Re: [dpdk-dev] [RFC] net/ixgbe: fix Tx descriptor status api

["Zhang, Qi Z" <qi.z.zhang@intel.com>]

Hi Oliver

> -----Original Message-----
> From: Olivier Matz [mailto:olivier.matz@6wind.com]
> Sent: Tuesday, June 26, 2018 4:46 PM
> To: Zhao1, Wei <wei.zhao1@intel.com>
> Cc: Zhang, Qi Z <qi.z.zhang@intel.com>; dev@dpdk.org; Lu, Wenzhuo
> <wenzhuo.lu@intel.com>
> Subject: Re: [RFC] net/ixgbe: fix Tx descriptor status api
> 
> Hi Wei,
> 
> On Tue, Jun 26, 2018 at 01:38:22AM +0000, Zhao1, Wei wrote:
> > Hi,  Olivier Matz
> >
> >  Will you commit fix patch for i40e and ixgbe and em?
> 
> If you think the patch are relevant, yes :)
> 
> Here is a pre-version (last 5 patches):
> http://git.droids-corp.org/?p=dpdk.git;a=shortlog;h=refs/heads/tx-desc
> 
> It still need to fix checkpatch issues, few more tests, and rebase on next-net.
> 
> > And the code " dd = (desc / txq->tx_rs_thresh + 1) * txq->tx_rs_thresh - 1;"
> > Is only proper for tx function ixgbe_xmit_pkts_simple  and
> ixgbe_xmit_pkts_vec ().
> > But not proper for ixgbe_xmit_pkts (), the RS bit set rule is different from all
> these two.
> 
> Can you please give more detail please?

I think the problem in ixgbe_xmit_pkts is, we cannot guarantee tx_rs_thresh *n -1 will always get RS bit.
Because the next tx_rs_thresh cycle counted start from the last segment of the packet that cross the boundary, it could be any value. 

So probably we should return -ENOSUP if pkt_tx_burst = ixgbe_xmit_pkts,  or we need to change the method that write RS in ixgbe_xmit_pkts.

Btw, yes, we should look before the tail but not after, you fix is correct.

Thanks
Qi  

> 
> Note this code, maybe you are talking about this?
> 
> +       /* In full featured mode, RS bit is only set in the last descriptor */
> +       /* of a multisegments packet */
> +       if (!((txq->offloads == 0) &&
> +             (txq->tx_rs_thresh >= RTE_PMD_IXGBE_TX_MAX_BURST)))
> +               dd = txq->sw_ring[dd].last_id;
> 
> Maybe there is something better to test?
> 
> Just to ensure we are on the same line, here are some more infos.
> 
> ===
> 
> - sw advances the tail pointer
> - hw advances the head pointer
> - the software populates the ring with full buffers to be sent by
>   the hw
> - head points to the in-progress descriptor.
> - sw writes new descriptors at tail
> - head == tail means that the transmit queue is empty
> - when the hw has processed a descriptor, it sets the DD bit if
>   the descriptor has the RS (report status) bit.
> - the driver never reads the head (needs a pci transaction), instead it monitors
> the DD bit of a descriptor that has the RS bit
> 
> txq->tx_tail: sw value for tail register
> txq->tx_free_thresh: free buffers if count(free descs) < this value
> txq->tx_rs_thresh: RS bit is set every rs_thresh descriptor
> txq->tx_next_dd: next desc to scan for DD bit
> txq->tx_next_rs: next desc to set RS bit
> txq->last_desc_cleaned: last descriptor that have been cleaned
> txq->nb_tx_free: number of free descriptors
> 
> Example:
> 
> |----------------------------------------------------------------|
> |               D       R       R       R
> |
> |        ............xxxxxxxxxxxxxxxxxxxxxxxxx                   |
> |        <descs sent><- descs not sent yet  ->                   |
> |        ............xxxxxxxxxxxxxxxxxxxxxxxxx                   |
> |----------------------------------------------------------------|
>         ^last_desc_cleaned=8                    ^next_rs=47
>                 ^next_dd=15                   ^sw_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
> 
> In this example, we have rs_thres=8. On next call to ixgbe_tx_free_bufs(),
> some buffers will be freed.
> 
> ===
> 
> Let's call ixgbe_dev_tx_descriptor_status(10):
> 
> 
> - original version:
> 
> desc = 45 + 10 = 55
> desc = ((55 + 8 - 1) / 8) * 8 = (62 / 8) * 8 = 56
> 
> wrong because it goes in the wrong direction, and because
> 56 does not have the RS bit
> 
> - after your patch:
> 
> desc = 45 + 10 = 55
> desc = (((55 / 8) + 1) * 8) - 1 = (7 * 8) - 1 = 55
> 
> wrong because it goes in the wrong direction
> 
> - after my patch
> 
> desc = 45 - 10 - 1 = 34
> desc = (((34 / 8) + 1) * 8) - 1 = (5 * 8) - 1 = 39
> 
> looks correct
> 
> 
> 
> Regards,
> Olivier