Re: [PATCH v2 1/2] net/fm10k: convert to new Rx offloads API

["Dai, Wei" <wei.dai@intel.com>]

Thanks, Konstantin and Zhang Qi for your feedback and support.
I have talked with Qi and know following:
To avoid packet dropping when FM10K_SRRCTL_BUFFER_CHAINING_EN is not set,
the queue can work with a different mempool which has larger mem buffer size.
For example, SCATTER is enabled on queue 0 with a small mem buffer size of mempool 0,
And SCATTER is disabled on queue 1 with a large mem buffer size of mempool 1,
Both queues can avoid packet dropping.
Indeed, current fm10k PMD also automatically set FM10K_SRRCTL_BUFFER_CHAINING_EN
If max_rx_pkt_len + 2 * VLAN_TAG_SIZE > mem_buf_size no matter whether SCATTER is enabled
on queue configuration or not.


> -----Original Message-----
> From: Ananyev, Konstantin
> Sent: Sunday, April 1, 2018 8:09 PM
> To: Zhang, Qi Z <qi.z.zhang@intel.com>; Dai, Wei <wei.dai@intel.com>;
> Wang, Xiao W <xiao.w.wang@intel.com>
> Cc: 'dev@dpdk.org' <dev@dpdk.org>
> Subject: RE: [dpdk-dev] [PATCH v2 1/2] net/fm10k: convert to new Rx
> offloads API
> 
> Hi Qi,
> 
> > > > > > >
> > > > > > > Hi Daiwei:
> > > > > > >
> > > > > > > > +static uint64_t fm10k_get_rx_queue_offloads_capa(struct
> > > > > > > > +rte_eth_dev
> > > > > > > > +*dev) {
> > > > > > > > +	RTE_SET_USED(dev);
> > > > > > > > +
> > > > > > > > +	return (uint64_t)(DEV_RX_OFFLOAD_SCATTER);
> > > > > > > > +}
> > > > > > >
> > > > > > > why per queue rx scattered feature here?
> > > > > > > My understanding is either we use scattered rx function that
> > > > > > > enable this feature for all queues or we use non-scattered
> > > > > > > rx function that disable this feature for all queues, right?
> > > > > >
> > > > > > Checked with Dai Wei offline, fm10k have per queue register
> > > > > > that can be configured to support rx scattered, So it is per queue
> offload.
> > > > >
> > > > > Ok, but these days we have one RX function per device.
> > > > > Looking at fm10k - it clearly has different RX function for
> > > > > scattered and non-scattered case.
> > > > > Yes, HW does support scatter/non-scatter selection per queue,
> > > > > but our SW - doesn't (same for ixgbe and i40e) So how it could
> > > > > be per queue
> > > offload?
> > > >
> > > > We saw the implementation of fm10k is a little bit different with i40e.
> > > > It set per queue register "FM10K_SRRCTL_BUFFER_CHAINING_EN" to
> > > > turn
> > > on multi-seg feature when offload is required.
> > > >
> > > > That means two queues can have different behavior when process a
> > > > packet that exceed the buffer size base on the register setting,
> > > > though we
> > > use the same rx scattered function, so we think this is per queue
> > > feature, is that make sense?
> > >
> > > Ok, suppose we have 2 functions configured.
> > > One with DEV_RX_OFFLOAD_SCATTER is on, second with
> > > DEV_RX_OFFLOAD_SCATTER is off.
> > > So scatter RX function will be selected, but for second queue  HW
> > > support will not be enabled, so packets bigger then RX buffer will
> > > be silently dropped by HW, right?
> >
> > Yes according to datasheet
> >
> > Bit FM10K_SRRCTL_BUFFER_CHAINING_EN:
> >
> > 0b = Any packet longer than the data buffer size is terminated with a
> > TOO_BIG error status in Rx descriptor write-back. The remainder of the
> > frame is not posted to host, it is silently dropped.
> > 1b = A packet can be spread over more than one single receive data
> > buffer
> >
> 
> Ok, that's a bit unusual approach but understandable.
> Thanks
> Konstantin