Re: [PATCH v3 0/4] Solve postboot supplier cleanup and optimize probe ordering

[Saravana Kannan]

On Tue, Jul 2, 2019 at 5:03 PM David Collins <collinsd@xxxxxxxxxxxxxx> 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
>
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