {
-		/* data_avail is 0 */
-		if (!vi->busy) {
-			/* no pending request, ask for more */
-			vi->busy = true;
-			reinit_completion(&vi->have_data);
-			register_buffer(vi);
-		}
+		/* data_avail is 0 but a request is pending */
 		ret = wait_for_completion_killable(&vi->have_data);
 		if (ret < 0)
 			return ret;
···
 {
 	struct virtrng_info *vi = (struct virtrng_info *)rng->priv;
 
-	if (vi->busy)
-		complete(&vi->have_data);
+	complete(&vi->have_data);
 }
 
 static int probe_common(struct virtio_device *vdev)
···
 		goto err_find;
 	}
 
+	/* we always have a pending entropy request */
+	request_entropy(vi);
+
 	return 0;
 
 err_find:
···
 	vi->data_idx = 0;
 	complete(&vi->have_data);
 	vdev->config->reset(vdev);
-	vi->busy = false;
 	if (vi->hwrng_register_done)
 		hwrng_unregister(&vi->hwrng);
 	vdev->config->del_vqs(vdev);