linux-kernel - Re: [PATCH v3] driver core: Break infinite loop when deferred probe can't be satisfied

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Message-ID: <20200326142350.GW1922688@smile.fi.intel.com>
Date:   Thu, 26 Mar 2020 16:23:50 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Grant Likely <grant.likely@....com>
Cc:     Peter Ujfalusi <peter.ujfalusi@...com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Saravana Kannan <saravanak@...gle.com>,
        Andrzej Hajda <a.hajda@...sung.com>,
        Artem Bityutskiy <artem.bityutskiy@...ux.intel.com>,
        Felipe Balbi <balbi@...nel.org>,
        Mark Brown <broonie@...nel.org>, Ferry Toth <fntoth@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Android Kernel Team <kernel-team@...roid.com>,
        nd <nd@....com>
Subject: Re: [PATCH v3] driver core: Break infinite loop when deferred probe
 can't be satisfied

On Thu, Mar 26, 2020 at 01:45:50PM +0000, Grant Likely wrote:
> On 26/03/2020 12:03, Andy Shevchenko wrote:
> > On Thu, Mar 26, 2020 at 11:45:18AM +0200, Peter Ujfalusi wrote:
> > > On 26/03/2020 10.39, Rafael J. Wysocki wrote:
> > > > On Wed, Mar 25, 2020 at 11:09 PM Saravana Kannan <saravanak@...gle.com> wrote:
> > > > > On Wed, Mar 25, 2020 at 5:51 AM Andy Shevchenko
> > > > > <andriy.shevchenko@...ux.intel.com> wrote:
> > 
> > ...
> > 
> > > > OK, so the situation right now is that commit 58b116bce136 has
> > > > introduced a regression and so it needs to be fixed or reverted.  The
> > > > cases that were previously broken and were unbroken by that commit
> > > > don't matter here, so you cannot argue that they would be "broken".
> > > 
> > > commit 58b116bce136 is from 2014 and the whole ULPI support for dwc3
> > > came in a year later.
> > > While I agree that 58b116bce136 fail to handle came a year later, but
> > > technically it did not introduced a regression.
> > > 
> > > The revert on the other hand is going to introduce a regression as
> > > things were working fine since 2014. Not sure why the dwc3 issue got
> > > this long to be noticed as the 58b116bce136 was already in kernel when
> > > the ULPI support was added...
> > 
> > I dare to say that is luck based on people's laziness to figure out the root
> > cause. As I pointed out in email to Saravana the issue is not limited to USB
> > case and, if my memory doesn't trick me out, I suffered from it approximately
> > in ~2014-2015 with pin control tables.
> 
> I've not been involved in this for a very long time, but from our past
> conversations and the description that is given here I still feel that this
> problem is a design bug on the dwc3 driver dependencies rather than a
> failure with driver core. dwc3 is doing something rather convoluted and it
> would be worth reevaluating how probe failures are unwound on that
> particular driver stack.

I disagree. Have you chance to look into another example I gave to Saravana?

The unbalanced increment is fragile per se, because you can't guarantee that
it will be no unsynchronization between probed successfully (unrelated!) and
deferred drivers.

-- 
With Best Regards,
Andy Shevchenko