Re: [dpdk-dev] [PATCH] parray: introduce internal API for dynamic arrays

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

> >>>>>> One more thought here - if we are talking about rte_ethdev[] in particular, I think  we can:
> >>>>>> 1. move public function pointers (rx_pkt_burst(), etc.) from rte_ethdev into a separate flat array.
> >>>>>> We can keep it public to still use inline functions for 'fast' calls rte_eth_rx_burst(), etc. to avoid
> >>>>>> any regressions.
> >>>>>> That could still be flat array with max_size specified at application startup.
> >>>>>> 2. Hide rest of rte_ethdev struct in .c.
> >>>>>> That will allow us to change the struct itself and the whole rte_ethdev[] table in a way we like
> >>>>>> (flat array, vector, hash, linked list) without ABI/API breakages.
> >>>>>>
> >>>>>> Yes, it would require all PMDs to change prototype for pkt_rx_burst() function
> >>>>>> (to accept port_id, queue_id instead of queue pointer), but the change is mechanical one.
> >>>>>> Probably some macro can be provided to simplify it.
> >>>>>>
> >>>>>
> >>>>> We are already planning some tasks for ABI stability for v21.11, I think
> >>>>> splitting 'struct rte_eth_dev' can be part of that task, it enables hiding more
> >>>>> internal data.
> >>>>
> >>>> Ok, sounds good.
> >>>>
> >>>>>
> >>>>>> The only significant complication I can foresee with implementing that approach -
> >>>>>> we'll need a an array of 'fast' function pointers per queue, not per device as we have now
> >>>>>> (to avoid extra indirection for callback implementation).
> >>>>>> Though as a bonus we'll have ability to use different RX/TX funcions per queue.
> >>>>>>
> >>>>>
> >>>>> What do you think split Rx/Tx callback into its own struct too?
> >>>>>
> >>>>> Overall 'rte_eth_dev' can be split into three as:
> >>>>> 1. rte_eth_dev
> >>>>> 2. rte_eth_dev_burst
> >>>>> 3. rte_eth_dev_cb
> >>>>>
> >>>>> And we can hide 1 from applications even with the inline functions.
> >>>>
> >>>> As discussed off-line, I think:
> >>>> it is possible.
> >>>> My absolute preference would be to have just 1/2 (with CB hidden).
> >>>
> >>> How can we hide the callbacks since they are used by inline burst functions.
> >>
> >> I probably I owe a better explanation to what I meant in first mail.
> >> Otherwise it sounds confusing.
> >> I'll try to write a more detailed one in next few days.
> >
> > Actually I gave it another thought over weekend, and might be we can
> > hide rte_eth_dev_cb even in a simpler way. I'd use eth_rx_burst() as
> > an example, but the same principle applies to other 'fast' functions.
> >
> >  1. Needed changes for PMDs rx_pkt_burst():
> >     a) change function prototype to accept 'uint16_t port_id' and 'uint16_t queue_id',
> >          instead of current 'void *'.
> >     b) Each PMD rx_pkt_burst() will have to call rte_eth_rx_epilog() function at return.
> >          This  inline function will do all CB calls for that queue.
> >
> > To be more specific, let say we have some PMD: xyz with RX function:
> >
> > uint16_t
> > xyz_recv_pkts(void *rx_queue, struct rte_mbuf **rx_pkts, uint16_t nb_pkts)
> > {
> >      struct xyz_rx_queue *rxq = rx_queue;
> >      uint16_t nb_rx = 0;
> >
> >      /* do actual stuff here */
> >     ....
> >     return nb_rx;
> > }
> >
> > It will be transformed to:
> >
> > uint16_t
> > xyz_recv_pkts(uint16_t port_id, uint16_t queue_id, struct rte_mbuf **rx_pkts, uint16_t nb_pkts)
