netdev - Re: question: tg3 driver/nics and inconsistent RX ring count

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Date:	Tue, 26 Jul 2016 13:06:21 -0700
From:	Alexander Duyck <alexander.duyck@...il.com>
To:	Michal Soltys <soltys@....info>
Cc:	Linux Netdev List <netdev@...r.kernel.org>
Subject: Re: question: tg3 driver/nics and inconsistent RX ring count

On Tue, Jul 26, 2016 at 12:52 PM, Michal Soltys <soltys@....info> wrote:
> Hi,
>
> I have a few of BCM5720 and BCM5719 kinds sitting in Dell R320 and R520
> servers - and all of them have certain peculiarity: they claim to have
> up to 4 TX and RX rings (and this can be set/verified just fine through
> ethtool -l/-L, with driver defaulting to 4 rings), indirection table
> (ethtool -x) looks fine as well:
>
> RX flow hash indirection table for eth1b with 3 RX ring(s):
>     0:      0     1     2     3     0     1     2     3
>     8:      0     1     2     3     0     1     2     3
> ......
>
> But this "3 RX ring(s)" is actually a real limit if I try to adjust
> anything, for example all those commands would fail:
>
> ethtool -X eth1b equal 4
> ethtool -X eth1b weight 1 1 1 1
>
> But would work fine for 3 and less rings. This was quickly tested with
> different kernel/ethtool combinations, from old 3.16 to 4.6.y with same
> exact results. Nothing fancier (-N/-U) is supported either.
>
> Any hints/comments about the cause of this and/or possible workarounds ?

Well a quick look at the driver code makes it seem the problem lies in
tg3_get_rxnfc.  I suspect the bug is related to the following lines:

                /* The first interrupt vector only
                 * handles link interrupts.
                 */
                info->data -= 1;

I'm not sure what the number of interrupt vectors has to do with the
number of rings.  Perhaps someone more familiar with the driver can
point out why you would subtract 1 from tp->rxq_cnt to get the number
of queues when it seems like it should be tp->rxq_cnt.

Hope that helps.

- Alex