Re: [PATCH 4/6] net/e1000: implement descriptor status API (em)

[Olivier Matz <olivier.matz@6wind.com>]

Hi Wenzhuo,

On Thu, 2 Mar 2017 01:22:25 +0000, "Lu, Wenzhuo" <wenzhuo.lu@intel.com> wrote:
> Hi Oliver,
> 
> > -----Original Message-----
> > From: Olivier Matz [mailto:olivier.matz@6wind.com]
> > Sent: Thursday, March 2, 2017 1:19 AM
> > To: dev@dpdk.org; thomas.monjalon@6wind.com; Ananyev, Konstantin; Lu,
> > Wenzhuo; Zhang, Helin; Wu, Jingjing; adrien.mazarguil@6wind.com;
> > nelio.laranjeiro@6wind.com
> > Cc: Yigit, Ferruh; Richardson, Bruce
> > Subject: [PATCH 4/6] net/e1000: implement descriptor status API (em)
> > 
> > Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
> > +int
> > +eth_em_tx_descriptor_status(struct rte_eth_dev *dev, uint16_t tx_queue_id,
> > +	uint16_t offset)
> > +{
> > +	volatile uint8_t *status;
> > +	struct em_tx_queue *txq;
> > +	uint32_t desc;
> > +
> > +	txq = dev->data->tx_queues[tx_queue_id];
> > +	if (unlikely(offset >= txq->nb_tx_desc))
> > +		return -EINVAL;
> > +
> > +	desc = txq->tx_tail + offset;
> > +	/* go to next desc that has the RS bit */
> > +	desc = ((desc + txq->tx_rs_thresh - 1) / txq->tx_rs_thresh) *
> > +		txq->tx_rs_thresh;  
> The descriptor may be changed here. So the return value may not be for the offset one. Why?

Yes, desc index can change. From what I understand, the
"descriptor done" (DD) bit is only set on descriptors which are
flagged with the "report status" (RS) bit.

Here is an example from:
http://www.dpdk.org/ml/archives/dev/2016-November/050679.html

|----------------------------------------------------------------|
|               D       R       R       R                        |
|        ............xxxxxxxxxxxxxxxxxxxxxxxxx                   |
|        <descs sent><- descs not sent yet  ->                   |
|        ............xxxxxxxxxxxxxxxxxxxxxxxxx                   |
|----------------------------------------------------------------|
        ^last_desc_cleaned=8                    ^next_rs=47
                ^next_dd=15                   ^tail=45
                     ^hw_head=20

                     <----  nb_used  --------->

The hardware is currently processing the descriptor 20
'R' means the descriptor has the RS bit
'D' means the descriptor has the DD + RS bits
'x' are packets in txq (not sent)
'.' are packet already sent but not freed by sw


Regards,
Olivier