[1/4] Revert "net/mlx5: fix Rx queue count calculation"

  This reverts commit d2d57605522d4a43be17e22e649e54033f6d8835.

This fix is uncorrect for at least two reasons.

First issue, when there are more than 8 CQEs to uncompress, the
computation done in this commit cannot work. Because the zip-ai
variable describes the current index inside the CQE8 array and thus is
limited from 0 to 7 included. So if we are decompressed the 9 packets,
ai is 0. So in this case, n is equals to cqe_cnt - 0.

Example with 11 packets we will have:
C | a | e0 | e1 | e2 | e3 | e4 | e5 | C | a | e0

c <-- CQE compressed
a <-- Array of minicqe
ex <-- emptry entry to store uncompressed CQE.

If the 9th packet is decompressed by the soft, n is equals to 9.
But with this commit, n is equals to 11 (i.e. 11 - 0).

Second issue is to count the next packet.

Example:

     packet 1                       |    packet 2
C | a | e0 | e1 | e2 | e3 | e4 | e5 | C | a | e0

There are 2 packets compressed in the first queue. For the first packet,
n is computed correctly.

But for the second, n is not computed properly. Because the zip context
is for the first packet. The  second packet is not yet decompressed, so
there are no context.

Signed-off-by: Maxime Leroy <maxime.leroy@6wind.com>
Signed-off-by: Nelio Laranjeiro <nelio.laranjeiro@6wind.com>
---
 drivers/net/mlx5/mlx5_rxtx.c | 27 +++++++++++++++------------
 1 file changed, 15 insertions(+), 12 deletions(-)
  

Hi, Maxime

Thanks a lot for the patch. There is the comment for the entire series.

[1]_____
> 
> First issue, when there are more than 8 CQEs to uncompress, the computation
> done in this commit cannot work. Because the zip-ai variable describes the
> current index inside the CQE8 array and thus is limited from 0 to 7 included. So
> if we are decompressed the 9 packets, ai is 0. So in this case, n is equals to
> cqe_cnt - 0.
> 
> Example with 11 packets we will have:
> C | a | e0 | e1 | e2 | e3 | e4 | e5 | C | a | e0
> 
1. ai is not index in the array (just tree lsbs of ai). It is an index of the miniCQE being processed
in the compressed session and is in the range [0 . .zip.cqe_cnt-1]. In your example there will be
two compressed sessions. The bug was we corrected each compressed session for the ai of the
first one (in processing that we were).

[2]_____
>	/* if we are processing a compressed cqe */
> 	if (zip->ai) {
>-		used = zip->cqe_cnt - zip->ca;
> 		cq_ci = zip->cq_ci;
>+		cq_end = cq_ci + zip->cqe_cnt;
>+		cq_cur = zip->ca + zip->ai;
>+		used = cq_end - cq_cur;
>	} else {
> 		used = 0;
> 		cq_ci = rxq->cq_ci;

Sorry, it seems to be incorrect.
zip->cq_ci is the index of the NEXT CQE, following the compressed session being processed.
zip->ai is index of miniCQE being processed. "used" should be calculated much simple:

    used = zip->cqe_cnt - zip->ai

[3]_____
-       if (dev->rx_pkt_burst == NULL ||
-           dev->rx_pkt_burst == removed_rx_burst) {
+       if (dev->rx_pkt_burst != mlx5_rx_burst) {

In this way, we cut the support for other rx_burst routines, we should restore.

[4]______
I'am OK with Didier patch "net/mlx5: fix Rx descriptor status returned value"

I see you wrote the luxury commit messages, and I'm crying with bloody tears about what I'm going to ask you for -
could we squash the series in to single commit? Or at least two - Didier and yours? 

With best regards, Slava
  

Hi Slava,

On Wed, Nov 11, 2020 at 8:51 PM Slava Ovsiienko <viacheslavo@nvidia.com> wrote:
>
> Hi, Maxime
>
> Thanks a lot for the patch. There is the comment for the entire series.
>
> [1]_____
> >
> > First issue, when there are more than 8 CQEs to uncompress, the computation
> > done in this commit cannot work. Because the zip-ai variable describes the
> > current index inside the CQE8 array and thus is limited from 0 to 7 included. So
> > if we are decompressed the 9 packets, ai is 0. So in this case, n is equals to
> > cqe_cnt - 0.
> >
> > Example with 11 packets we will have:
> > C | a | e0 | e1 | e2 | e3 | e4 | e5 | C | a | e0
> >
> 1. ai is not index in the array (just tree lsbs of ai). It is an index of the miniCQE being processed
> in the compressed session and is in the range [0 . .zip.cqe_cnt-1]. In your example there will be
> two compressed sessions. The bug was we corrected each compressed session for the ai of the
> first one (in processing that we were).

The name of the variable (i.e. array index) has confused me. But you are right.
>
> [2]_____
> >       /* if we are processing a compressed cqe */
> >       if (zip->ai) {
> >-              used = zip->cqe_cnt - zip->ca;
> >               cq_ci = zip->cq_ci;
> >+              cq_end = cq_ci + zip->cqe_cnt;
> >+              cq_cur = zip->ca + zip->ai;
> >+              used = cq_end - cq_cur;
> >       } else {
> >               used = 0;
> >               cq_ci = rxq->cq_ci;
>
> Sorry, it seems to be incorrect.
> zip->cq_ci is the index of the NEXT CQE, following the compressed session being processed.
> zip->ai is index of miniCQE being processed. "used" should be calculated much simple:
>
>     used = zip->cqe_cnt - zip->ai

You are right.

>
> [3]_____
> -       if (dev->rx_pkt_burst == NULL ||
> -           dev->rx_pkt_burst == removed_rx_burst) {
> +       if (dev->rx_pkt_burst != mlx5_rx_burst) {
>
> In this way, we cut the support for other rx_burst routines, we should restore.
>
> [4]______
> I'am OK with Didier patch "net/mlx5: fix Rx descriptor status returned value"
>
> I see you wrote the luxury commit messages, and I'm crying with bloody tears about what I'm going to ask you for -
> could we squash the series in to single commit? Or at least two - Didier and yours?
>
> With best regards, Slava
>

I have just sent a V2 version fixing all these points.

Best regards,

Maxime Leroy

>
>