Questions on i40e TX path

[Xing, Beilei]

Hi Honnappa,

> -----Original Message-----
> From: Honnappa Nagarahalli <Honnappa.Nagarahalli at arm.com>
> Sent: Wednesday, February 9, 2022 12:36 PM
> To: Xing, Beilei <beilei.xing at intel.com>; dev at dpdk.org
> Cc: Feifei Wang <Feifei.Wang2 at arm.com>; Ruifeng Wang
> <Ruifeng.Wang at arm.com>; Yigit, Ferruh <ferruh.yigit at intel.com>; Richardson,
> Bruce <bruce.richardson at intel.com>; nd <nd at arm.com>; nd <nd at arm.com>
> Subject: Questions on i40e TX path
> 
> Hi Beilei,
> 	I want to make sure my understanding of the TX path is correct.
> Following is my understanding.
> 
> 1) The RS bit must be set in the TX descriptors to ask the NIC to report back the
> send status.
Not for each Tx descriptor.
According to the datasheet, " The RS flag can be set only on a last Transmit Data
Descriptor of a packet or last Transmit Data Descriptor of a TSO or last Transmit
Data Descriptor of a filter." 

> 2) The NIC reports the send completion by setting the DTYPE field to 0xf. This
> also indicates that all the earlier descriptors are also done sending the packets.
Yes.

> 3) The check "if (txq->nb_tx_free < txq->tx_free_thresh)" is mainly to ensure that
> we do not check the "descriptor done" status too often.
This condition is to ensure there're enough free descriptors for Tx, avoid Tx ring full.

> 
> Is my understanding correct?
> 
> Thank you,
> Honnappa