[dpdk-dev] MLX5 driver - number of descriptors overflow

[Yongseok Koh]

> On Nov 21, 2018, at 1:01 AM, Amedeo Sapio <amedeo.sapio at gmail.com> wrote:
> 
> Hello,
> 
> I experienced a problem with the MLX5 driver running a code that is working
> fine with an Intel card. I have found that the reason of this error is an
> overflow of the uint16_t number of descriptors in the mlx driver.
> 
> Here the details:
> 
> - The NIC is a Mellanox ConnectX-5 100G.
> 
> - This is a summary code that I run to initialize the port:
> 
>        ret = rte_eth_dev_configure(dpdk_par.portid, 1, 1, &port_conf);
> 
>        dpdk_par.port_rx_ring_size = dev_info.rx_desc_lim.nb_max;
>        dpdk_par.port_tx_ring_size = dev_info.tx_desc_lim.nb_max;
> 
>        ret = rte_eth_dev_adjust_nb_rx_tx_desc(dpdk_par.portid,
> &dpdk_par.port_rx_ring_size, &dpdk_par.port_tx_ring_size);
> 
>        ret = rte_eth_rx_queue_setup(dpdk_par.portid, 0,
> dpdk_par.port_rx_ring_size, rte_eth_dev_socket_id(dpdk_par.portid),
> &rx_conf, dpdk_data.pool);
>        ret = rte_eth_tx_queue_setup(dpdk_par.portid, 0,
> dpdk_par.port_tx_ring_size, rte_eth_dev_socket_id(dpdk_par.portid),
> &tx_conf);
> 
>        ret = rte_eth_dev_start(dpdk_par.portid);
> 
> - The "rte_eth_dev_start" function returns  -ENOMEM  =  -12 (Out of memory)
> 
> - I see that "dev_info.rx_desc_lim.nb_max" is 65535.  This value is rounded
> to the next power of 2 in "mlx5_rx_queue_setup",  which overflows and
> becomes 0.
> 
> I thought that  "rte_eth_dev_adjust_nb_rx_tx_desc" should have adjusted the
> value, but clearly it has not.

Nice catch! You are right. We should've set it appropriately.
Will come up with a patch. Meanwhile, can you please use a workaround?, e.g.,

rte_log2_u32(rte_align32prevpow2(dev_info.rx_desc_lim.nb_max));

Will let you know when we push a patch.

Thanks,
Yongseok