On Mon, Oct 20, 2025 at 03:09:58PM +0800, Jason Wang wrote:
> 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 <[email protected]>
> Reviewed-by: Xuan Zhuo <[email protected]>
> Signed-off-by: Jason Wang <[email protected]>
> ---
> drivers/virtio/virtio_ring.c | 19 ++++++-------------
> 1 file changed, 6 insertions(+), 13 deletions(-)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 37b16ef906a4..20bc48b1241e 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, c = 0;
> int head;
> bool indirect;
>
c is not a great variable name. Maybe sg_count?
same in patch 19 actually.
> @@ -639,12 +639,11 @@ static inline int virtqueue_add_split(struct
> vring_virtqueue *vq,
> 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,
> + ++c == total_sg ? 0 :
> VRING_DESC_F_NEXT,
> premapped);
> }
> }
> @@ -656,21 +655,15 @@ static inline int virtqueue_add_split(struct
> vring_virtqueue *vq,
> 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,
> + (++c == total_sg ? 0 : VRING_DESC_F_NEXT) |
> + VRING_DESC_F_WRITE, premapped);
this continuation line should be indented more,
and maybe premapped on a line by itself.
Alternatively use a variable for flags.
> }
> }
> - /* 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
