Re: [PATCH v1 1/5] drivercore: Revert "deferral race condition fix"

From: Rob Herring <robh@kernel.org>
To: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: 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@ti.com, a.hajda@samsung.com,
	Grant Likely <grant.likely@arm.com>
Subject: Re: [PATCH v1 1/5] drivercore: Revert "deferral race condition fix"
Date: Sun, 11 Nov 2018 13:26:30 -0600	[thread overview]
Message-ID: <CABGGisw-eid3Cq0HRu_Bw720JVy56jDCiSNyOY2s-JU6X5wRWg@mail.gmail.com> (raw)
In-Reply-To: <CAHp75VfwKbzLNrxWRhoGW3aLN1PLWhx7sXWMgyFw2kS+4KZcuQ@mail.gmail.com>

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.

> 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?

Rob