[PATCH V10 14/19] virtio_ring: determine descriptor flags at one time

[Jason Wang <jasowang@redhat.com>]

Let's determine the last descriptor by counting the number of sg. This
would be consistent with packed virtqueue implementation and ease the
future in-order implementation.

Acked-by: Eugenio Pérez <eperezma@redhat.com>
Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
---
 drivers/virtio/virtio_ring.c | 28 +++++++++++++---------------
 1 file changed, 13 insertions(+), 15 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 4e905dc7b6cb..dbcd6f26df7d 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -575,7 +575,7 @@ static inline int virtqueue_add_split(struct vring_virtqueue *vq,
 	struct vring_desc_extra *extra;
 	struct scatterlist *sg;
 	struct vring_desc *desc;
-	unsigned int i, n, avail, descs_used, prev, err_idx;
+	unsigned int i, n, avail, descs_used, err_idx, sg_count = 0;
 	int head;
 	bool indirect;
 
@@ -635,42 +635,40 @@ static inline int virtqueue_add_split(struct vring_virtqueue *vq,
 		for (sg = sgs[n]; sg; sg = sg_next(sg)) {
 			dma_addr_t addr;
 			u32 len;
+			u16 flags = 0;
+
+			if (++sg_count != total_sg)
+				flags |= VRING_DESC_F_NEXT;
 
 			if (vring_map_one_sg(vq, sg, DMA_TO_DEVICE, &addr, &len, premapped))
 				goto unmap_release;
 
-			prev = i;
 			/* Note that we trust indirect descriptor
 			 * table since it use stream DMA mapping.
 			 */
-			i = virtqueue_add_desc_split(vq, desc, extra, i, addr, len,
-						     VRING_DESC_F_NEXT,
-						     premapped);
+			i = virtqueue_add_desc_split(vq, desc, extra, i, addr,
+						     len, flags ,premapped);
 		}
 	}
 	for (; n < (out_sgs + in_sgs); n++) {
 		for (sg = sgs[n]; sg; sg = sg_next(sg)) {
 			dma_addr_t addr;
 			u32 len;
+			u16 flags = VRING_DESC_F_WRITE;
+
+			if (++sg_count != total_sg)
+				flags |= VRING_DESC_F_NEXT;
 
 			if (vring_map_one_sg(vq, sg, DMA_FROM_DEVICE, &addr, &len, premapped))
 				goto unmap_release;
 
-			prev = i;
 			/* Note that we trust indirect descriptor
 			 * table since it use stream DMA mapping.
 			 */
-			i = virtqueue_add_desc_split(vq, desc, extra, i, addr, len,
-						     VRING_DESC_F_NEXT |
-						     VRING_DESC_F_WRITE,
-						     premapped);
+			i = virtqueue_add_desc_split(vq, desc, extra, i, addr,
+						     len, flags, premapped);
 		}
 	}
-	/* Last one doesn't continue. */
-	desc[prev].flags &= cpu_to_virtio16(vq->vq.vdev, ~VRING_DESC_F_NEXT);
-	if (!indirect && vring_need_unmap_buffer(vq, &extra[prev]))
-		vq->split.desc_extra[prev & (vq->split.vring.num - 1)].flags &=
-			~VRING_DESC_F_NEXT;
 
 	if (indirect) {
 		/* Now that the indirect table is filled in, map it. */
-- 
2.31.1