Re: [dpdk-dev] Regarding rte_eth_rx_burst

["Wang, Haiyue" <haiyue.wang@intel.com>]

> -----Original Message-----
> From: Prashant Upadhyaya <praupadhyaya@gmail.com>
> Sent: Monday, May 11, 2020 13:58
> To: Wang, Haiyue <haiyue.wang@intel.com>
> Cc: dev@dpdk.org
> Subject: Re: [dpdk-dev] Regarding rte_eth_rx_burst
> 
> Thanks Wang !
> ...which begs the proverbial question -- is this a bug or a feature ?
> I would say it is a bug as the polling for 1 mbuf works for the other
> PMD's, worse still the setting to zero is done in a quiet manner
> leading to entire rx blockage if the caller keeps calling with polling
> for 1 mbuf and keeps wondering why the rx is not working.

Maybe a feature, this is good for vector handling best performance.
A slow Rx ' i40e_recv_pkts' in 'drivers/net/i40e/i40e_rxtx.c' can handle
1 mbuf, detail please refer to 'i40e_set_rx_function'

Why 1 mbuf ? Why not loop one by one each received buffer after
calling rte_eth_rx_burst ?

> 
> Regards
> -Prashant
> 
> 
> On Sun, May 10, 2020 at 10:46 PM Wang, Haiyue <haiyue.wang@intel.com> wrote:
> >
> > > -----Original Message-----
> > > From: dev <dev-bounces@dpdk.org> On Behalf Of Prashant Upadhyaya
> > > Sent: Sunday, May 10, 2020 22:06
> > > To: dev@dpdk.org
> > > Subject: [dpdk-dev] Regarding rte_eth_rx_burst
> > >
> > > Hi,
> > >
> > > I recently started using X722 NIC which uses i40 PMD of DPDK.
> > > I am on DPDK 20.02.
> > > I am seeing that when I call the rte_eth_rx_burst with last argument
> > > as 1 (polling for 1 mbuf), then I am not receiving data via repeated
> > > calls.
> >
> > I saw this kind of issue many times. ;-)
> >
> > This is *burst* for vector mode:
> >
> > drivers/net/i40e/i40e_rxtx_vec_sse.c
> >
> > static inline uint16_t
> > _recv_raw_pkts_vec(struct i40e_rx_queue *rxq, struct rte_mbuf **rx_pkts,
> >                    uint16_t nb_pkts, uint8_t *split_packet)
> > {
> >       ....
> >
> >         /* nb_pkts shall be less equal than RTE_I40E_MAX_RX_BURST */
> >         nb_pkts = RTE_MIN(nb_pkts, RTE_I40E_MAX_RX_BURST);
> >
> >         /* nb_pkts has to be floor-aligned to RTE_I40E_DESCS_PER_LOOP */
> >         nb_pkts = RTE_ALIGN_FLOOR(nb_pkts, RTE_I40E_DESCS_PER_LOOP);   <--- nb_pkts = 0, if you
> passed 1.
> >
> >
> > > When I go for calls to rte_eth_rx_burst with last argument as 32, the
> > > function does return the mbuf's as received data.
> > >
> > > Is this expected ? Or this is a bug in the i40 driver handling this NIC ?
> > >
> > > The polls to rte_eth_rx_burst with last argument as 1 works well for
> > > the ixgbe PMD for sure since I have been using X520 successfully with
> > > last argument as 1.
> > >
> > > Regards
> > > -Prashant