Re: [dpdk-dev] [PATCH] net/ixgbe: fix Tx check descriptor status APIs error

["Zhao1, Wei" <wei.zhao1@intel.com>]

Hi,

> -----Original Message-----
> From: Zhang, Qi Z
> Sent: Friday, June 22, 2018 9:47 PM
> To: Zhao1, Wei <wei.zhao1@intel.com>; dev@dpdk.org
> Cc: Lu, Wenzhuo <wenzhuo.lu@intel.com>; stable@dpdk.org
> Subject: RE: [PATCH] net/ixgbe: fix Tx check descriptor status APIs error
> 
> Hi Wei:
> 
> > -----Original Message-----
> > From: Zhao1, Wei
> > Sent: Friday, June 22, 2018 4:39 PM
> > To: dev@dpdk.org
> > Cc: Lu, Wenzhuo <wenzhuo.lu@intel.com>; Zhang, Qi Z
> > <qi.z.zhang@intel.com>; stable@dpdk.org; Zhao1, Wei
> > <wei.zhao1@intel.com>
> > Subject: [PATCH] net/ixgbe: fix Tx check descriptor status APIs error
> >
> > This is a issue involve RS bit set rule in ixgbe.
> > Let us take function ixgbe_xmit_pkts_vec () as an example, in this
> > function RS bit will be set for descriptor with index
> > txq->tx_next_rs, and also descriptor free function
> > ixgbe_tx_free_bufs() also check RS bit for descriptor with index
> > txq->tx_next_rs, This is perfect ok. Let us take an example,
> > if app set tx_rs_thresh = 32 and nb_desc = 512, then ixgbe PMD code
> > will init
> > txq->tx_next_rs = 31 in function ixgbe_reset_tx_queue when tx queue
> setup.
> > And also txq->tx_next_rs will be update as 63, 95 and so on. But, in
> > the function ixgbe_dev_tx_descriptor_status(), the RS bit to check is
> > " desc = ((desc
> > + txq->tx_rs_thresh - 1) /
> > txq->tx_rs_thresh) * txq-tx_rs_thresh", which is 32 ,64, 96 and so on.
> > So, they are all wrong! In tx function of ixgbe_xmit_pkts_simple, the
> > RS bit rule is also the same, it also set index 31 ,64, 95.
> > we need to correct it.
> 
> One question:
> does only the descriptor with RS bit will have DD status, or NIC will always
> update all descriptor's DD status but this happens when the next descriptor
> with RS bit has been sent?
> If it is the first case, I think you fix still have problem, because multi-seg mbuf
> or tso offload will break the 31, 63, 95 pattern
> See:
> 						nb_used = (uint16_t)(tx_pkt-
> >nb_segs + new_ctx);
> 
> 						if (txp != NULL &&
>                                 nb_used + txq->nb_tx_used >= txq->tx_rs_thresh)
>                         /* set RS on the previous packet in the burst */
>                         txp->read.cmd_type_len |=
>                                 rte_cpu_to_le_32(IXGBE_TXD_CMD_RS);
> 
> so the possible solution is store each RS position in a list at tx, and find the
> next RS from the list in ixgbe_dev_tx_descriptor_status
> 
> If it is the second case, it will be simple we don't need to check forward with
> tx_rs_thresh, just check the exact position ( I hope it is this case :))

In this patch, code "desc = txq->sw_ring[desc].last_id;" will get the last index for several segments packet, that  solve the case when packet contain more than one segment.

> 
> Regards
> Qi
> >
> > Fixes: a2919e13d95e ("net/ixgbe: implement descriptor status API")
> >
> > Signed-off-by: Wei Zhao <wei.zhao1@intel.com>
> > ---
> >  drivers/net/ixgbe/ixgbe_rxtx.c | 12 ++++++------
> >  1 file changed, 6 insertions(+), 6 deletions(-)
> >
> > diff --git a/drivers/net/ixgbe/ixgbe_rxtx.c
> > b/drivers/net/ixgbe/ixgbe_rxtx.c index 3e13d26..f185219 100644
> > --- a/drivers/net/ixgbe/ixgbe_rxtx.c
> > +++ b/drivers/net/ixgbe/ixgbe_rxtx.c
> > @@ -3146,15 +3146,15 @@ ixgbe_dev_tx_descriptor_status(void
> *tx_queue,
> > uint16_t offset)
> >  		return -EINVAL;
> >
> >  	desc = txq->tx_tail + offset;
> > +	if (desc >= txq->nb_tx_desc)
> > +		desc -= txq->nb_tx_desc;
> >  	/* go to next desc that has the RS bit */
> > -	desc = ((desc + txq->tx_rs_thresh - 1) / txq->tx_rs_thresh) *
> > -		txq->tx_rs_thresh;
> > -	if (desc >= txq->nb_tx_desc) {
> > +	desc = (desc  / txq->tx_rs_thresh + 1) *
> > +			txq->tx_rs_thresh - 1;
> > +	if (desc >= txq->nb_tx_desc)
> >  		desc -= txq->nb_tx_desc;
> > -		if (desc >= txq->nb_tx_desc)
> > -			desc -= txq->nb_tx_desc;
> > -	}
> >
> > +	desc = txq->sw_ring[desc].last_id;
> >  	status = &txq->tx_ring[desc].wb.status;
> >  	if (*status & rte_cpu_to_le_32(IXGBE_ADVTXD_STAT_DD))
> >  		return RTE_ETH_TX_DESC_DONE;
> > --
> > 2.7.5