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Date:   Tue, 30 Jun 2020 19:15:42 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Saravana Kannan <saravanak@...gle.com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        "Cc: Android Kernel" <kernel-team@...roid.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1] driver core: Fix suspend/resume order issue with
 deferred probe

On Tue, Jun 30, 2020 at 7:11 PM Saravana Kannan <saravanak@...gle.com> wrote:
>
> On Tue, Jun 30, 2020 at 9:11 AM Rafael J. Wysocki <rafael@...nel.org> wrote:
> >
> > On Tue, Jun 30, 2020 at 5:39 PM Greg Kroah-Hartman
> > <gregkh@...uxfoundation.org> wrote:
> > >
> > > On Tue, Jun 30, 2020 at 03:50:58PM +0200, Rafael J. Wysocki wrote:
> > > > On Fri, Jun 26, 2020 at 10:53 PM Geert Uytterhoeven
> > > > <geert@...ux-m68k.org> wrote:
> > > > >
> > > > > Hi Saravana,
> > > > >
> > > > > On Fri, Jun 26, 2020 at 10:34 PM Saravana Kannan <saravanak@...gle.com> wrote:
> > > > > > On Fri, Jun 26, 2020 at 4:27 AM Rafael J. Wysocki <rafael@...nel.org> wrote:
> > > > > > > On Thu, Jun 25, 2020 at 7:52 PM Saravana Kannan <saravanak@...gle.com> wrote:
> > > > > > > > On Thu, Jun 25, 2020 at 10:47 AM Rafael J. Wysocki <rafael@...nel.org> wrote:
> > > > > > > > > Note that deferred probing gets in the way here and so the problem is
> > > > > > > > > related to it.
> > > > > > > >
> > > > > > > > I mean, we officially support deferred probing. Shouldn't we fix it so
> > > > > > > > that it doesn't break suspend/resume?
> > > > > > >
> > > > > > > Yes, we should fix deferred probing.
> > > > >
> > > > > Please take into account that breakage is an actual regression.
> > > > >
> > > > > > > > Also, it's pretty easy to have
> > > > > > > > cases where one module probes multiple device instances and loading it
> > > > > > > > in one order would break dpm_list order for one device and loading it
> > > > > > > > in another order would break it for another device. And there would be
> > > > > > > > no "proper" order to load modules (because module order != device
> > > > > > > > order).
> > > > > > >
> > > > > > > I'm not saying that the current code is perfect.  I'm saying that the
> > > > > > > fix as proposed adds too much cost for everybody who may not care IMO.
> > > > > >
> > > > > > Ok, how about I don't do this reordering until we see the first
> > > > > > deferred probe request? Will that work for you? In that case, systems
> > > > > > with no deferred probing will not incur any reordering cost. Or if
> > > > > > reordering starts only towards the end, all the previous probes won't
> > > > > > incur reordering cost.
> > > > >
> > > > > That first deferred probe request is more or less as of the first probe,
> > > > > since commit 93d2e4322aa74c1a ("of: platform: Batch fwnode parsing when
> > > > > adding all top level devices"), at least on DT systems.
> > > >
> > > > The deferred probe reordering of devices to the end of dpm_list
> > > > started in 2012, so it is nothing new, and it demonstrably works for
> > > > devices where the dependencies are known to the driver core.
>
> Isn't "where the dependencies are known to the driver core" this a big caveat?
>
> > > >
> > > > That said, in the cases when the dependencies are known to the driver
> > > > core, it is also unnecessary to reorder dpm_list in
> > > > deferred_probe_work_func(), because the right ordering of it is going
> > > > to be determined elsewhere.
>
> Until driver core knows about 100% of the dependencies, we still need
> to do some kind of dpm_list reordering to have correct ordering. Even
> with fw_devlink=on, I'd imagine it'd be difficult to achieve 100%
> dependency being known to driver core.
>
> > > >
> > > > Also commit 494fd7b7ad10 ("PM / core: fix deferred probe breaking
> > > > suspend resume order") is not the source of the problem here, because
> > > > the problem would have still been there without it, due to the
> > > > device_pm_move_last() that was there before, so the Fixes: tag
> > > > pointing to that commit is misleading.
> > > >
> > > > Now, because 716a7a259690 ("driver core: fw_devlink: Add support for
> > > > batching fwnode parsing") is an optimization and the regression is
> > > > present because of it AFAICS, the best way to address it at that point
> > > > would be to revert commit 716a7a259690 for 5.8 and maybe do the
> > > > optimization more carefully.
>
> No, this patch is not adding any new issues to deferred probe. It just
> increases the probability of reproducing the issue. That's exactly why
> I wrote the commit text for this patch without the fwnode batch
> processing example. Even if you revert the patch, suspend/resume
> ordering is broken if deferred probe happens.
>
> > > >
> > > > Greg, what do you think?
> > >
> > > I've been ignoreing this and letting you all sort it out :)
> > >
> > > But if you think that patch should be reverted, I'll not object and will
> > > be glad to to it if this solves the issue.
> >
> > Well, if Geert can confirm that reverting commit 716a7a259690 makes
> > the problem go away, IMO this would be the most reasonable thing to do
> > at this stage of the cycle without risking that more regressions will
> > be introduced.
>
> I already have a patch to avoid deferred probe during batch fwnode
> parsing. I'm trying to do a few more tests before I send it out. So,
> it'd be nice if we don't revert it right now and give me some time to
> finish testing.

If you have an alternative fix, let's see it before deciding what to do.

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