On Mon, Jun 16, 2025 at 04:25:12PM +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.
>
> Reviewed-by: Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
> ---
> drivers/virtio/virtio_ring.c | 25 +++++++++++++------------
> 1 file changed, 13 insertions(+), 12 deletions(-)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index af32d1a1a1db..d5e4d4cd2487 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -570,7 +570,7 @@ 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, c, avail, descs_used, err_idx;
> int head;
> bool indirect;
>
> @@ -626,46 +626,47 @@ static inline int virtqueue_add_split(struct vring_virtqueue *vq,
> return -ENOSPC;
> }
>
> + c = 0;
initialize at point of declaration?
> for (n = 0; n < out_sgs; n++) {
> + sg = sgs[n];
> for (sg = sgs[n]; sg; sg = sg_next(sg)) {
> dma_addr_t addr;
> u32 len;
> + u16 flags = 0;
>
> if (vring_map_one_sg(vq, sg, DMA_TO_DEVICE, &addr, &len, premapped))
> goto unmap_release;
>
> - prev = i;
> + if (++c != total_sg)
> + flags = VRING_DESC_F_NEXT;
> +
Don't like it how the logic is split.
flags isn't used before that.
So I prefer:
flags = ++c == total_sg ? 0 : VRING_DESC_F_NEXT;
and at that point, we do not really need flags anymore:
> /* 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,
> + flags,
So just:
i = virtqueue_add_desc_split(vq, desc, extra, i, addr, len,
++c == total_sg ? 0 : VRING_DESC_F_NEXT,
here and we are done.
> premapped);
> }
> }
> for (; n < (out_sgs + in_sgs); n++) {
> for (sg = sgs[n]; sg; sg = sg_next(sg)) {
> + u16 flags = VRING_DESC_F_WRITE;
> dma_addr_t addr;
> u32 len;
>
> if (vring_map_one_sg(vq, sg, DMA_FROM_DEVICE, &addr, &len, premapped))
> goto unmap_release;
>
> - prev = i;
> + if (++c != total_sg)
> + flags |= VRING_DESC_F_NEXT;
> +
Don't like it that above it's "=" here it is "|=".
And flags isn't used before that.
So I prefer:
flags = ++c == total_sg ? VRING_DESC_F_WRITE : VRING_DESC_F_WRITE | VRING_DESC_F_NEXT;
and again we don't really need the variable:
> /* 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,
so just:
i = virtqueue_add_desc_split(vq, desc, extra, i, addr, len,
(++c == total_sg ? 0 : VRING_DESC_F_NEXT) |
VRING_DESC_F_WRITE,
is clearer, and the patch smaller.
> - premapped);
> + 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. */
> --
> 2.34.1
