Re: [PATCH rdma-core] mlx5: Return pointer to CQ doorbell

[Jason Gunthorpe <jgunthorpe-ePGOBjL8dl3ta4EC/59zMFaTQe2KTcn/@public.gmane.org>]

On Thu, Aug 17, 2017 at 04:36:38PM +0300, Yishai Hadas wrote:
> From: Leon Romanovsky <leonro-VPRAkNaXOzVWk0Htik3J/w@public.gmane.org>
> 
> The returned UAR pointer is actually void ** and points to whole
> UAR database.
> 
> Because user is not supposed to access it and expected to use
> the CQ doorbell, we will return uar[0] (CQ doorbell) directly
> and eliminate the following logic in the applications:
> 
> uint64_t cq_db_reg = (uint64_t *)(((uint64_t)(uint64_t *)rxq->cq_uar) +
>                      MLX5_CQ_DOORBELL;

NAK, at least, as is..

This changes the ABI and mlx5dv is a public interface now, so you have
to do it properly.

Looking at it, my advice is to rev the symbol version of
mlx5dv_init_obj and consider providing a compat symbol.

Also, you need to change the name of the 'uar' field:

> -	cq_out->uar	  = mctx->uar;
> +	cq_out->uar	  = mctx->uar[0];

To something else, to make it clear, which of the two ABIs the
application code is coded to - for a change like this, you *MUST*
force old apps to have a compile failure.

Jason
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