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

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

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?

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