Re: [dpdk-dev] [PATCH] net/mlx5: fix Rx descriptor status returned value

[Slava Ovsiienko <viacheslavo@mellanox.com>]

>> From: Didier Pallard <didier.pallard@6wind.com> 
>> Sent: Monday, March 16, 2020 19:24
>> To: Slava Ovsiienko <viacheslavo@mellanox.com>
>> Cc: dev@dpdk.org; stable@dpdk.org; Matan Azrad <matan@mellanox.com>
>> Subject: Re: [dpdk-dev] [PATCH] net/mlx5: fix Rx descriptor status returned value
>> Well, you're right, another way to fix the problem could be to set up the queue size assuming the provided number
>> is a number of packets in queue rather than a number of mbufs in queue.

Yes, it is intended in queue setup routine. But, for mlx5 we have a bug for regular mlx5_rx_burst if scattering is enabled,
the Rx queue is created with size wqe_n elements, should be wqe_n << sges_n instead, to be able to receive the requested
number of packets (wqe_n). I think we must fix. Would you like to update your patch, or should I provide mine? 

>> But not sure it's better, it's also important for the application/user to know the number of mbufs that could fit in a rx/tx queue,
>> whatever the number of packets that it covers, since it is very important to size the memory pools correctly to avoid any
>> mbuf shortage during system life.
>> Thanks
>> Didier
To estimate - the number of "DPDK descriptors" should be multiplied by the maximal length of scattered packet chain.

With best regards, Slava

> -----Original Message-----
> From: dev <mailto:dev-bounces@dpdk.org> On Behalf Of Didier Pallard
> Sent: Friday, March 13, 2020 11:57
> To: mailto:dev@dpdk.org
> Cc: mailto:stable@dpdk.org
> Subject: [dpdk-dev] [PATCH] net/mlx5: fix Rx descriptor status returned value
> 
> Two bugs in rx_queue_count function:
> - One entry may contain several segments, so 'used' must be multiplied
>   by number of segments per entry to properly reflect the queue usage.
> - rx_queue_count returns the number of entries used in queue, so it ranges
>   from 0 to max number of entries in queue, not this number minus
>   one.
> 
> Fixes: 8788fec1f269 ("net/mlx5: implement descriptor status API")
> Cc: mailto:stable@dpdk.org
> Signed-off-by: Didier Pallard <mailto:didier.pallard@6wind.com>
> ---
>  drivers/net/mlx5/mlx5_rxtx.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/net/mlx5/mlx5_rxtx.c b/drivers/net/mlx5/mlx5_rxtx.c index
> 5ac63da8039d..17f80c25443e 100644
> --- a/drivers/net/mlx5/mlx5_rxtx.c
> +++ b/drivers/net/mlx5/mlx5_rxtx.c
> @@ -500,7 +500,7 @@ rx_queue_count(struct mlx5_rxq_data *rxq)
>               used += n;
>               cqe = &(*rxq->cqes)[cq_ci & cqe_cnt];
>       }
> -     used = RTE_MIN(used, (1U << rxq->elts_n) - 1);
> +     used = RTE_MIN(used * (1 << rxq->sges_n), 1U << rxq->elts_n);
>       return used;
>  }
> 
> --
> 2.24.1