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From:   Saravana Kannan <saravanak@google.com>
Date:   Wed, 3 Jul 2019 15:26:46 -0700
Message-ID: <CAGETcx_by9aShONfSAR8rfhC69nBzeEhrZSHhOb7HuUBeCu=JA@mail.gmail.com>
Subject: Re: [PATCH v3 0/4] Solve postboot supplier cleanup and optimize probe ordering
To:     David Collins <collinsd@codeaurora.org>
Cc:     Rob Herring <robh+dt@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Frank Rowand <frowand.list@gmail.com>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>,
        Android Kernel Team <kernel-team@android.com>
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On Tue, Jul 2, 2019 at 5:59 PM Saravana Kannan <saravanak@google.com> wrote:
>
> On Tue, Jul 2, 2019 at 5:03 PM David Collins <collinsd@codeaurora.org> wrote:
> >
> > Hello Saravana,
> >
> > On 7/1/19 5:48 PM, Saravana Kannan wrote:
> > ...
> > > TODO:
> > > - For the case of consumer child sub-nodes being added by a parent
> > >   device after late_initcall_sync we might be able to address that by
> > >   recursively parsing all child nodes and adding all their suppliers as
> > >   suppliers of the parent node too. The parent probe will add the
> > >   children before its probe is completed and that will prevent the
> > >   supplier's sync_state from being executed before the children are
> > >   probed.
> > >
> > > But I'll write that part once I see how this series is received.
> >
> > I don't think that this scheme will work in all cases.  It can also lead
> > to probing deadlock.
> >
> > Here is an example:
> >
> > Three DT devices (top level A with subnodes B and C):
> > /A
> > /A/B
> > /A/C
> > C is a consumer of B.
> >
> > When device A is created, a search of its subnodes will find the link from
> > C to B.  Since device B hasn't been created yet, of_link_to_suppliers()
> > will fail and add A to the wait_for_suppliers list.  This will cause the
> > probe of A to fail with -EPROBE_DEFER (thanks to the check in
> > device_links_check_suppliers()).  As a result device B will not be created
> > and device A will never probe.
> >
> > You could try to resolve this situation by detecting the cycle and *not*
> > adding A to the wait_for_suppliers list.  However, that would get us back
> > to the problem we had before.  A would be allowed to probe which would
> > then result in devices being added for B and C.  If the device for B is
> > added before C, then it would be allowed to immediately probe and
> > (assuming this all takes place after late_initcall_sync thanks to modules)
> > its sync_state() callback would be called since no consumer devices are
> > linked to B.
> >
> > Please note that to change this example from theoretical to practical,
> > replace "A" with apps_rsc, "B" with pmi8998-rpmh-regulators, and "C" with
> > pm8998-rpmh-regulators in [1].
>
> Interesting use case.
>
> First, to clarify my TODO: I was initially thinking of the recursive
> "up-heritance" of suppliers from child to parent to handle cases where
> the supplier is a device from some other top level device (or its
> child). My thinking has evolved a bit on that. I think the parent
> needs to inherit only from it's immediate children and not its
> grandchildren (the child is responsible for handling grandchildren
> suppliers). I'll also have to make sure I don't try to create a link
> from a parent device to one of its child device nodes (should be easy
> to check).
>
> Anyway, going back to your case, for dependencies between child nodes
> of a parent, can't the parent just populate them in the right order?
> You can loop through the children and add them in multiple stages.
>
> I'll continue to think if I can come up with anything nicer on the
> drivers, but even if we can't come up with anything better, we can
> still make sync_state() work.

There's actually a much better way to handle this case where you won't
have to handle ordering on the driver side. I just need to add one or
two patches to my patch series. I'll send that out sometime next week.


-Saravana