Re: Configuration of cq->cqe is lower than entries by 1

[Leon Romanovsky <leon-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>]

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On Mon, Nov 14, 2016 at 11:07:53AM +0000, Amrani, Ram wrote:
> Hi Leon, All,
> While inspecting MLX code as well as other vendors' I see that the actual number of cq->cqe is configured to be less by 1 than 'entries'. Why is that?

There is addition of 1 in mlx4_ib_create_cq():
192         entries      = roundup_pow_of_two(entries + 1);
193         cq->ibcq.cqe = entries - 1;

The same goes for mlx4_alloc_resize_buf, just earlier in the stack.

>
> e.g.
> struct ib_cq *mlx4_ib_create_cq(struct ib_device *ibdev,
>                                 const struct ib_cq_init_attr *attr,
>                                 struct ib_ucontext *context,
>                                 struct ib_udata *udata)
> {
>         ...
>         cq->ibcq.cqe = entries - 1;
>         ...
> }
>
>
> static int mlx4_alloc_resize_buf(struct mlx4_ib_dev *dev, struct mlx4_ib_cq *cq,
>                                   int entries)
> {
>         ...
>         cq->resize_buf->cqe = entries - 1;	// this is later copied to cq->ibcq.cqe
>         ...
> }
>
> Thanks,
> Ram
>

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