Re: [linuxwifi] [RFC] iwlwifi: enable TX AMPDU for some iwldvm

["Grumbach, Emmanuel" <emmanuel.grumbach@intel.com>]

On Tue, 2019-03-12 at 14:31 -0600, Kevin Locke wrote:
> Thanks Luca and Emmanuel,
> 
> On Tue, 2019-03-12 at 19:47 +0000, Grumbach, Emmanuel wrote:
> > On Tue, 2019-03-12 at 21:38 +0200, Luciano Coelho wrote:
> > > If this feature was explicitly disabled, it certainly means that
> > > something was causing problems with it, so I'd be wary to enable
> > > it
> > > for all DVM devices.
> > > 
> > > I guess we could enable it by default for devices that work fine,
> > > but
> > > we would have to run it in real life for a lot longer with a lot
> > > more
> > > different APs to be sure it won't cause any problems.
> 
> Makes sense to me.  I can start testing with available APs.  Let me
> know if there is anything specific I should look for or catalog
> beyond
> latency, packet drop rates, and bandwidth.
> 
> > > Emmanuel, do you happen to remember what was the issue, so Kevin
> > > could test that specific scenario with this specific NIC?
> > 
> > long long nights of despair.
> > 
> > We had issues with reclaim path upon BACK. This is of course a
> > firmware
> > problem...
> 
> Does that suggest the issue may have been fixed by a firmware update?
> For reference, I'm currently using "firmware version 9.221.4.1 build
> 25532" from the firmware-iwlwifi Debian package (version 20190114-1).
> 
> If it would be helpful, I could attempt to bisect the firmware
> revisions to find the one that fixed it (assuming I can reproduce the
> issue with a previous firmware version).

Well.. Sorry, I wasn't very "technical".
So the problem was really that we stopped getting BACK notifications
from the firmware and that caused a reclaim stall which in turn was
caught by a Tx queue stuck timer firing in the driver.
I was never able to reproduce this. What I can do is to enable A-MPDU
on my old system that has this same device, just to see what happens.
While chasing this bug, I even found another one which bought me a few
moments of fame:

commit d6ee27eb13beab94056e0de52d81220058ca2297
Author: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Date:   Wed Jun 6 09:13:36 2012 +0200

    iwlwifi: don't mess up the SCD when removing a key

and in the commit message of that very commit:

    This doesn't seem to fix the higher queues that get stuck
    from time to time.

There were no new versions of the firmware released since then.
I tried to skim through bugzilla, but couldn't find the bugs I was
handling then.

> 
> Thanks again,
> Kevin