> > Hi Thomas, > > > -----Original Message----- > > From: Thomas Monjalon [mailto:thomas.monja...@6wind.com] > > Sent: Thursday, December 1, 2016 20:52 > > To: Kulasek, TomaszX <tomaszx.kula...@intel.com> > > Cc: dev@dpdk.org; Ananyev, Konstantin <konstantin.anan...@intel.com>; > > olivier.m...@6wind.com; Richardson, Bruce <bruce.richard...@intel.com> > > Subject: Re: [dpdk-dev] [PATCH v12 1/6] ethdev: add Tx preparation > > > > 2016-12-01 19:20, Kulasek, TomaszX: > > > Hi Thomas, > > > > > > Sorry, I have answered for this question in another thread and I missed > > about this one. Detailed answer is below. > > > > Yes you already gave this answer. > > And I will continue asking the question until you understand it. > > > > > > 2016-11-28 11:54, Thomas Monjalon: > > > > > Hi, > > > > > > > > > > 2016-11-23 18:36, Tomasz Kulasek: > > > > > > --- a/config/common_base > > > > > > +++ b/config/common_base > > > > > > @@ -120,6 +120,7 @@ CONFIG_RTE_MAX_QUEUES_PER_PORT=1024 > > > > > > CONFIG_RTE_LIBRTE_IEEE1588=n > > > > > > CONFIG_RTE_ETHDEV_QUEUE_STAT_CNTRS=16 > > > > > > CONFIG_RTE_ETHDEV_RXTX_CALLBACKS=y > > > > > > +CONFIG_RTE_ETHDEV_TX_PREPARE=y > > > > > > > > > > Please, remind me why is there a configuration here. > > > > > It should be the responsibility of the application to call > > > > > tx_prepare or not. If the application choose to use this new API > > > > > but it is disabled, then the packets won't be prepared and there is > > no error code: > > > > > > > > > > > +#else > > > > > > + > > > > > > +static inline uint16_t > > > > > > +rte_eth_tx_prepare(__rte_unused uint8_t port_id, __rte_unused > > > > uint16_t queue_id, > > > > > > + __rte_unused struct rte_mbuf **tx_pkts, uint16_t > > > > > > +nb_pkts) { > > > > > > + return nb_pkts; > > > > > > +} > > > > > > + > > > > > > +#endif > > > > > > > > > > So the application is not aware of the issue and it will not use > > > > > any fallback. > > > > > > tx_prepare mechanism can be turned off by compilation flag (as discussed > > with Jerin in http://dpdk.org/dev/patchwork/patch/15770/) to provide real > > NOOP functionality (e.g. for low-end CPUs, where even unnecessary memory > > dereference and check can have significant impact on performance). > > > > > > Jerin observed that on some architectures (e.g. low-end ARM with > > embedded NIC), just reading and comparing 'dev->tx_pkt_prepare' may cause > > significant performance drop, so he proposed to introduce this > > configuration flag to provide real NOOP when tx_prepare functionality is > > not required, and can be turned on based on the _target_ configuration. > > > > > > For other cases, when this flag is turned on (by default), and > > tx_prepare is not implemented, functional NOOP is used based on comparison > > (dev->tx_pkt_prepare == NULL). > > > > So if the application call this function and if it is disabled, it simply > > won't work. Packets won't be prepared, checksum won't be computed. > > > > I give up, I just NACK. > > It is not to be turned on/off whatever someone wants, but only and only for > the case, when platform developer knows, that his platform > doesn't need this callback, so, he may turn off it and then save some > performance (this option is per target). > > For this case, the behavior of tx_prepare will be exactly the same when it is > turned on or off. If is not the same, there's no sense to turn it > off. There were long topic, where we've tried to convince you, that it should > be turned on for all devices.
As Tomasz pointed out the RTE_ETHDEV_TX_PREPARE was introduced to fulfill Jerin request. >From here: "Low-end ARMv7,ARMv8 targets may not have PCIE-RC support and it may have only integrated NIC controller. On those targets/configs, where integrated NIC controller does not use tx_prep service it can made it as NOOP to save cycles on following "rte_eth_tx_prep" and associated "if (unlikely(nb_prep < nb_rx))" checks in the application." According to the measurements he done it can save ~7% on some low-end ARM machine. You can read whole story here: http://dpdk.org/dev/patchwork/patch/15770/ Though, if now you guys believe that this is not good enough reason, I have absolutely no problem to remove the RTE_ETHDEV_TX_PREPARE and associated logic. I personally don't use ARM boxes and don't plan to, and in theory users can still do conditional compilation at the upper layer, if they want to. Konstantin
