Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
MIME-Version: 1.0
References: <20190702004811.136450-1-saravanak@google.com> <7900c670-5b3a-f950-dec9-70d98d94a84f@codeaurora.org>
In-Reply-To: <7900c670-5b3a-f950-dec9-70d98d94a84f@codeaurora.org>
From:   Saravana Kannan <saravanak@google.com>
Date:   Tue, 2 Jul 2019 17:59:54 -0700
Message-ID: <CAGETcx--+3BNjYZ6cgirNr_uZjU0464UHSUcaVHh_uTO2yWTCQ@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>,
        devicetree@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
        Android Kernel Team <kernel-team@android.com>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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.

Cheers,
Saravana

>
> Take care,
> David
>
> [1]
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/arm64/boot/dts/qcom/sdm845-mtp.dts?h=v5.2-rc7#n55
>
> --
> The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
> a Linux Foundation Collaborative Project