RE: Questions on i40e TX path

["Xing, Beilei" <beilei.xing@intel.com>]

> -----Original Message-----
> From: Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>
> Sent: Thursday, February 10, 2022 12:38 PM
> To: Xing, Beilei <beilei.xing@intel.com>; dev@dpdk.org
> Cc: Feifei Wang <Feifei.Wang2@arm.com>; Ruifeng Wang
> <Ruifeng.Wang@arm.com>; Yigit, Ferruh <ferruh.yigit@intel.com>; Richardson,
> Bruce <bruce.richardson@intel.com>; nd <nd@arm.com>; nd <nd@arm.com>
> Subject: RE: Questions on i40e TX path
> 
> <snip>
> 
> Thank you for your input. Please see few comments inline.
> 
> > > 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."
> Yes, understood.
> When combined with #2 below, we are asking the NIC to report back the
> send/completion status for a set of packets. This allows for amortization of cost
> of reporting over the set of packets.
> 
> >
> > > 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.
> Ok. I think this check has another purpose as well, though I am not sure if it is
> intentional. I see that the descriptors are initialized with DTYPE set to 0xF (in
> function i40e_reset_tx_queue). So, in the very first call to transmit function (for
> ex: i40e_xmit_fixed_burst_vec), the 'i40e_tx_free_bufs' function would end up
> checking the DTYPE field, if the above check was not there.
> 
> In the data sheet, in section 8.4.2.1.1 (transmit data descriptor format), the RS
> field is described as follows:
> 
> " Report Status. When set, the hardware reports the DMA completion of the
> transmit descriptor and its data buffer. Completion is reported by descriptor
> write back or by head write back as configured by the HEAD_WBEN flag in the
> transmit context. When it is reported by descriptor write back, the DTYP field is
> set to 0xF and the RS flag is set."
> 
> Considering the last sentence, should the code check for both DTYP field and RS
> field for completion? Currently, the code checks for just the DTYP field (in the
> function i40e_tx_free_bufs).

In my understanding, RS bit is set by SW to indicate HW needs to write back. So I don't
think PMD needs to check RS bit.
Besides, PMD should have ensured PMD checks DTYPE of the descriptor with RS bit only.

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