Hi Thomas, > -----Original Message----- > From: Thomas Monjalon [mailto:thomas.monja...@6wind.com] > Sent: Friday, December 2, 2016 00:51 > 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 22:31, Kulasek, TomaszX: > > From: Thomas Monjalon [mailto:thomas.monja...@6wind.com] > > > 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). > > How may he know? There is no comment in the config file, no documentation. > > > 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. > > Really? You tried to convince me to turn it on? > No you were trying to convince Jerin. > I think it is a wrong idea to allow disabling this function. > I didn't comment in first discussion because Jerin told it was really > important for small hardware with fixed NIC, and I thought it would be > implemented in a way the application cannot be misleaded. > > The only solution I see here is to add some comments in the configuration > file, below the #else and in the doc. > Have you checked doc/guides/prog_guide/poll_mode_drv.rst?
I can change the name of CONFIG_RTE_ETHDEV_TX_PREPARE=y to something like CONFIG_RTE_ETHDEV_TX_PREPARE_NOOP=n to made it less confusing, and add comments to describe why it is introduced and how it use safely. I can also remove it at all if you don't like it. As for doc/guides/prog_guide/poll_mode_drv.rst, do you mean, to add new section describing this feature? Tomasz
