[dpdk-dev] [PATCH v12 1/6] ethdev: add Tx preparation

[Jerin Jacob]

On Thu, Dec 01, 2016 at 08:52:22PM +0100, Thomas Monjalon wrote:
> 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.
The use case I was referring  was "integrated NIC" case where
- DPDK target with no external NW PCI card support
AND
- The "integrated NIC" does not need tx_prepare