> > {
> >          struct xyz_rx_queue *rxq;
> >          uint16_t nb_rx;
> >
> >          rxq = _rte_eth_rx_prolog(port_id, queue_id);
> >          if (rxq == NULL)
> >              return 0;
> >          nb_rx = _xyz_real_recv_pkts(rxq, rx_pkts, nb_pkts);
> >          return _rte_eth_rx_epilog(port_id, queue_id, rx_pkts, nb_pkts);
> > }
> >
> > And somewhere in ethdev_private.h:
> >
> > static inline void *
> > _rte_eth_rx_prolog(uint16_t port_id, uint16_t queue_id);
> > {
> >    struct rte_eth_dev *dev = &rte_eth_devices[port_id];
> >
> > #ifdef RTE_ETHDEV_DEBUG_RX
> >         RTE_ETH_VALID_PORTID_OR_ERR_RET(port_id, NULL);
> >         RTE_FUNC_PTR_OR_ERR_RET(*dev->rx_pkt_burst, NULL);
> >
> >         if (queue_id >= dev->data->nb_rx_queues) {
> >                 RTE_ETHDEV_LOG(ERR, "Invalid RX queue_id=%u\n", queue_id);
> >                 return NULL;
> >         }
> > #endif
> >   return dev->data->rx_queues[queue_id];
> > }
> >
> > static inline uint16_t
> > _rte_eth_rx_epilog(uint16_t port_id, uint16_t queue_id, struct rte_mbuf **rx_pkts, const uint16_t nb_pkts);
> > {
> >     struct rte_eth_dev *dev = &rte_eth_devices[port_id];
> >
> > #ifdef RTE_ETHDEV_RXTX_CALLBACKS
> >         struct rte_eth_rxtx_callback *cb;
> >
> >         /* __ATOMIC_RELEASE memory order was used when the
> >          * call back was inserted into the list.
> >          * Since there is a clear dependency between loading
> >          * cb and cb->fn/cb->next, __ATOMIC_ACQUIRE memory order is
> >          * not required.
> >          */
> >         cb = __atomic_load_n(&dev->post_rx_burst_cbs[queue_id],
> >                                 __ATOMIC_RELAXED);
> >
> >         if (unlikely(cb != NULL)) {
> >                 do {
> >                         nb_rx = cb->fn.rx(port_id, queue_id, rx_pkts, nb_rx,
> >                                                 nb_pkts, cb->param);
> >                         cb = cb->next;
> >                 } while (cb != NULL);
> >         }
> > #endif
> >
> >         rte_ethdev_trace_rx_burst(port_id, queue_id, (void **)rx_pkts, nb_rx);
> >         return nb_rx;
> >  }
> >
> > Now, as you said above, in rte_ethdev.h we will keep only a flat array
> > with pointers to 'fast' functions:
> > struct {
> >      eth_rx_burst_t             rx_pkt_burst
> >       eth_tx_burst_t             tx_pkt_burst;
> >       eth_tx_prep_t              tx_pkt_prepare;
> >      .....
> > } rte_eth_dev_burst[];
> >
> > And rte_eth_rx_burst() will look like:
> >
> > static inline uint16_t
> > rte_eth_rx_burst(uint16_t port_id, uint16_t queue_id,
> >                  struct rte_mbuf **rx_pkts, const uint16_t nb_pkts)
> > {
> >     if (port_id >= RTE_MAX_ETHPORTS)
> >         return 0;
> >    return rte_eth_dev_burst[port_id](port_id, queue_id, rx_pkts, nb_pkts);
> > }
> >
> > Yes, it will require changes in *all* PMDs, but as I said before the changes will be a mechanic ones.
> >
> 
> I did not like the idea to push to calling Rx/TX callbacks responsibility to the
> drivers, I think it should be in the ethdev layer.

Well, I'd say it is an ethdev layer function that has to be called by PMD 😊

> 
> What about making 'rte_eth_rx_epilog' an API and call from 'rte_eth_rx_burst()',
> which will add another function call for Rx/Tx callback but shouldn't affect the
> Rx/Tx burst.

But then we either need to expose call-back information to the user or pay the penalty
for extra function call, correct?