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From:   Andy Shevchenko <andy.shevchenko@gmail.com>
Date:   Mon, 12 Nov 2018 01:53:02 +0200
Message-ID: <CAHp75VeKqijzEiP5Kf0+Xoem_3QByZL3aF9c4bbzF68_E=1uWA@mail.gmail.com>
Subject: Re: [PATCH v1 1/5] drivercore: Revert "deferral race condition fix"
To:     Rob Herring <robh@kernel.org>
Cc:     Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Hans de Goede <hdegoede@redhat.com>,
        Peter Ujfalusi <peter.ujfalusi@ti.com>,
        Andrzej Hajda <a.hajda@samsung.com>, grant.likely@arm.com
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On Sun, Nov 11, 2018 at 9:26 PM Rob Herring <robh@kernel.org> wrote:
> On Sun, Nov 11, 2018 at 7:04 AM Andy Shevchenko
> <andy.shevchenko@gmail.com> wrote:

> > I seems Grant's mail delivery bounces messages. I delibirately reduced
> > the Cc list for sake of ping Grant in case it would pass.
>
> That would be because he is not at Linaro anymore. Added his ARM account.

Thanks!

> > On Sat, Nov 10, 2018 at 8:12 PM Andy Shevchenko
> > <andriy.shevchenko@linux.intel.com> wrote:
> > >
> > > Consider the following scenario.
> > >
> > > There are two independent devices coupled together by functional dependencies:
> > >  - USB OTG (dwc3-pci)
> > >  - extcon (tested with extcon-intel-mrfld, not yet in upstream)
> > >
> > > Each of the driver services a corresponding device is built as a module. In the
> > > Buildroot environment the modules are probed by alphabetical ordering of their
> > > modaliases. The latter comes to the case when USB OTG driver will be probed
> > > first followed by extcon one.
> > >
> > > So, if the platform anticipates extcon device to be appeared, in the above case
> > > we will get deferred probe of USB OTG, because of ordering.
> > >
> > > Now, a cherry on top of the cake, the deferred probing list contains
> > > the only two modules, i.e. USB OTG and extcon. Due to above circumstances,
> > > values in the local_trigger_count and deferred_trigger_count are not the same,
> > > and thus provokes deferred probe triggering again and again.
> > >
> > > ...
> > > [   20.678332] platform dwc3.0.auto: Retrying from deferred list
> > > [   20.694743] platform dwc3.0.auto: Driver dwc3 requests probe deferral
> > > [   20.701254] platform dwc3.0.auto: Added to deferred list
> > > [   20.706620] platform dwc3.0.auto: driver_deferred_probe_add_trigger 1 2
> > > [   20.713732] platform dwc3.0.auto: Retrying from deferred list
> > > [   20.730035] platform dwc3.0.auto: Driver dwc3 requests probe deferral
> > > [   20.736540] platform dwc3.0.auto: Added to deferred list
> > > [   20.741889] platform dwc3.0.auto: driver_deferred_probe_add_trigger 3 4
> > > [   20.748991] platform dwc3.0.auto: Retrying from deferred list
> > > [   20.765416] platform dwc3.0.auto: Driver dwc3 requests probe deferral
> > > [   20.771914] platform dwc3.0.auto: Added to deferred list
> > > [   20.777279] platform dwc3.0.auto: driver_deferred_probe_add_trigger 5 6
> > > ...
> > >
> > > Deeper investigation shows the culprit commit 58b116bce136
> > > ("drivercore: deferral race condition fix") which was dedicated to fix some
> > > other issue while bringing a regression.
> > >
> > > This reverts commit 58b116bce13612e5aa6fcd49ecbd4cf8bb59e835 for good until
> > > we will have better solution.
>
> How is behavior that's been there for 4.5 years a regression? Aren't
> we going to break Davinci that this commit was supposed to fix? What
> else could be relying on current behavior?

I guess we are purerly on a side of luck. I don't remember people
build usb and extcon as modules
and using Buildroot, or anything that probes modules let's say,in
"arbitrary" way.
(That is one of the reason why I didn't dare to put Fixes tag there).
Nevertheless, I have actual issue. I'm all ears to know about another
possibility
how to fix the issue on my side.

-- 
With Best Regards,
Andy Shevchenko