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

[Slava Ovsiienko <viacheslavo@mellanox.com>]

Hi, Didier

First, thank you for the patch.

If we have a look at the description of rte_eth_rx_queue_count(): "Get the number of used descriptors of a rx queue".
It means the DPDK generic descriptors, not PMD specific ones. "DPDK descriptor" means the entity which can handle one packet.
rte_eth_rx_queue_count() should return the potential number of packets could be fetched from the Rx queue on the next rx_burst() call.
Application should know anything about PMD descriptors, it must be isolated. So, rx_queue_count() should return the number of expected
packets not hardware entries. That's why the value being returned is compared with elts, not with HW desctriptors.

As for -1 - I agree, should be fixed.

It seems we have another bug - the Rx queue is created with number of hardware descriptors which does not correspond the requested packets
in case of multi-segment packets  (requested desc is divided by (1<<sges_n), it seems it should not).
 
With best regards, Slava

> -----Original Message-----
> From: dev <dev-bounces@dpdk.org> On Behalf Of Didier Pallard
> Sent: Friday, March 13, 2020 11:57
> To: dev@dpdk.org
> Cc: 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: stable@dpdk.org
> Signed-off-by: Didier Pallard <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